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Graduate Theses and Dissertations Iowa State University Capstones, Theses and Dissertations
2021
An evaluation of tillage radish (Raphanus sativus L.) to alleviate An evaluation of tillage radish (Raphanus sativus L.) to alleviate
post-construction soil compaction and germination potential post-construction soil compaction and germination potential
under varying environmental conditions under varying environmental conditions
Marcus David Jansen Iowa State University
Follow this and additional works at: https://lib.dr.iastate.edu/etd
Recommended Citation Recommended Citation Jansen, Marcus David, "An evaluation of tillage radish (Raphanus sativus L.) to alleviate post-construction soil compaction and germination potential under varying environmental conditions" (2021). Graduate Theses and Dissertations. 18517. https://lib.dr.iastate.edu/etd/18517
This Thesis is brought to you for free and open access by the Iowa State University Capstones, Theses and Dissertations at Iowa State University Digital Repository. It has been accepted for inclusion in Graduate Theses and Dissertations by an authorized administrator of Iowa State University Digital Repository. For more information, please contact [email protected].
An evaluation of tillage radish (Raphanus sativus L.) to
alleviate post-construction soil compaction and germination potential under varying
environmental conditions
by
Marcus David Jansen
A thesis submitted to the graduate faculty
in partial fulfillment of the requirements for the degree of
MASTER OF SCIENCE
Major: Horticulture
Program of Study Committee:
Grant L. Thompson, Major Professor
Ajay Nair
Marshall McDaniel
The student author, whose presentation of the scholarship herein was approved by the program
of study committee, is solely responsible for the content of this thesis. The Graduate College will
ensure this thesis is globally accessible and will not permit alterations after a degree is conferred.
Iowa State University
Ames, Iowa
2021
Copyright © Marcus David Jansen, 2021. All rights reserved.
ii
DEDICATION
This thesis is dedicated to my late grandfather, John “Dave” Duncan. You instilled in me
a love for the land and shared a passion for trees. Like the trees, your roots were strong, your life
was abundant, and your legacy will continue to grow.
iii
TABLE OF CONTENTS
Page
LIST OF FIGURES .........................................................................................................................v
LIST OF TABLES ......................................................................................................................... vi
ACKNOWLEDGMENTS ............................................................................................................ vii
ABSTRACT ................................................................................................................................. viii
CHAPTER 1. GENERAL INTRODUCTION ................................................................................1 Introduction................................................................................................................................ 1 Effects of Compaction on Soil ................................................................................................... 2 Effects of Compaction on Plant Development .......................................................................... 5 Causes of Compaction ............................................................................................................... 8 Conclusion ............................................................................................................................... 11 References................................................................................................................................ 11
CHAPTER 2. EVALUATION OF TILLAGE RADISH COVER CROP TO ALLEVIATE SOIL
COMPACTION AND SUBSEQUENT ESTABLISHMENT OF A MODEL ORNAMENTAL
PERENNIAL .................................................................................................................................17 Abstract .................................................................................................................................... 17 Introduction.............................................................................................................................. 18 Methods and Materials ............................................................................................................ 20
Phase 1 – Radish establishment.......................................................................................... 21 Phase 2 – Radish termination ............................................................................................. 22 Phase 3 – Coral bells establishment ................................................................................... 23
Results...................................................................................................................................... 24 Radish growth and harvest ................................................................................................. 24 Coral bells growth and harvest following radish termination ............................................ 25
Discussion ................................................................................................................................ 30 Radish growth and harvest ................................................................................................. 30 Coral bells growth and harvest ........................................................................................... 31 Radish termination effects on coral bells ........................................................................... 32 Radish allelopathy nutrient unavailability .......................................................................... 33 Limitations and future research .......................................................................................... 34
Conclusion ............................................................................................................................... 36 Acknowledgements.................................................................................................................. 37 References................................................................................................................................ 37 Appendix A. Tillage Radish Aboveground Growth at Harvest ............................................... 41 Appendix B. Tillage Radish Belowground Growth at Harvest ............................................... 41 Appendix C. Coral Bells Aboveground Growth at Harvest .................................................... 42
iv
CHAPTER 3. EVALUATION OF GERMINATION PERCENTAGE OF FIVE TILLAGE
RADISH CULTIVARS AT VARYING AIR TEMPERTURES AND WATER AMOUNTS ....43 Abstract .................................................................................................................................... 43 Introduction.............................................................................................................................. 44 Methods & Materials ............................................................................................................... 46
Treatments .......................................................................................................................... 47 Blotter Paper Germination Trial ......................................................................................... 47 Media Germination Trial .................................................................................................... 48
Results...................................................................................................................................... 50 Discussion ................................................................................................................................ 58
Air Temperature ................................................................................................................. 58 Water .................................................................................................................................. 59 Cultivar ............................................................................................................................... 60 Blotter Paper vs. Media Substrate ...................................................................................... 61 Application ......................................................................................................................... 62 Limitations and Future Research ........................................................................................ 63
Conclusion ............................................................................................................................... 64 Acknowledgements.................................................................................................................. 65 References................................................................................................................................ 65
CHAPTER 4. GENERAL CONCLUSIONS .................................................................................69 General Discussion .................................................................................................................. 69 Recommendations for Future Research ................................................................................... 72 References................................................................................................................................ 74
v
LIST OF FIGURES
Page
Chapter 1
Figure 1. Major interactions between soil properties and plant root function at the root-soil
interface in a compacted soil. ..................................................................................... 8
Chapter 2
Figure 1. (A) Mean calculated ellipsoidal volume (cm3) of radishes (Raphanus sativus L.
‘Nitro’) at each compaction level across the 8 weeks following seeding in
containers in the greenhouse. ................................................................................... 27
Figure 2. Mean calculated ellipsoidal volume (cm3) of coral bells (Heuchera micrantha var.
diversifolia ‘Palace Purple’) at each compaction level across the 8 weeks
following plug transplanting in containers in the greenhouse for the (A) cold
termination radish (CTR) treatment, (C) herbicide termination radish (HTR)
treatment, and (E) no radish control (NRC) treatment. ............................................ 29
Figure 3. Aboveground growth of tillage radishes (Raphanus sativus L. ‘Nitro’) in the
Harvested Radish (HR) treatment at week (56 days) arranged with increasing
soil compaction level bulk density from left to right: (CL 1) 0.96 g*cm-3, (CL
2) 1.11 g*cm-3, (CL 3) 1.28 g*cm-3, (CL4) 1.44 g*cm-3, and (CL 5)1.6 g*cm-3. .... 41
Figure 4. Belowground growth of tillage radishes (Raphanus sativus L. ‘Nitro’) in the
Harvested Radish (HR) treatment at week (56 days) arranged with increasing
soil compaction level bulk density from left to right: (CL 1) 0.96 g*cm-3, (CL
2) 1.11 g*cm-3, (CL 3) 1.28 g*cm-3, (CL4) 1.44 g*cm-3, and (CL 5)1.6 g*cm-3. .... 41
Figure 5. Aboveground growth of coral bells (Heuchera micrantha var. diversifolia ‘Palace
Purple’) in the (A) cold termination radish (CTR) treatment, (B) herbicide
termination radish (HTR) treatment, and (C) no radish control (NRC) treatment
at week (56 days) arranged with increasing soil compaction level bulk density
from left to right: (CL 1) 0.96 g*cm-3, (CL 2) 1.11 g*cm-3, (CL 3) 1.28 g*cm-3,
(CL4) 1.44 g*cm-3, and (CL 5)1.6 g*cm-3................................................................ 42
Chapter 3
Figure 1. Mean percent germination of radish cultivars across tested air temperatures (C )
throughout the blotter paper trial in the growth chambers. ...................................... 55
Figure 2. Mean percent germination of radish cultivars across tested water amounts (mL )
throughout the blotter paper trial in the growth chambers. ...................................... 57
Figure 3. Regression model of percent germination between radish cultivars in the blotter
paper germination trial and the jiffy tray media germination trial. .......................... 58
vi
LIST OF TABLES
Page
Chapter 2
Table 1. Summary of planting treatment groups showing abbreviations and actions during
the phases 1, 2, and 3 in a greenhouse. ..................................................................... 24
Table 2. Analysis of variance table for radish germination, aboveground radish growth,
radish leaf area, radish aboveground biomass, and radish belowground biomass
in radish growth phase (phase 1) in a greenhouse. ................................................... 26
Table 3. Analysis of variance table for coral bells aboveground growth, volume at harvest,
leaf area, and aboveground biomass between growth planting treatment groups
in the coral bells growth phase (phase 3) in a greenhouse. ...................................... 28
Chapter 3
Table 1. Mean air temperatures (C and F) and rainfall (mL per month and per day) for the
months of the growing season (April – November) in Polk County, Iowa form
1990-2019. ................................................................................................................ 49
Table 2. Analysis of variance table for radish germination within and between treatments
throughout the blotter paper trial in the growth chambers and between
treatments within specific cultivars. ......................................................................... 51
Table 3. Tukey test comparison between cultivars within the air temperature treatments in
the blotter paper trial in the growth chambers. ......................................................... 53
Table 4. Tukey test comparison between cultivars with the water amount treatments in the
blotter paper trial in the growth chambers. ............................................................... 55
vii
ACKNOWLEDGMENTS
I express my gratitude towards my advisor and committee chair, Dr. Grant Thompson for
his mentorship, collaboration, and investment in me as his first graduate student. I also extend
my appreciation towards my committee members, Dr. Ajay Nair and Dr. Marshall McDaniel for
their contributions to this thesis along with their guidance throughout the course of this research.
Furthermore, I recognize Dr. Diana Cochran for her support through my start as a graduate
student.
Additionally, I thank my fellow researchers in the Sustainable Landscapes and
Management Lab, Abigail Enos, Cody McKune, Connor Evers, Elizabeth Hurley-Blewett, Emily
Meader, and Justin Wigdahl for their assistance with the execution of my studies and data
collection. I acknowledge Pete Lawlor for his guidance and expertise with the work that I
conducted in the greenhouse. I also share appreciation for the community of faculty and staff in
the Department of Horticulture and the Horticulture Research Station for their continued support
throughout my time as a student at Iowa State University. Lastly, I profess my gratefulness
towards my family, friends, and fellow graduate students for their encouragement, fellowship,
and memories throughout this experience.
viii
ABSTRACT
Tillage radish (Raphanus sativus L.) has been utilized as a cover crop for alleviating soil
compaction and scavenging nutrients in agricultural crop productions systems. Recognizing the
improvements of compacted soils from a tillage radish cover crop in agricultural cropping
systems and the need for compaction remediation in post-construction urban soils where
landscape plants will be established, we sought to test the potential of tillage radish for post-
construction applications. Unlike the more predictable seasonal periods of sewing, terminating,
and incorporating a tillage radish cover crop in an annual agricultural crop rotation, the timing
and environmental conditions of building and landscape construction is more variable, which
may have an effect on cover crop radish seed germination.
In the first study, a simulated landscape system in a controlled environment was used to
evaluate the performance of tillage radish (Raphanus sativus L. ‘Nitro’) at five soil compaction
levels and the impacts of that cover crop on the growth of the following establishment of a model
herbaceous perennial – coral bells (Heuchera micrantha var. diversifolia (Rydb.) Rosend.,
Buttters & Lakela ‘Purple palace’). Radishes produced comparable amounts of mean
aboveground and belowground dry biomass through a range of compacted soils but showed
reduced mean biomass at the highest soil compaction level. Coral bells following the no radish
control treatment produced more aboveground dry mean biomass than coral bells following the
cold termination radish treatment, which is speculated to be a result of allelopathic effects and/or
nutrient unavailability. The findings of this study provide evidence that tillage radishes have the
capacity to grow successfully at a range of soil bulk densities, but more work is needed better
understand the management considerations for adapting agricultural cover crops, such as tillage
radish, for use in ornamental landscapes and in post-construction soils.
ix
The aim of the second study was to test the effects of temperature and soil water
availability on germination of five tillage radish cultivars. We used a growth chamber to evaluate
germination rates of these cultivars at eight air temperatures (8 – 38 ºC) crossed factorially with
four soil water contents (25 – 55ml). We found that air temperature significantly affected the
germination percentage across all cultivars, and the optimal range for germination was 23-33 ℃.
Water rates significantly affected the germination of Nitro, Smart, and Badger radish cultivars,
suggesting that irrigation management during the germination of these cultivars may be crucial
to their success. In our study conditions, the Nitro radish cultivar has the highest overall mean
germination percentage and proved to be successful across a range of air temperatures and water
treatment combinations. The findings of this study have potential value in providing estimated
germination rates of these cultivars across a range of environmental conditions, including sub-
optimal air temperatures and water levels.
1
CHAPTER 1. GENERAL INTRODUCTION
Introduction
Soil compaction continues to be one of the leading threats to soil health (McGarry, 2003)
and plant performance, impacting the many industries across agriculture that rely on soils to
support plant growth. A broad field of research spanning managed landscapes rom row-crop
fields to turf grass systems has developed around understanding the phenomenon of soil
compaction, identifying the causes, and quantifying the effect on plant growth and production
(Nawaz, Bourrié and Trolard, 2013; Hu et al., 2021). The influence of soil compaction on soil
properties and crop growth are complex (Batey, 1990). Reviews have highlighted the range of
research addressing the modeling of soil compaction (O’Sullivan and Simota, 1995; Lipiec and
Hatano, 2003) along with the physical (Soane, Dickson and Campbell, 1982; Horn et al., 1995),
biological (Frey et al., 2009; Pupin, Freddi and Nahas, 2009), and chemical (De Neve and
Hofman, 2000; Tamene et al., 2019) properties of soil affected by forces resulting in compaction.
With a growing understanding of soil compaction, research has continued to identify and
quantify the causes of compaction (Alakukku et al., 2003; Nawaz, Bourrié and Trolard, 2013;
Shah et al., 2017; Keller et al., 2019), effects on crop growth (Drewry, Cameron and Buchan,
2001; Unger and Kaspar, 1994; Nawaz, Bourrié and Trolard, 2013), generalized impacts on the
environment (Batey, 2009; Palmer and Smith, 2013; Schjønning et al., 2015; Hu et al., 2021),
and economic costs of soil compaction (Graves et al., 2015). This review will explore the
fundamentals of this issue by defining and quantifying soil compaction by outlining the wholistic
effects of compaction on physical soil properties. It will also address the physiological reactions
of plants in a compacted soil, examining the response of root (belowground) development and
the subsequent effect on shoot (aboveground) growth and the success or yield of plants. Lastly,
2
connections will be made from these fundamental principles to real-world applications,
discussing the major causes and implications of soil compaction.
Effects of Compaction on Soil
Soil compaction occurs when soil particles are compressed together by external forces,
decreasing the pore space between these particles and creating a denser media. (Soil Science
Society of America, 2008). Soil compaction occurs in topsoil (Bouwman and Arts, 2000), but is
most detrimental to subsoils (Jones, Spoor and Thomasson, 2003). Soil compaction is often
quantified in terms of bulk density or resistance to penetration (Passioura, 2002).
Bulk density is defined as the mass of particles divided by the total volume they occupy
(Soil Science Society of America, 2008). Soil bulk density is determined by dividing the dry
weight of soil by the total volume of the sample, but is also estimated with a variety of direct and
indirect methods (Al-Shammary et al., 2018) and is typically expressed in units of grams per
cubic centimeter (gcm-3, Blake, 1965). Compaction increases soil bulk density and impacts
nearly all other soil physical properties (Shah et al., 2017). In a typical loam soil, bulk density is
usually ~1.3 g cm-3 and root growth is severely impaired or stops when soil bulk density reaches
the range of 1.5-1.7 g cm-3 (Jin et al., 2017). Soil texture alters the range of values at which soil
bulk density becomes root limiting. (Jones, 1983; Pierce et al., 1983; Unger and Kaspar, 1994).
Soils with a higher clay content become more inhibitory to root growth when compacted, thus
are problematic at lower soil bulk densities, whereas sandy soils can maintain root growth at
higher soil bulk densities (Atwell, 1993). Different from soil bulk density, the resistance of soil
to penetration by an applied force provides a relative measure of the difficulty of roots to grow in
compacted soils. This metric can be measured with a penetrometer or more specifically, a cone
3
penetrometer (Herrick and Jones, 2002; Mome Filho et al., 2014), and is measured in
megapascals (MPa). Soil penetration resistance and the depth at which threshold pressures are
reached has become a standard measurement within soil management and soil physics research
(Medeiros et al., 2011, 2013; Mome Filho et al., 2014), because of the ease and efficiency of
testing (Busscher, Frederick and Bauer, 2000), although results may be more variable than soil
bulk density. Research by Chaney, Hodgson and Braim (1985) suggests that compaction causes a
significant increase in penetration resistance. While soil bulk density provides a direct
representation of the physical compactness of the soil, penetration resistance has shown to be a
strong predictor of the ability of roots to frow in compacted soils (Busscher and Bauer, 2003;
Otto et al., 2011; Bécel, Vercambre and Pagès, 2012).
A related factor when considering compaction, is soil porosity. Compacted soils show a
decrease in soil porosity or a change in the amount, size, and connectedness of pores that may
hold water or air (Dexter, 1988). Changes in soil porosity from compaction results in decreased
pore size and connectivity, restricting the permeability of air and water into and out of the soil
(Richard et al., 2001). Decrease in macropores can result in the development of anoxic
conditions (Correa et al., 2019), interfering with crop growth and development. This reduction in
pore space also has a negative impact on the water availability of the soil (Katou, Miyaji and
Kubota, 1987; Dexter, 2004). Alteration in pore size distribution due to compaction results in
increased runoff, decreased infiltration, and higher erosion losses (Shah et al., 2017). Along with
this chain of negative effects of compaction on soil physical properties, soil aggregate stability
index and soil hydraulic conductivity are also reduced (Potocka and Szymanowska-Pulka, 2018).
Aggregate stability refers to the ability of soil aggregates to resist disintegration when disruptive
forces are applied (Shah et al., 2017). Disturbed soils lose stability to resist further compaction,
4
causing the structure of soil aggregates to collapse and further degrade. Hydraulic conductivity
describes the ease with which water can move through pore spaces or fractures with respect to
the intrinsic permeability of the material and the degree of saturation (Soil Science Society of
America, 2008). Hydraulic conductivity is highly responsive to soil degradation resulting from
compaction (Whalley, Dumitru and Dexter, 1995) and consequently reduced porosity (Matthews
et al., 2010). Furthermore, hydraulic conductivity varies with different soil depths even within
the same soil profiles, typically declining with depth (Nakano and Miyazaki, 2005). With the
reduced pore space of compacted soils, water less able to move downwards in the soil and
waterlogging can become an issue (Batey, 2009). A compacted soil can locally become an
anaerobic environment due to the reduction in gas diffusion through a soil (Hamamoto et al.,
2012). A compacted soil subject to waterlogged and reduced gas exchange resulting in anaerobic
conditions has substantial ramifications. Anoxia impedes aerobic microbial mineralization and
encourages anaerobic denitrification activity. The anerobic microbial activity leads to reduced
nitrogen availability in the soil.
Other factors intertwined with compaction effects include the amount of soil organic
matter and soil water content. When soil organic matter decreases, there is a loss of aggregate
stability and thus structural stability, resulting in a higher susceptibility to compaction (Casanova
et al., 2013). Increased soil mechanical resistance under a variety of water potential levels is also
a consequence of reduced organic matter (To and Kay, 2005). Soil organic matter has been
identified as a key contributor to the formation of soil pores and soil aggregates, consequently
affecting soil gas exchange (Hamamoto et al., 2012). Soil strength increases with decreasing soil
water content (Bengough et al., 2011). Therefore, in dryer soils root growth can be limited by a
combination of increased resistance to root penetration and water deficiency (Kolb, Legué and
5
Bogeat-Triboulot, 2017). While we may conceptualize soil compaction as a singular problem,
this issue has many contributing factors and results in numerous changes to soil physical
properties that result in cascading outcomes for soil chemistry, biology, and overall soil health.
Effects of Compaction on Plant Development
Soil compaction lowers plant performance by altering and largely hindering belowground
root growth and consequently reducing aboveground plant growth (Figure 1). Root growth is
often slowed by a combination of soil physical stresses, including mechanical impedance, water
stress, and oxygen deficiency. The stress on the plant may vary significantly depending on the
location of the root in the soil profile, the prevailing soil water conditions, and the degree to
which the soil has been compacted (Bengough et al., 2006). In a compacted soil, roots are
continuously exposed to mechanical pressure and this often results in morphological
modifications. The most notable of these root changes are in the overall architecture of the root
system and shape of particular roots. These changes are often accompanied by modifications of
the cell structure and cell morphology (Potocka and Szymanowska-Pulka, 2018). Root system
architecture (RSA) refers to the specific arrangement of root components, encompassing the
overall form of the root system and shape of particular roots. The development of a particular
root system architecture pattern results from the processes of root tip extension, lateral root
formation, and root tropism (Correa et al., 2019). Considering the general influence of root
system architecture on the plant’s exploration of soil for water and nutrients, detriments to proper
architectural development resulting from compaction can have deleterious outcomes for the
overall plant. A review conducted by Potocka and Szymanowska-Pulka (2018) summarizes the
specific effects of mechanical stress on root systems architecture across many plant species.
6
The primary influence of soil compaction or soil strength on root system architecture is a
decrease in total root length (Pfeifer et al., 2014) parallel to an increase in root diameter (Popova
et al., 2016). At a bulk density of ~1.5-1.7 gcm-3 (Pierce et al., 1983) or a penetrometer
resistance of ~ 5MPa (Bengough et al., 2011), shorter and thicker roots have been observed. This
increase in root diameter in response to compacted soils is a mechanism that the plant uses to
push through substrates with higher penetration resistance at the same penetration pressure
imposed by root extension (Popova et al., 2016). Roots must exert a pressure in length and girth
in order to displace soil particles, overcome friction, and elongate through the soil. If a plant is
able to maintain a larger root system with more roots in compacted soil, it would result in greater
soil exploration than a plant with a severely stunted root system with fewer roots. However, the
potential for root penetration and growth into a compacted soil layer is also dependent on how
plastic the root angles and diameter are. The angle at which the roots grows also determines the
direction of root elongation and the soil volume the root system can occupy for water and
nutrient uptake. Roots in a compacted soil tend to forge a shallower growth angle as the
mechanical force and strength of the soil deters them from growing down at a steeper angle.
When roots hit a compacted layer of soil such as a hardpan, there can be different outcomes on
growth: roots may avoid the deleterious effects of compaction by inducing a more lateral growth
direction; roots may penetrate the hardpan and grown down deeper into the soil profile; or root
growth may stop completely (Clark, Whalley and Barraclough, 2003). Root growth response to a
compacted layer is variable and depends on species and situational conditions, but ultimately
plasticity of root diameter and steepness of angle in response to encountering higher density soils
results in the capacity of a root system to cope with hardpan layers. The property of root
tortuosity, or waviness of growth along the root length, is also influenced by soil compaction. An
7
increase in root tortuosity is a mechanism that plants use to avoid soil frictional resistance to root
tip penetration and increase the potential for exploring larger volumes of soil (Popova et al.,
2016).
Plants root-shoot ratios change as a function of plant size and stage of development.
Typically, younger and smaller plants have a higher ratio of roots to shoots. With more biomass
belowground, younger plants are more susceptible to stunting and less capable of transferring
energy when growing in compacted soils. The root system architecture in compacted soil will
often respond to the restriction of apical roots from compaction by increasing lateral root growth
that is less subject to compaction with depth. Compensatory growth may be a strategy of
adaptive plasticity to counter the limited function of a root system impeded by compaction by
growing less in areas with high soil strength and growing more where soil strength is lower
(Correa et al., 2019). Besides directly interfering with root proliferation, soil compaction also
causes numerous indirect effects on aboveground plant growth. Lower tissue nutrient
concentration has been observed in plants growing in compacted soils. This is likely due to a
combination of reduced nutrient availability in compacted soils and reduced root exploration in
the soil. Low yields under severely compacted soils are correlated with low concentrations of
nitrogen, phosphorus, and potassium (Arvidsson, 1999). Some research has found that in
compacted soils, reduced water uptake resulted in reduced stomatal conductance and higher
accumulation of abscisic acid in roots (Tardieu et al., 1992). Research by Young et al., (1997)
attributed the reduction in leaf appearance rate to a hormonal signal generated by impeded roots.
Reduced photosynthesis from stomatal or non-stomatal inhibition can also be attributed to
conditions of compacted soil. Research suggest that negative effect on photosynthesis, water
8
relations, and shoot growth caused by soil compaction could be closely related to the sensitivity
of root system architecture with high mechanical impedance of soil (Tubeileh et al., 2003).
Figure 1. Major interactions between soil properties and plant root function at the root-soil
interface in a compacted soil.Arrows indicate the influence of one property on another and
circles indicate a combined influence. Adapted from Correa et al., 2017.
Causes of Compaction
Soil compaction reduces agriculture productivity and a major concern for growers.
Intensive agricultural practices are beginning to be recognized as the cause of major soil
structure degradation (Palmer and Smith, 2013; Tamene et al., 2019), though compacted soils
can also occur under natural conditions without human or animal involvement (Potocka and
Szymanowska-Pulka, 2018). High mechanical load, less crop diversity, intensive grazing, and
irrigation methods contribute to soil compaction. Soil compaction is further magnified when
9
these management practices are accompanied by low soil organic matter, animal traffic, engine
vibrations, and tillage at high moisture contents (Batey, 2009). Efforts have been made to
quantify these effects of soil compaction and to analyze spatial and temporal relationship
between extent of compaction and its causes (Nawaz, Bourrié and Trolard, 2013).
Many agricultural and industrial operations require the use of heavy machinery for
farming practices or construction activities. Mechanically induced soil compaction has been
identified and characterized (Smith, Johnston and Lorentz, 1997), and research has also
addressed the interactions and compounded effects of a variety of factors including soil physical
properties, wheeling, the number of passes, specific farming practices, soil structure, soil water
status, and crop rotations (Hamza and Anderson, 2005). The severity and implications of
compaction can be affected by the size and weight of the machinery used, an issue that is
becoming more prevalent with the increase in industrialized agriculture and the use of larger
farming and construction implements (Keller et al., 2019). Axel load has been pinpointed as a
major cause of compaction, making it the focus of many studies, and a reliable measure to gauge
compaction potential. Ground contact pressure can be determined by axle load divided by the
surface area of contact between machine and soil. Ground pressure contributes to top soil
compaction, while high axle load leads to subsoil compaction (Botta et al., 1999). Different soil
textures result in variation in compaction and compaction resistance to different pressure
magnitudes (Ellies Sch. et al., 2000). Vibration due to heavy mechanical implements can
compact soils effectively at higher moisture contents. Beyond simple axel load, vibrations
actually impose additional impact and high intensity pressure on soil particles. The speed of a
implement together with vibration intensity can cause significant soil compaction. The threat of
10
heavy machinery certainly is a multi-faceted issue with many points of contention to be
considered (Soane, Dickson and Campbell, 1982).
Livestock production is a fundamental component of agriculture and certainly provides
its own contributions to soil compaction. Soil type and soil moisture are factors that determine
the magnitude of soil compaction as a result of livestock grazing. For example, fine-textured
soils are more vulnerable to trampling by grazing animals than coarse-textured soil (Batey,
2009). Furthermore, dry soils experience less trampling action due to higher aggregate stability
index, while moist soils are more vulnerable to compaction (Mosaddeghi et al., 2000). Similar to
the issue of axel load in machinery, livestock concentrate a substantial weight on a small surface
area with their hooves. Compaction by animals proves to be more destructive than machinery.
Implement tires are typically wider than an animal’s hoof, thus there is a decreased ground
pressure with an increase contact surface area.
While water is one of the most essential resources for plant growth, it can also contribute
to soil compaction. Soil moisture content is the most influential factor of the soil itself that that
increases compaction potential, as soil moisture reduces soil strength and allows soil particles to
slip and move more easily (Lipiec and Hatano, 2003). Due to this, soil water content becomes
crucial during soil tillage to reduce the potential for compaction. The effect of soil moisture
content is strongest in the subsoil, while the effect of rain impact is more detrimental in the
surface of topsoil. The direct impacts raindrops can disperse soil particles by breaking the soil
surface crust. Through this dispersal, fine particles become separated from soil clods, which
aided by the movement of infiltrating water settle down into soil pores make a hard layer of
compacted soil. Soil compaction thus results from many contributing and interrelated factors.
While many of these factors cannot be avoided, completely managed, or entirely mitigated,
11
identifying limits and relative importance of these various factors may help in a holistic approach
at reducing soil compaction.
Conclusion
Soil compaction continues to be one of the leading contributors to soil degradation,
warranting the need for further research. Numerous factors ranging from soil moisture content to
mechanical load are responsible for soil compaction threatening soil health via directly or
indirectly modifying soil physical, chemical, and biological properties. Beyond soil properties,
compaction reduces plant performance by negatively influencing the growth and the
development of plants. Stunted growth, leaf discoloration, reduced plant height, and shallow root
system are predominant morphological effects of soil compaction. Reduced nutrient uptake,
reduced leaf gas exchange, carbon assimilation, and less translocation of photosynthates are all
detrimental effects that round out the negative ramifications of soil compaction. While
sustainable practices such as alternative tillage systems, use of cover crops, and remediation of
soil are gaining in popularity and proving effective, compaction continues to be global problem
across many sectors. Through continuous research there is potential enhance the understanding
and interrelationship of these factors towards developing a holistic approach for reducing or
mitigating soil compaction.
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17
CHAPTER 2. EVALUATION OF TILLAGE RADISH COVER CROP TO
ALLEVIATE SOIL COMPACTION AND SUBSEQUENT ESTABLISHMENT OF A
MODEL ORNAMENTAL PERENNIAL
Marcus D. Jansen1 and Grant L. Thompson1
1Iowa State University Department of Horticulture
Modified from a manuscript to be submitted to HortTechnology (2021)
Abstract
Recognizing the successful reduction of compaction from a tillage radish cover crop in
agricultural cropping systems and the need for compaction remediation in post-construction
urban soils where landscape plants will be established, we sought to test the potential of tillage
radish (Raphanus sativus L.) for such uses. This study used a simulated landscape system in a
controlled environment to evaluate the performance of a tillage radish (Raphanus sativus L.
‘Nitro’) cover crop at five soil compaction levels and the impacts of that tillage radish cover crop
on the growth of the following establishment of a model herbaceous perennial – coral bells
(Heuchera micrantha var. diversifolia ‘Purple palace’). We found that tillage radish germination
was unaffected by soil compaction level. Radishes produced comparable amounts of mean
biomass through a range of compacted soils (15.7-19.9g) but showed reduced mean biomass at
the highest soil compaction level (3.1g). Coral bells following an herbicide termination radish
treatment where stunted, likely due to herbicide residues in the soil. Coral bells following the no
radish control treatment produced more mean biomass (9.2g) than coral bells following the cold
termination radish treatment (7.8g). We speculate that allelopathic effects and nutrient
unavailability may have contributed to the reduced amount of growth following the cold
termination treatment. The findings of this study confirmed that tillage radishes have the capacity
to grow successfully at a range of soil bulk densities, but more work is needed better understand
18
the managements consideration for adapting agricultural cover crops for use in ornamental
landscapes.
Introduction
Soil compaction is a major issue for post-construction urban soils and can negatively
affect the establishment and growth of ornamental landscape plants. Residential and commercial
construction process, often intentionally compact soils for foundations, pavement, and other
structures (Strom, Nathan and Woland, 2013) and the general operation of heavy machinery
utilized for construction can unintentionally compact site soil, including those areas that will
become landscaping or lawns (Randrup and Dralle, 1997). Substantial effort has addressed soil
compaction and its impacts on plant growth and development (Batey, 2009). Soil compaction is a
result of soil particles being compressed together by external forces, decreasing the pore space
between particles and creating a denser growing media(Soil Science Society of America,
2008)(Soil Science Society of America, 2008)(Soil Science Society of America, 2008)(Soil
Science Society of America, 2008)(Soil Science Society of America, 2008)(Soil Science Society
of America, 2008)(Soil Science Society of America, 2008)(Soil Science Society of America,
2008). Soil compaction is often quantified in terms of bulk density and resistance to penetration
(Passioura, 2002). Reduced poor space inhibits the movement of air and water within the soil
profile (Dexter, 2004) and can affect both gravitational water and plant unavailable water, thus
affecting plant growth. Soil compaction has shown to deter root exploration and establishment of
landscape plants since increased soil bulk density results in fewer, smaller, and less well-
connected soil pores for roots to inhabit (Kozlowski, 1999; Day, Seiler and Persaud, 2000). The
negative impact of soil disruption on urban plants has been recognized, and there has been a push
for urban soil remediation to improve plant growth (Sloan et al., 2012). Some soil rehabilitation
19
or amendment practices including top soil replacement (Layman et al., 2016), mechanical tillage
or physical fracturing (Somerville, May and Livesley, 2018), and compost incorporation (Sax et
al., 2017; McGrath et al., 2020; Somerville et al., 2020) have shown to accelerate establishment
and improved success of urban landscape plants.
Agricultural soils are better studied than urban soils and may provide insights for means
of mitigating and managing deleterious soil conditions, such as compaction, that are suitable for
direct use or use with modification in urban and constructed landscapes. Soil compaction has
also proven to be a substantial concern in agricultural systems due to use of heavy machinery,
working wet fields, intensive cropping, short crop rotations, grazing, and inappropriate soil
management (Hamza and Anderson, 2005). Cover crops have been widely utilized as a soil
management practice in conventional and organic agricultural systems for numerous benefits
including soil stabilization, weed suppression, and nutrient management. (Fageria, Baligar and
Bailey, 2005; Justes, 2017). Cover crop species with extensive tap roots have been shown to be
effective at reducing soil compaction, scavenging nutrients from deep in the soil and bringing
them into the root zone, and providing root channels for subsequent crops, such that the term
“bio-drillers” is sometimes applied to these tap rooted cover crops (Williams and Weil, 2004;
Chen and Weil, 2010). Within the Brassica plant family, Raphanus sativus L. var. longipinnatus
(Daikon radish) along with comparable varieties (tillage radish, forage radish, oilseed radish) and
other radish cultivars are valued for significantly higher soil penetrating capacity (Chen and
Weil, 2010) and capacity for nutrient accumulation (Toom et al., 2019).
20
The urban application of cover crops and short-term vegetative cover have been used for
erosion control and weed suppression on exposed soil along roadways and other construction
areas (By and Busby, 2014). There is less published research about the potential use of cover
crops for amending compacted landscape soils. Recognizing the successful reduction of
compaction from a tillage radish cover crop in agricultural cropping systems and the need for
compaction remediation in post-construction urban soils where landscape plants will be
established, we sought to test the potential of tillage radish for such uses. Our objectives in this
study were to quantify the growth of tillage radishes at varying levels of landscape soil
compaction and measure the subsequent establishment of a model perennial plant following the
tillage radish cover crop.
Methods and Materials
The study was initiated on December 8, 2019 and concluded on April 30, 2020 and was
conducted in three phases: (1) radish establishment; (2) radish termination; and (3) coral bell
establishment. The experiment consisted of four planting treatments (Table 1), three of which
were planted with Raphanus sativus L. ‘Nitro’ (tillage radish, Green Cover Seed, Bladen, NE)
seeds and one of which remained as an unplanted control. Within each planting treatment, there
were five soil compaction levels and five replicates of each for a full factorial design (4
treatments * 5 experimental soil compaction treatments * 5 replicates = 100 total experimental
units). At the end of phase 1, radishes in the harvested radish (HR) treatment group were
destructively harvested. All remaining units proceeded to phase 2 where the cold termination
radish (CTR) and the no radish control (NRC) treatment groups were terminated with a
simulated winter and the herbicide termination radish (HTR) treatment group was chemically
terminated. In phase 3 the CTR, HTR, and NRC treatment sets were planted with Heuchera
21
micrantha var. diversifolia (Rydb.) Rosend., Butters, & Lakela ‘Palace Purple’ (coral bells)
plugs as a model landscape perennial to evaluate establishment subsequent to tillage radish use
or the unplanted control. Palace Purple coral bells was chosen because of its sustained
popularity, rosette and mounding growth form, ease of growth, and prior selection as a Plant of
the Year (Perennial Plant Association, 1991). Throughout the experiment, containers were
randomized by replicate group (planting treatment * soil compaction treatment) on the
greenhouse benches.
Phase 1 – Radish establishment
Field soil was collected from the Iowa State University Horticulture Research Station
(Ames, Iowa, USA). The soil type was a Clarion series, which is a fine-loamy, Typic Hapludoll
with a slope of 2-6%. Field soil was sieved through 500 µm mesh to remove large soil
aggregates and other debris. Cylindrical polyvinyl chloride (PVC) columns with an interior
height of 208 mm and radius 75 mm were fitted with bottoms with an 18 mm drainage hole and
used as containers for the experiment to simulate the soil compaction effects of typical
construction practices. Each PVC container was divided into a compacted sublayer (138mm
deep) and top-dressed with a noncompacted layer of the same soil (70mm deep), representing
respread topsoil. To achieve the desired range of soil bulk densities amongst the compaction
sublayer experimental treatment groups, different masses of soil were compacted with a hand
tamper into the set volume of the lower portion of the PVC containers. The levels of were
compacted to bulk densities of 0.96 g*cm-3 (Compaction Level 1), 1.11 g*cm-3 (Compaction
Level 2), 1.28 g*cm-3 (Compaction Level 3), 1.44 g*cm-3 (Compaction Level 4), and 1.6 g*cm-3
(Compaction Level 5). Three seeds of Raphanus sativus L. ‘Nitro’ (radish) were planted at a
depth of 2cm and 10cm apart from one another in a triangular pattern centered in each container.
22
Containers were grown in the greenhouse with a 12 hour day length and a constant temperature
of 21 5°C. An automated (Model 24600, Orbit Irrigation Products LLC, North Salt Lake, UT)
drip irrigation system with pressure compensating spray stakes (Netafim USA, 3.2 GPH) were
run for 60 seconds every other day at a rate of 300mL to each container. Radish germination was
counted when the first true leaf appeared above the soil. Containers were monitored daily at the
same time until the seeds were completely germinated. On day 15 the containers were thinned,
leaving the single strongest seedling representative in each container. Dimensional
measurements of the radish plants were taken at the same time each week. Weekly, a ruler and
digital caliper were used to measure aboveground radish height and bidirectional width (widest
point then perpendicular). The calculated ellipsoidal aboveground volume (Thorne et al., 2002)
of plants was calculated from the dimensional measurements(2/3𝜋H (A/2 x B/2), H=height,
A=width 1, B=width 2). After 56 days of growth, plants from the HR treatment group were
harvested. Aboveground and belowground biomass was calculated from fresh and dry weights.
Leaf count and leaf area (cm2) (LI-3100C; LI-COR Biosciences, Lincoln, NE) were also
documented.
Phase 2 – Radish termination
Following the radish establishment phase, plants from the CTR treatment group were
terminated with a cold period and plants from the HTR treatment group were terminated with a
chemical herbicide. Cold and herbicide termination methods were utilized as they are the
standard for cover crop radish termination in farming practice. The simulated cold winter period
was created utilizing a large storage freezer to chill the plants at -20 5C for 30 days.
Replicates in the CTR and NRC treatment groups were returned to the greenhouse for 5 days
following the cold period to thaw the soil and radish root before transplanting of the coral bells.
23
Plants from the HTR treatment group were sprayed with RoundUp® glyphosate herbicide per
label instructions (Monsanto Company, St. Louis, MO, glyphosate isopropylamine salt) and
remained in the greenhouse for 35 days before installation of the coral bells. At the end of Phase
2, foliage was removed from the terminated radishes in the CTR and HTR treatment groups and
the tap root was left to deteriorate in the soil.
Phase 3 – Coral bells establishment
Heuchera micrantha var. diversifolia ‘Purple palace’ (coral bells) plugs in 50-cell trays
were obtained from a wholesale greenhouse (Swift Greenhouses, Inc., Gilman, IA). Plugs were
installed in the uncompacted soil layer in the containers of the CTR, HTR, and NRC treatment
groups. An automated (Model 24600, Orbit Irrigation Products LLC, North Salt Lake, UT) drip
irrigation system with pressure compensating spray stakes (Netafim USA, 3.2 GPH) were run for
60 seconds every third day at a rate of 300 mL to each container. Dimensional measurements of
the coral bells were taken at the same time each week to document plant growth. Weekly height
and bidirectional width measurements were taken to track coral bell growth and determine the
aboveground ellipsoidal volume using the same formula as for the radishes. After 52 days of
growth, plants from the CTR, HTR, and NRC treatment groups were harvested. Above-ground
biomass was calculated from fresh and dry weights. Leaf count and leaf area (leaf area meter
model) were also documented. Regression analysis was utilized to identify significance at P <
0.05 between treatment. Means were separated by Tukey’s significant difference test at P < 0.05.
24
Table 1. Summary of planting treatment groups showing abbreviations and actions during the
phases 1, 2, and 3 in a greenhouse.
Treatment Group Abbreviation Phase 1 Phase 2 Phase 3
Harvested Radish HR radish harvested
--- ---
Cold Termination
Radish
CTR not harvested cold treatment coral bell
harvested
Herbicide Termination
Radish
HTR not harvested herbicide treatment coral bell
harvested
No Radish Control NRC not harvested cold treatment coral bell
harvested
Results
Radish growth and harvest
Radish seed germination was unaffected by compaction (P = 0.91) or treatment group (P
= 0.63) (Table 2). Following germination, there were differences in radish growth over time or at
harvest between all compaction levels (CL) (Figure 1, Table 2). There were significant
interactions of treatment * compaction level (P = 0.0007) and compaction * week (P < 0.0001),
along with the main effects of treatment, compaction level, and week of growth (all P < 0.001)
for the calculated aboveground volumetric size of the radishes. Radishes grown at a soil bulk
density of 1.6 g*cm-3 (CL 5) were the most negatively affected in calculated volumetric size
followed by radishes grown at 1.44 g*cm-3 (CL 4, Figure 1A). Radishes grown soil bulk density
of 0.96 g*cm-3,1.11 g*cm-3, and 1.28 g*cm-3 (CL 1, 2, and 3, respectively) produced significantly
greater calculated volumes than the higher compaction levels and were not significantly different
from each other. At harvest, radishes grown at CL 1 (bulk density: 0.96 g*cm-3), CL 2
(1.11 g*cm-3), CL 3 (1.28 g*cm-3), and CL 4 (1.44 g*cm-3) had produced comparable amounts of
aboveground biomass and belowground biomass, while radishes grown at a higher soil bulk
25
density of (CL5: 1.6 g*cm-3) produced less aboveground and belowground biomass (Figure 1C,
1D and Appendix 1 and 2). More variation was observed in leaf area at harvest. Radishes grown
at a CL 3 (1.28 g*cm-3) produced the greatest leaf area at harvest (mean SE:451.51 28.7
cm-2). Radishes grown at a soil bulk density of CL 5 (1.6 g*cm-3) produced the least leaf area at
harvest (79.98 15.0 cm-2, Figure 1B).
Coral bells growth and harvest following radish termination
There were treatment group and compaction resulted in drastic differences in Heuchera
micrantha var. diversifolia ‘Purple palace’ (coral bells) growth after transplant (Table 2).
Positive volumetric growth of coral bells was observed with the no radish control (NTC)
treatment and the cold termination radish (CTR) treatment, but little growth was observed with
the herbicide termination radish (HTR) treatment (Figure 2A, 2C, and 2E). Stunted growth in the
HTR treatment is speculated to be a result of residual herbicide effects, therefore subsequent
comparisons will predominantly be made between the NTC and CTR treatments. Calculated
volume at harvest of coral bells in the NTC treatment (1665.6 ± 154.4 cm3) was 34.1% greater
than that of coral bells in the CTR treatment (1097.6 ± 83.3 cm3, Figure 2B). Leaf area at harvest
of coral bells in the NTC treatment (460.1 ± 31.7 g*cm-2) was 20.0% greater than that of coral
bells in the CTR treatment (368.1 ± 24.9 g*cm-2, Figure 2D). Aboveground dry biomass at
harvest of coral bells in the NTC treatment (9.3 ± 0.6 g) was 15.5% greater than that of coral
bells in the CTR treatment (7.9 ± 0.6 g, Figure 2F, Appendix 3). For coral bells in the NRC
treatment, there were significant interactions with compaction level for calculated volume at
harvest (P = 0.0001), leaf area (P < 0.0001), and aboveground dry biomass (P < 0.0001, Table 3).
For coral bells in the CTR treatment, there was a significant interaction with compaction level
26
for calculated volume at harvest (P = 0.0002), but there were not significant interactions with
compactions level for leaf area (P = 0.1780) and aboveground dry biomass (P = 0.0562). For
coral bells in the HTR treatment, there was a significant interaction with compaction level for
calculated volume at harvest (P = 0.0135) and leaf area (P = 0.0239), but there were not
significant interactions with compactions level for aboveground dry biomass (P = 0.1970).
Aboveground dry biomass increased with declining soil bulk density for coral bells in the NRC
treatment: CL5 (5.26 g), CL4(7.23 g), CL3(9.90 g), CL2(11.75 g), CL1(12.32 g). Aboveground
biomass did not follow this same trend for coral bells in the CTR treatment: CL4(5.07 g),
CL5(7.05 g), CL3(8.45 g), CL3(9.14 g), CL1(9.57 g).
Table 2. Analysis of variance table for radish germination, aboveground radish growth, radish
leaf area, radish aboveground biomass, and radish belowground biomass in radish growth phase
(phase 1) in a greenhouse.
Growth Response Model Term n DF F p-value
Radish Germination (days to
germination)
compaction 5 4 0.25 0.9099
treatment 3 2 0.46 0.6335
treatment *compaction 15 8 1.31 0.2579
Aboveground Radish Growth
(cm3)*
compaction 5 4 103.68 <.0001
treatment 3 2 10.43 <.0001
week 8 7 93.20 <.0001
treatment*compaction 15 8 3.47 0.0007
treatment*week 24 14 1.18 0.2842
compaction*week 40 28 4.86 <.0001
treatment*compaction*week 120 56 0.47 0.9996
27
Table 2 (Continued)
Growth Response Model Term n DF F p-value
Radish Leaf Area (g*cm-2) compaction 5 4 32.07 <.0001
Radish Aboveground Biomass (g) compaction 5 4 15.60 <.0001
Radish Belowground Biomass (g) compaction 5 4 6.40 0.0017
*calculated ellipsoidal volume
Figure 1. (A) Mean calculated ellipsoidal volume (cm3) of radishes (Raphanus sativus L. ‘Nitro’)
at each compaction level across the 8 weeks following seeding in containers in the
greenhouse.Data was pooled across the replicates in all planting treatment groups with radishes.
Each symbol represents the mean of 15 replicates. Error bars represent the standard errors of the
mean of the 15 replicates. (B) Leaf area (g*cm-2), (C) aboveground biomass (g), and (D)
belowground biomass (g) of radishes at each compaction level at harvest. Each box represents
the mean of 5 replicates. Error bars represent the standard errors of the mean of the 5 replicates.
Week
Cal
cula
ted v
olu
me
(cm
3)
Lea
f ar
ea (
g*cm
-2)
500
400
300
200
100
0
1 2 3 4 5Compaction level
1 2 3 4 5Compaction level
1 2 3 4 5Compaction level
Ab
ov
egro
und
bio
mas
s (g
)
25
20
15
10
5
0
15
10
5
0
A B
C D
Bel
ow
gro
un
d b
iom
ass
(g)
28
Table 3. Analysis of variance table for coral bells aboveground growth, volume at harvest, leaf
area, and aboveground biomass between growth planting treatment groups in the coral bells
growth phase (phase 3) in a greenhouse.
Growth Response Model Term n DF F p-value
Heuchera Aboveground Growth (cm3)
Cold Termination Radish (CTR) compaction 5 4 22.19 <.0001
week 8 7 83.26 <.0001
compaction*week 40 28 3.07 <.0001
Herbicide Termination Radish (HTR) compaction 5 4 16.21 <.0001
week 8 7 1.07 0.3883
compaction*week 40 28 1.26 0.1922
No Radish Control (NRC) compaction 5 4 32.20 <.0001
week 8 7 85.28 <.0001
compaction*week 40 28 4.14 <.0001
Coral Bells Volume at Harvest (cm3)
Cold Termination Radish (CTR) compaction 5 4 9.33 0.0002
Herbicide Termination Radish (HTR) compaction 5 4 4.12 0.0135
No Radish Control (NRC) compaction 5 4 10.41 0.0001
Coral Bells Leaf Area (g*cm-2)
Cold Termination Radish (CTR) compaction 5 4 1.75 0.1780
Herbicide Termination Radish (HTR) compaction 5 4 3.56 0.0239
No Radish Control (NRC) compaction 5 4 14.87 <.0001
Coral Bells Aboveground Biomass (g)
Cold Termination Radish (CTR) compaction 5 4 2.76 0.0562
Herbicide Termination Radish (HTR) compaction 5 4 1.67 0.1970
No Radish Control (NRC) compaction 5 4 12.87 <.0001
*calculated ellipsoidal volume
29
Figure 2. Mean calculated ellipsoidal volume (cm3) of coral bells (Heuchera micrantha var.
diversifolia ‘Palace Purple’) at each compaction level across the 8 weeks following plug
transplanting in containers in the greenhouse for the (A) cold termination radish (CTR)
treatment, (C) herbicide termination radish (HTR) treatment, and (E) no radish control (NRC)
treatment. Each symbol represents the mean of 5 replicates. Error bars represent the standard
Week
Cal
cula
ted v
olu
me
(cm
3)
Week
Cal
cula
ted
vo
lum
e (c
m3)
Week
Cal
cula
ted
volu
me
(cm
3)
CTR HTR NRC
Planting treatment group
CTR HTR NRC
Planting treatment group
CTR HTR NRC
Planting treatment group
3000
2000
1000
0
Cal
cula
ted
volu
me
(cm
3)
Lea
f ar
ea (
g*
cm-2
)
800
600
400
200
0
15.0
10.0
7.5
5.0
2.5
0.0
12.5
Abo
veg
rou
nd b
iom
ass
(g)
A B
C D
E F
30
errors of the mean of the 5 replicates. (B), Calculated volume at harvest (cm3, ellipsoidal
volume), (D) Leaf area (g*cm-2), and (F) aboveground biomass (g) of coral bells at each
compaction level at harvest. Each box represents the mean of 5 replicates. Error bars represent
the standard errors of the mean of the 5 replicates.
Discussion
Radish growth and harvest
We observed that the radish seed germination was not significantly different among the
treatment groups with nearly all seeds germinating within 7 to 8 days, while radish seed has been
known to germinate in as little as 3 days in ideal conditions (Jacobs, 2012). The similarity in
germination was expected as all treatments received the simulated uncompacted topsoil layer
(70mm deep) in which the seeds germinated. A reduction in the calculated volumetric growth
was observed during the radish growth phase at higher soil bulk densities (CL 4: 1.44 g*cm-3 and
CL 5: 1.6 g*cm-3), which is consistent with previous work suggesting that root development and
overall plant growth is negatively impacted when soil reaches a bulk density above 1.5 g*cm-3
(Jin et al., 2017). In this simulated system, radishes at lower compaction levels (CL 1: 0.96
g*cm-3, CL 2: 1.11 g*cm-3, CL 3: 1.28 g*cm-3) reached peak growth approximately 5 weeks after
sowing (35 days), while radishes in a fall to winter cover crop rotation typically reach peak
growth in eight to twelve weeks (56-84 days) (Lawley, Weil and Teasdale, 2011). The rate of
development for tillage radish is influenced by day length, so the reduced time to maturity in this
experiment was likely influenced by the twelve-hour day length. At harvest, we found that
radishes grown at CL 1 (0.96 g*cm-3), CL 2 (1.11 g*cm-3), CL 3 (1.28 g*cm-3), and CL 4 (1.44
g*cm-3) all produced similar rates of aboveground and belowground dried biomass. This finding
agrees with previous research (Williams and Weil, 2004; Chen and Weil, 2010, 2011)
concluding that a tillage radish cover crop has the capacity to grow successfully and produce a
31
sufficient tap root at a range of soil bulk densities. We found that our range of aboveground
biomass (3.1 - 19.9 g) from high to low levels of compaction in this greenhouse container study
was lower than but somewhat comparable the range of aboveground biomass (18.8 - 27.2g) from
high to low levels of compaction in a field study (Chen and Weil, 2010), a difference likely
influenced by the environmental conditions of the study and the duration of the crop time.
Coral bells growth and harvest
It was evident early in the coral bells growth phase that the plugs transplanted in the
herbicide radish termination (HTR) treatment were uniformly stunted and were likely negatively
impacted by the herbicide termination treatment. Therefore, the growth and harvest data from
this treatment does not provide a suitable comparison to the other treatments. Coral bells
following the no radish control (NRC) treatment produced more growth (calculated aboveground
volume, leaf area, and above dry ground biomass) than coral bells following the cold radish
termination (CTR) treatment. This finding was contrary to our expectations, as we expected the
tillage radish cover crop to increase the growth of the subsequent coral bells. The unexpected
underperformance of the coral bells in the cold terminated radish treatment containers compared
to the no-radish control This outcome may have been associated with reported allelopathy
properties of tillage radish, nutrient removal with the aboveground radish biomass following
termination, or nutrients retained in the radish root that were not decomposing at a rate suitable
for coral bells update, which are not mutually exclusive and require further research to
substantiate or disentangle. These will be discussed in greater detail below.
32
Radish termination effects on coral bells
If a tillage radish cover crop is to be adopted as a potential soil amendment practice for
landscape soils, then termination and carry over effects of the cover crop must be addressed.
Radish and other Brassica cover crops must be terminated to keep them from progressing to
flower and seed set and to initiate decomposition before the planting of the following crop
(Ferrell et al., 2018; Askew et al., 2019). Cold temperatures over winter (i.e. winter-kill) and
herbicide applications are both standard methods for terminating a radish cover crop (Jacobs,
2012; Oliveira, Butts and Werle, 2019). In this greenhouse study, we attempted to simulate these
methods of termination, which may have also inadvertently negatively impacted the subsequent
coral bells growth.
We suspect that residual herbicide is what stunted the coral bells in the HRT treatment
group, since coral bells in this treatment group put on minimal growth while coral bells in the
other treatments did. Despite following the manufacturer’s recommended application rate and
residual period, the herbicide may have persisted in the PVC containers and reacted differently
than it would have in a field setting. Soil microbes have the capacity to degrade glyphosate
herbicide in aerobic and anerobic conditions but reduced microbial activity can result in
glyphosate accumulation in the soil (Daniele & Federico, 2017). Productive farm field soil was
used for this study that was presumed to have adequate microbial activity. Following the
glyphosate application, the herbicide terminated radish (HTR) treatment group remained in the
controlled greenhouse conditions for 30 days before planting the coral bell plugs. These
containers were not watered until a week before the coral bells plugs were transplanted, so there
is a possibility that dry soil did not allow for sufficient microbial activity necessary to breakdown
the glyphosate (la Cecilia and Maggi, 2018; Kanissery et al., 2019). Residual herbicide may have
then caused the observed stunting of coral bells in the chemical termination treatment. Dry
33
conditions and compaction do occur under field conditions and herbicide residual periods
account for adverse conditions, therefore, more investigation is needed under field conditions to
determine if herbicide termination of tillage radish prior to ornamental plant installation is a
viable practice. Due to the stunting of the coral bells, it is challenging to tell if the radishes
themselves would have had an effect on the coral bells transplants.
Radish allelopathy nutrient unavailability
The herbicide termination treatment aside, the coral bells transplanted into the cold
terminated radish (CRT) treatment containers grew less than those in the no-radish control
(NRC) treatment. We speculate that reported allelopathic effects of radish or nutrient
unavailability, or a combination of those causes, may have contributed to reduced coral bells
growth following the cold termination treatment.
Allelopathy is one possible factor that reduced coral bells growth, since tillage radishes
and other Brassica species have shown to have allelopathic effects on other plant species
(Rehman et al., 2013). All Brassica species contain glucosinolates, which are hydrolyzed to
isothiocyanates (ITC), thiocyanates, and nitriles in the soil (Boydston & Hang, 1995). There are
numerous examples of ITC’s inhibition of plant growth or germination (Evenari, 1949; Bell and
Muller, 1973; Teasdale, Taylorson and Taylorson2, 1986). While much work has addressed the
use of allelopathic effects of Brassica for weed suppression (Norsworhty, 2003; Uremis et al.,
2009; Rehman et al., 2019), Lawley, Teasdale and Weil (2012) found that winter radish cover
crops inhibit weed growth by competition and canopy establishment and not through allelopathic
mechanisms. Despite research on the disputed allelopathic effects of Brassica family members
for weed control, less research has been conducted on Raphanus sativus allelopathy affecting
subsequent crops instead of weeds (Uygur et al., 1970).
34
Another possible cause of the reduced coral bells growth after tillage radish in this study
is linked to nutrient availability. The deep taproot and ability to accumulate biomass rapidly
allow tillage radish to accumulate nutrients that are deeper in the soil and bring them towards the
soil surface, within the rootzone of other plants (Dean and Weil, 2009a; Hirsh et al., 2021). It is
possible that radishes in the cold termination treatment (CRT) scavenged nutrients from the soil
and incorporating them into the radish biomass, thus making nutrients unavailable to the
subsequent coral bells and this did not occur in the no-radish control (NRC) treatment. Unlike
weed seeds that may use nitrogen availability as a signal to germinate (Lawley, Teasdale and
Weil, 2012), decomposing radish root biomass may not have supplied sufficient nitrogen for
coral bells growth since the radish roots were not tilled in after termination and did not have a
sustained winter period as under field conditions to begin decomposition. In the short duration of
the termination period (35 days) and since the tillage radish was not incorporated into the soil
prior to the coral bells planting, it is possible that scavenged nutrients by the radish were not
made available through decomposition at a rate that would have made those nutrients available to
the coral bells. Additionally, radish aboveground biomass in the CRT treatment was removed for
this study and not supplemented, which represents a net export of nutrients that were not
available for the coral bells compared to the NRC treatment that experienced no nutrient loss or
uptake prior to the transplanting of the coral bells.
Limitations and future research
This preliminary study testing tillage radish at various compaction levels and subsequent
model ornamental plant establishment exposed some limitations that should be addressed in
future research. The use of a controlled greenhouse environment for the study allowed for
35
conducting the experimental soil compaction treatments but imposed some challenges in
mimicking field conditions and management practices better suited to field research. Since the
tillage radishes were successful at all but the highest compaction levels, future research is needed
into how tillage radish termination practices at any time of year, using chemical products, or
relying on winter-kill can be combined with a subsequently installed ornamental landscape.
Specifically, with regards to incorporating or not incorporating the terminated tillage radish into
the soil before adding landscape plants; the availability and timing of nutrients with biomass
decomposition; and residual effects of radish termination practices. Removal of aboveground
radish growth removed nutrients that were not supplemented in the radish treatment vs. the no
radish control. There are management practices used for the implementation of a tillage radish
cover crop in an agricultural crop system that would not be feasible in a perennial ornamental
landscape. With regards to establishing landscape perennials, the great variety in herbaceous and
woody plants precludes the cropping system refinements as in agricultural settings, thus even
with more research into subsequent establishment, there may remain the potential for tillage
radish to negatively affect the establishment of landscape plants. In an agricultural production
system, a tillage radish cover crop is used to compliment a monocultural annual cash crop such a
corn, soybeans, small grains, or vegetables; is most typically planted in the fall season following
cash crop harvest; and can be implemented once or more per year and over a period of many
years. Due to the semi-permanent nature of perennial landscapes, a tillage radish cover crop can
only be implemented once before the installation of that landscape, thus some benefits gained
from long-term use of cover crops may not be possible in ornamental landscapes. Additionally,
unlike relatively predictable seasonal planting and harvest periods for agricultural settings, urban
and landscape construction projects may be completed at nearly any point during the year. Thus,
36
tillage radish performance across varying seasonal conditions should be studied. Using cover
crops in mixes has shown to be a valuable strategy in agricultural systems (Blesh, 2018), and
could be another area of research for tillage radish to be used in conjunction with other cover
crop species which may result in multifunctional outcomes to aid in the establishment of
perennial ornamental landscapes.
Conclusion
This study used a simulated landscape system in a controlled environment to evaluate the
performance of a tillage radish cover crop at a variety of soil compaction levels and the impacts
of that tillage radish cover crop on the growth of the following establishment of a model
herbaceous perennial – coral bells. We found that tillage radish germination was unaffected by
soil compaction level. Radishes produced comparable amounts of growth through a range of
compacted soils but showed reduced growth at the highest soil compaction level. Coral bells
following an herbicide termination radish treatment where stunted, likely due to herbicide
residues in the soil. Coral bells following the no radish control treatment produced more growth
than coral bells following the cold termination radish treatment. We speculate that allelopathic
effects and nutrient unavailability may have contributed to the reduced amount of growth
following the cold termination treatment. Further research is needed to better understand the
effects of a tillage radish cover crop on the performance of a variety of ornamental landscape
plants. We recognize an opportunity for future research to address the managements
consideration for adapting agricultural cover crops for use in ornamental landscapes.
Understanding the planting and termination methods for a tillage radish cover crop, potential of a
tillage radish cover crop at various environmental conditions, the effects of a tillage radish cover
37
crop on perennial landscape plants, and combination effects of tillage radishes in cover crop
mixes are all areas for consideration for the future of using cover crops in ornamental landscapes.
Acknowledgements
The authors gratefully acknowledge Connor Evers, Emily Meader, and Pete Lawlor for
greenhouse and laboratory assistance; Alex Lindsey for statistical consultation; Dr. Ajay Nair
and Dr. Marshall McDaniel for guidance and project design; support from the Iowa State
University Horticulture Research Station; and funding from the Iowa State University
Department of Horticulture. This research was also funded by NIFA Hatch grant IOW03657, “In
search of sustainable landscape and horticultural production systems.” The authors appreciate
the generosity of green Cover Seed for donating seed for this research project. Data and results
are independent and not influenced by the material support from green Cover Seed. Mention of
trade names in this publication does not imply endorsement by Iowa State University of products
named, nor criticism of similar products not named.
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Uremis, I, M Arslan, A Uludag, and M Sangun. 2009. Allelopathic Potentials of Residues of 6
Brassica Species on Johnsongrass [Sorghum Halepense (L.) Pers.].” African Journal of
Biotechnology 8 (15).
Uygur, F. N., F. Köseli, A. Çınar, and W. Koch. 1970. The Allelopathic Effect of Raphanus
Sativus L. Journal of Plant Diseases and Protection. E. Ulmer. 12: 259-264.
Williams, Stacey M., and Ray R. Weil. 2004. Crop Cover Root Channels May Alleviate Soil
Compaction Effects on Soybean Crop.” Soil Science Society of America Journal 68(4): 1403–
1409.
41
Appendix A. Tillage Radish Aboveground Growth at Harvest
Figure 3. Aboveground growth of tillage radishes (Raphanus sativus L. ‘Nitro’) in the Harvested
Radish (HR) treatment at week (56 days) arranged with increasing soil compaction level bulk
density from left to right: (CL 1) 0.96 g*cm-3, (CL 2) 1.11 g*cm-3, (CL 3) 1.28 g*cm-3, (CL4)
1.44 g*cm-3, and (CL 5)1.6 g*cm-3.
Appendix B. Tillage Radish Belowground Growth at Harvest
Figure 4. Belowground growth of tillage radishes (Raphanus sativus L. ‘Nitro’) in the Harvested
Radish (HR) treatment at week (56 days) arranged with increasing soil compaction level bulk
42
density from left to right: (CL 1) 0.96 g*cm-3, (CL 2) 1.11 g*cm-3, (CL 3) 1.28 g*cm-3, (CL4)
1.44 g*cm-3, and (CL 5)1.6 g*cm-3.
Appendix C. Coral Bells Aboveground Growth at Harvest
Figure 5. Aboveground growth of coral bells (Heuchera micrantha var. diversifolia ‘Palace
Purple’) in the (A) cold termination radish (CTR) treatment, (B) herbicide termination radish
(HTR) treatment, and (C) no radish control (NRC) treatment at week (56 days) arranged with
increasing soil compaction level bulk density from left to right: (CL 1) 0.96 g*cm-3, (CL 2)
1.11 g*cm-3, (CL 3) 1.28 g*cm-3, (CL4) 1.44 g*cm-3, and (CL 5)1.6 g*cm-3.
43
CHAPTER 3. EVALUATION OF GERMINATION PERCENTAGE OF FIVE
TILLAGE RADISH CULTIVARS AT VARYING AIR TEMPERTURES AND WATER
AMOUNTS
Marcus D. Jansen1 and Grant L. Thompson1
1Iowa State University Department of Horticulture
Modified from a manuscript to be submitted to HortTechnology (2021)
Abstract
Tillage radish (Raphanus sativus L.) has but utilized as a cover crop for alleviating soil
compaction and scavenging nutrients in agricultural crop productions systems and has potential
as a soil amendment practice to address compacted soils on landscape construction sites. Unlike
the more predictable seasonal periods of sewing, terminating, and incorporating a tillage radish
cover crop in an annual agronomic crop rotation, the timing and environmental condition of
building and landscape construction is more variable, which may have an effect on cover crop
radish seed germination. The aim of this study was to determine the percentages of germination
produced by different tillage radish cultivars at a range of temperatures and water conditions.
Our study utilized a germination box in growth chamber system and a soilless media jiffy tray in
growth chamber system to quantify percent germination of five cover crop radish cultivars across
eight air temperature and four water amounts. We found that air temperature significantly
affected germination percentage across all cultivars, and the optimal range for germination was
23-33℃ where the cultivars produced a mean germination range of 91.10 - 79.14%. Water
amount significantly affected Nitro, Smart, and Badger radish cultivars, suggesting that irrigation
management during the germination of these cultivars may be crucial to their success. In our
study conditions, Nitro radish cultivar has the highest overall mean germination percentage
(68.03%) and the highest germination percentage at many of the air temperature and water
44
treatment combinations. Germination percentages were not particularly comparable between the
blotter paper and soilless media substrate at the optimal air temperature ranges. The findings of
this study have potential value in providing estimated germination rates of these cultivars across
a range of conditions, including sub-optimal air temperatures and water levels.
Introduction
Soil compactions contributes to soil degradation and a major concern for agriculture
(Hamza and Anderson, 2005) and other uses across the world (Batey, 2009). Soil compaction
occurs when mechanical force is applied to the soil increasing soil bulk density and decreasing
soil porosity (Soil Science Society of America, 2008). Plant growth is negatively impacted when
reduced poor space suppresses the movement of air and water within the soil profile impacting
both gravitational and plant available water (Dexter, 2004). The breakdown of soil aggregates,
reduced pore spaces, and reduced pore space connectivity associated with soil compaction also
directly affects the ability of plant roots to explore the soil and grow (Bengough et al., 2006). In
agricultural production systems, research has substantially characterized mechanically induced
soil compaction (Shah et al., 2017). Furthermore, efforts have been made to understand
interactions and compound effects of a variety of factors including soil physical properties,
wheeling, number, of passes, production practices, soil structure, soil water status, and crop
rotations (Hamza and Anderson, 2005). In respond to the negative effects on crop yield (Nyéki et
al., 2017), the industry has considered solutions to soil compaction including tillage, crop
rotation, and cover crops. (Forte et al., 2018; Mirzavand and Moradi-Talebbeigi, 2020)
Raphanus sativus L. var. longipinnatus (Daikon radish) along with its comparable varieties
(tillage radish, forage radish, oilseed radish) and cultivars have proven their worth as a cover
crop species and are valued for having a substantial taproot with the capacity to penetrate
45
compacted soils (Williams and Weil, 2004; Chen and Weil, 2010) and accumulate nutrients from
deep within the soil profile to shallower depths where nutrients may be made available to other
plants (Toom et al., 2019). A tillage radish cover crop is typically planted by seed in the fall
(Charles et al., 2006) and terminated by winter kill or spring herbicide application (Dean and
Weil, 2009b; Chen and Weil, 2011). This soil amendment practice can be implemented annually
and provide compounded benefits from year to year in an annual cropping system. (Hodgdon et
al., 2016)
The deleterious effects of soil compaction and their causes in agricultural production are
largely similar to soil compaction resulting from construction activities associated with urban
development and built landscapes. In the landscape construction and installation process,
mitigation strategies are required to alleviate compaction for landscape plant establishment
(Sloan et al., 2012). Conventional practice utilized in the landscape management industry
includes tillage (Layman et al., 2010), soil fracturing (Somerville, May and Livesley, 2018), and
the incorporation of compost (Sax et al., 2017; McGrath et al., 2020; Somerville et al., 2020).
While there has been less published research on the use of cover crops for amending compacted
landscape soils, there is increasing interest in the potential for tillage radish to be used for
decompaction in post-construction urban soils since it can be easily seeded and would result in
less traffic and shipping costs compared to compost application. However, unlike the more
predictable seasonal periods of sewing, terminating, and incorporating a tillage radish cover crop
in an annual agronomic cropping system, the timing of building and landscape construction is
more variable. Thus, if tillage radish is to see wide-spread use in constructed landscapes to
remediate deleterious post-construction effects, a necessary initial step is needed to determine the
46
range of temperature and water conditions that will allow for acceptable percentages of tillage
radish seed germination. While much work has looked into germination of vegetable radishes
(Raphanus sativus L.) for edible consumption (Kordon, 2010; Bakhshandeh and
Gholamhossieni, 2019) and germination of other brassica species (Russo, Bruton and Sams,
2010; Alias et al., 2018), less work has addressed the germination of cover crop radishes
(Raphanus sativus L. var. longipinnatus) at a range of environmental conditions. Therefore, the
objective of this study is to determine the percentages of germination produced by different
tillage radish cultivars at a range of temperatures and water conditions.
Methods & Materials
This study was conducted to determine the variation of seed germination rates amongst
different cultivars of cover crop radish when grown at varying levels of moisture and air
temperature. Five cultivars of cover crop radish seed were tested: TapMaster Brand Radish
(Albert Lea Seeds, Albert Lea, MN), Tillage RadishⓇ (La Crosse Seed, LaCrosse, WI), Nitro
Radish (Green Cover Seed, Bladen, NE), Smart Radish (Green Cover Seed, Bladen, NE), and
Badger Brand Daikon Radish (Hood River Seed Company, Evansville, IN). These cover crop
radish cultivars were selected based on their popularity and availability amongst cover crop
growers in the Midwest. Sample seeds were obtained from the various manufacturers in the
summer of 2020 for this study. Following the protocol used by the Iowa State University Seed
Testing Lab under the guidelines of AOSA Rules for Testing Seed (Association of Official Seed
Analysts, 2017), with treatment modifications described below, radish seeds were placed on
blotter paper (AHL628-838880, Anchor Paper Co, St. Paul, MN) in clear plastic germination
47
boxes (26.67 x 15.72 x 3.97cm, K601 box, Flambeau, Middlefield, OH) and germinated in
controlled environment growth chambers (PGC 10;Percival Scientific, Perry, Iowa).
Treatments
Following established procedures of the Iowa State University Seed Testing Lab, it was
experimentally determined that the blotter paper saturation water volume was 35 ml per
container (0.08mL * cm-2). Two water volume treatments above and one below the blotter paper
saturation point were selected to represent germination under wetter and dryer conditions
respectively, resulting in four water rates (55, 45, 35, 25 mL per area of the container). Seven air
temperatures (8, 13, 18, 23, 28, 33, and 38℃) were tested and were simulated in growth
chambers; one temperature at a time. Ranges of variables were selected to replicate the variety of
environmental conditions that cover crop radish seeds would experience while being sown at
different times of the year. Historical environmental data was references to determine increments
that would be representative of conditions in Iowa (Table 1). Treatment combinations were
repeated across 4 replications. All cultivars were tested across all combination of water amount
and temperature (5 cultivars * 4 water amounts * 7 temperatures * 4 replicates = 560
experimental units)
Blotter Paper Germination Trial
Blotter paper in the germination boxes was hydrated with the respective water treatment
immediately prior to seeding. One hundred seeds were placed on the hydrated blotter paper using
a custom trip tray to fit the germination boxes. Seeds were organized into 5 rows of 20 seeds
with 12.3 mm spacing within row and 25.4 mm spacing between rows. Each germination box
was an experimental unit. Immediately following seed placement, germination boxes were
48
closed, sealed in clear plastic bags for moisture retention, and placed in the growth chambers.
Four identical growth chambers were used for this experiment with each growth chamber
representing one of the 4 temperature treatment repetitions. Experimental units were randomized
within each chamber. Growth chambers were set to a twelve-hour day length and temperature
was set based on the seven treatment temperatures. Light was supplied through fluorescent bulbs
at a target intensity of 250 molm-2s-1. Seeds were left to germination in the growth chamber
for seven days, and percent germination was manually counted on day seven. Identification of
seed germination status was influenced by AOSA guidelines (Association of Official Seed
Analysts, 2017). Seedlings that possessed both a radical and cotyledons were identified as
germinated. Seedlings that did not possess all structures needed for successful plant
establishment were identified as abnormal. Seedlings that had not produced any part of a
seedling were identified as dead seeds. Abnormal seedlings and dead seeds were combined into a
classification of ungerminated seeds for statistical analysis. Regression analysis was utilized to
identify significance at P < 0.05 between treatment. Means were separated by Tukey’s
significant difference test at P < 0.05.
Media Germination Trial
A secondary germination trail was executed with comparable methodology to the original
trial with the exception of soilless potting media as the substrate instead of blotter paper. This
trial was executed to observe the effect of these treatment combinations on germination rates
when subject to a media substrate that may more closely emulate the seed to soil contact and
water relations of field conditions. For this trial, only the most optimal air temperatures of 23, 28,
33℃ were tested. With the change in substate conditions, water amounts were amplified from
49
55, 45, 35, 25 mL per area of the container in the original trial to water amounts with comparable
water percentages in media. It was experimentally determined that the media saturation water
volume was 459 mL (0.16mL * cm-3). Two water volume treatments above and one below the
blotter paper saturation point were selected to represent germination under wetter and dryer
conditions respectively, resulting in four water rates (323, 459, 588, and 717 ml per volume of
container). Rectangular (35.6cm x 12.8cm x 6.4cm) growing trays (Jiffy tray 51, 505, Jiffy
Products, Zwijndrecht, Netherlands) were filled with a peat-based germination potting mix
media (Pro-Mix PGX with Biofungicide, Premier Tech Horticulture, Rivière-du-Loup, Quebec,
Canada). Media was hydrated with the respective water treatments prior to seeding. One hundred
seeds were sown at a depth of 2cm in the media in individual trays for each experimental unit.
Seeds were organized into 4 rows of 25 seeds with 13.1 mm spacing within row and 25.4 mm
spacing between rows. Seeded trays were sealed with plastic bags for moisture retention and
placed in the growth chambers. Chamber conditions and data collection procedures were
identical to the original trial.
Table 1. Mean air temperatures (C and F) and rainfall (mL per month and per day) for the
months of the growing season (April – November) in Polk County, Iowa form 1990-2019.
Month Mean
Temperature (C)
Mean
Temperature (F)
Mean Rainfall
per Month (mL)
Mean Rainfall
per Day (mL)
April 9.47 49.05 3900.88 130.03
May 15.71 60.28 5179.11 172.64
June 21.19 70.14 5707.87 190.26
July 23.04 73.47 4503.88 150.13
August 21.75 71.15 4470.62 149.02
September 17.83 64.10 3581.07 119.37
50
Table 1 (Continued)
Month Mean
Temperature (C)
Mean
Temperature (F)
Mean Rainfall
per Month (mL)
Mean Rainfall
per Day (mL)
October 10.80 51.44 2656.57 88.55
November 2.86 37.15 1813.71 60.46
*mean of environmental data for Polk County Iowa 1990-2019
Results
There were significant interactions of cultivar*air temperature (P < 0.001) and water*air
temperature (0.0004) along with the main effects of cultivar, water, and air temperature (all P <
0.001, Table 2). When averaged across all temperature treatments, water level treatments
resulted in significant differences for Badger (P = 0.0218), Nitro (P = 0.0008), and Smart (P =
0.0355) cultivars (Table 2). When averaged across all water level treatments, temperature
treatments resulted in significant differences for all cultivars (all P < 0.001, Table 1). Across all
air temperatures and water levels, Nitro produced the highest mean germination percentages
(68.03 2.65%), followed by Smart (64.63 2.87%), Badger (60.14 3.23%), Lacrosse (49.71
3.39%), and TapMaster (47.48 3.27%). Across all cultivars and water levels, the highest
mean germination was achieved at 28℃ (91.10 0.73%), 23℃ (87.49 1.17%), 33℃ (79.41
1.24%), 18℃ (62.60 2.71%), 13℃ (34.33 2.62%), 38℃ (33.91 3.46%), and 8℃ (17.15
1.99%). Across all cultivars and air temperatures, the highest mean germination was achieved at
the 45ml (66.19 2.70%) water level, followed by 55ml (65.19 2.65%), 35ml (55.94%
2.78%), and 25ml (44.67 2.88%). At the optimum air temperature (28℃), Badger (93.50%
1.24%) and Smart (92.56 1.67%) produced statistically comparable and higher percentages of
mean germination than Nitro (91.13 2.10% and Lacrosse (90.50 0.89%) produced
significantly lower percentages of mean germination; and TapMaster (87.81 1.77%) produced
51
the lowest percentage of mean germination (Table 3, Figure 1). At the least optimal air
temperature (8℃), Nitro (38.88 5.35%) produced the highest mean germination; Smart (22.44
2.59%) produced a significantly lower percentages of mean germination; and Badger (13.06
2.59%), Lacrosse (6.38 2.98%) TapMaster (5.00 0.98%) produced statistically comparable
percentage of mean germination that were the lowest amongst the cultivars (Table 3, Figure 1).
At the optimum water amount (45mL), all of the cultivars produced statistically different mean
germination percentages descending from Smart (79.43 4.23%), Nitro (71.11 5.26%),
Badger (68.89 5.93%), Lacrosse (57.21 6.89%), and TapMaster (54.29 6.63%, Table 4,
Figure 2). At the least optimum water amount (25mL), all of the cultivars had statistically
comparable mean germination percentages with Nitro (52.14 5.96%), Smart (51.96 5.77%),
Badger (44.93 6.90 %), Lacrosse (39.29 7.00%), and TapMaster (35.04 6.31% Table 4,
Figure 2). In the regression analysis between germination percentages of radish cultivars in the
blotter paper germination trial and the jiffy tray media germination trial, the coefficient of
determination (R2) was fairly low and comparable between cultivars with Badger (R2 = 0.2362),
Lacrosse (R2 = 0.2483), Nitro (R2 = 0.1672), Smart (R2 = 0.2645), and TapMaster (R2 = 0.2348,
Figure 3)
Table 2. Analysis of variance table for radish germination within and between treatments
throughout the blotter paper trial in the growth chambers and between treatments within specific
cultivars.
Growth Response Model Term n DF F p-value
Radish Germination (%)
cultivar 5 4 48.17 <.0001
water 4 3 73.52 <.0001
air temperature 7 6 377.15 <.0001
52
Table 2 (Continued)
Growth Response Model Term n DF F p-value
cultivar*water 20 12 0.73 0.7192
cultivar*air temperature 35 24 7.52 <.0001
water*air temperature 28 18 2.60 0.0004
cultivar*water* air
temperature
140 72 0.61 0.9943
Radish Germination (%)
Badger water 4 3 3.35 0.0218
Lacrosse water 4 3 1.29 0.2808
Nitro water 4 3 6.01 0.0008
Smart water 4 3 2.96 0.0355
TapMaster water 4 3 2.13 0.1005
Radish Germination (%)
Badger air temperature 7 6 48.38 <.0001
Lacrosse air temperature 7 6 87.67 <.0001
Nitro air temperature 7 6 28.75 <.0001
Smart air temperature 7 6 49.03 <.0001
TapMaster air temperature 7 6 84.42 <.0001
53
Table 3. Tukey test comparison between cultivars within the air temperature treatments in the
blotter paper trial in the growth chambers.
Treatment Cultivar Mean N Tukey Grouping
8C
Nitro 38.88 16 A
Smart 22.44 16 B
Badger 13.06 16 C
LaCrosse 6.38 16 C
TapMaster 5.00 16 C
13C
Nitro 60.56 16 A
Smart 50.13 16 B
Badger 30.38 16 C
LaCrosse 16.13 16 D
TapMaster 14.44 16 D
18C
Nitro 81.75 16 A
Smart 80.31 16 A
Badger 68.88 16 B
LaCrosse 44.31 16 C
TapMaster 37.75 16 C
23C
Nitro 91.75 16 A
Badger 90.94 16 A
Smart 90.56 16 AB
LaCrosse 84.13 16 BC
TapMaster 80.06 16 C
54
Table 3 (Continued)
Treatment Cultivar Mean N Tukey Grouping
28C
Badger 93.50 16 A
Smart 92.56 16 A
Nitro 91.13 16 AB
LaCrosse 90.50 16 AB
TapMaster 87.81 16 B
33C
Badger 81.00 16 A
LaCrosse 80.94 16 A
Smart 80.00 16 A
Nitro 77.56 16 A
TapMaster 77.56 16 A
38C
Badger 43.25 16 A
Smart 36.38 16 A
Nitro 34.56 16 A
TapMaster 29.81 16 A
LaCrosse 25.56 16 A
55
Figure 1. Mean percent germination of radish cultivars across tested air temperatures (C )
throughout the blotter paper trial in the growth chambers.Letters indicate Tukey test statistical
groups
Table 4. Tukey test comparison between cultivars with the water amount treatments in the blotter
paper trial in the growth chambers.
Treatment Cultivar Mean N Tukey Grouping
25ml
Smart 52.14 28 A
Nitro 51.96 28 A
Badger 44.93 28 A
LaCrosse 39.29 28 A
TapMaster 35.04 28 A
Air Temperature (ºC)
Cultivar: Badger Lacrosse Nitro Smart TapMaster
Ger
min
atio
n (
%)
a
b
c
c
c
a
b
c
d
d
a
a
b
c
c
a
a
ab
c
bc
a
ab
bab
a
aa
aa
a
aaa
aa
56
Table 4 (Continued)
Treatment Cultivar Mean N Tukey Grouping
35ml
Nitro 65.43 28 A
Smart 62.04 28 AB
Badger 57.64 28 AB
LaCrosse 49.21 28 AB
TapMaster 45.39 28 B
45ml
Nitro 79.43 28 A
Smart 71.11 28 AB
Badger 68.89 28 ABC
LaCrosse 57.21 28 BC
TapMaster 54.29 28 C
55ml
Nitro 75.29 28 A
Smart 73.21 28 A
Badger 69.11 28 AB
TapMaster 55.25 28 B
LaCrosse 53.11 28 B
57
Figure 2. Mean percent germination of radish cultivars across tested water amounts (mL )
throughout the blotter paper trial in the growth chambers.Letters indicate Tukey test statistical
groups
Water amount (mL)
Cultivar: Badger Lacrosse Nitro Smart TapMaster
Ger
min
atio
n (
%)
a
a
a
a
a
a
abc
c
bc
ab
ab
a
b
b
a
aa
a
b
ab
ab
ab
58
Figure 3. Regression model of percent germination between radish cultivars in the blotter paper
germination trial and the jiffy tray media germination trial.R2 models for each cultivar are
indicated
Discussion
Air Temperature
Air temperature was a highly significant factor affecting germination for all cultivars,
which aligns with findings from vegetable radish research (Bakhshandeh and Gholamhossieni,
2019). As a cool season crop, radishes are tolerant of lower temperatures (15-20C) during
growth (Gunay, 2005), but benefit from higher temperatures (20-28C) for seed germination.
Our study found 28C to be the optimum temperature for seed germination across the radish
cultivars, which is higher than that found by Bakhshandeh and Gholamhossieni, 2019 (21.9C)
but comparable to that found by Lindgren and Browning, 2011 (27C) with Raphanus sativus L.
y = 0.9335x + 0.2367
R² = 0.2362
y = 1.1836x - 22.663
R² = 0.2483
y = 0.4943x + 36.675
R² = 0.1672
y = 0.8941x + 2.8076R² = 0.2645
y = 0.9393x + 4.2792
R² = 0.2348
0
10
20
30
40
50
60
70
80
90
100
0 20 40 60 80 100
Jiff
y t
ray
med
ia g
erm
inat
ion (
%)
Blotter paper germination (%)
Badger
LaCrosse
Nitro
Smart
TapMaster
Linear (Badger)
Linear (LaCrosse)
Linear (Nitro)
Linear (Smart)
TapMaster
Badger
LaCrosse
Nitro
Smart
TapMaster
59
This optimum temperatures is also similar to other Brassica germination studies concluding that
Brassica spp. has an optimum germination temperature of 25C (Alias et al., 2018) and Brassica
rapachinenis has an optimum germination temperature of 29C (Motsa et al., 2015). Within seed
germination research, effects of changes in soil temperature on germination are often determined
by studies that draw conclusions from changes in air temperature in a growth chamber without
soil. While daily soil temperate can be predicted from daily air temperature (Zheng, Hunt and
Running, 1993; Barman et al., 2017), soil temperature and air temperature do not change at the
same rate (Zhan et al., 2019). In our study, the blotter paper substrate temperature was more
likely similar to the air temperature due to its lower thermal mass when compared to that of soil
in field conditions. The blotter paper did not provide the same insulting capacity that a field soil
would, therefore air temperature may have had a stronger effect on germination in this study than
under typical field soil conditions. Additionally, the relatively small volume of the jiffy trays
would not provide the same insulating and temperature buffering capacity of a larger volume of
soil that would occur in field conditions. Germination trends may be observed based on studies
like ours, that manipulate air temperature, but may not directly reflect germination trends in the
field due to this different between air and soil temperature.
Water
Water level was less significant than air temperature in affecting germination. Research
has indicated a reduction in germination rate and germination percentage with a decrease in
water potential for edible vegetable radish seeds (Bakhshandeh and Gholamhossieni, 2019) and
other species (Atashi et al., 2015; Bakhshandeh and Gholamhossieni, 2018). While we did see an
overall similar trend amongst our water level treatments, there were some cultivars in our study
60
that showed non-significant responses to water across the ranges tested. Water amount was
significant for germination rates of Nitro, Smart, and Badger cultivars, indicating that irrigation
management during the germination of these cultivars may be important for their performance.
Under lower water conditions, all cultivars produced similar germination percentages. The
significant interaction between water and air temperature indicates that the effect of water was
not uniform amongst all temperatures. At optimum temperatures a change in water amount may
have less of an effect on germination, yet at the higher and lower temperature extremes an
increase in water has the potential to increase germination.
Cultivar
While germination of edible vegetable radishes cultivar have been studied extensively
(George and Evans, 1981; Martinez-Villaluenga et al., 2010), little published research has
compared cultivars of cover crop radishes. Within the germination environment of our study,
there were some prominent trends in cultivar performance. Within our study, Nitro had the
overall highest germination percentage and consistently had the highest germinations percentage
in many of the temperature and water treatment combinations. Nitro germinated well across a
range of air temperatures and had higher germination rates than the other cultivars at lower air
temperatures in particular from 8-18C (Nitro means: 38.88 – 81.75%, Bager means: 13.06 –
68.88%, Tapmaster means: 5.00 – 37.75% ). The ability of a cultivar to germinate in a range of
temperatures would be valuable in post construction landscapes since construction schedules
may begin early or end late in the season when temps are less predictable. Within more ideal air
temperatures (23-33C) all cultivar produced mean germination rates that were above 75% and
statically comparable to one other, so the difference between these cultivars may be negligible at
these air temperatures compared to cooler air temperatures. At 38C all cultivars produced
61
consistently lower germination rate (25.56 - 43.25%), which aligns with finding suggesting that
Raphanus sativus L. (edible vegetable radish) shows insufficient levels of germination above
35C air temperatures (Lindgren and Browning, 2011; Bakhshandeh and Gholamhossieni, 2019).
Germination percentages may not be identical in field conditions, but we can suggest that trends
between cultivars may be comparable.
Blotter Paper vs. Media Substrate
The trial using soilless media substrate in Jiffy trays was conducted to compare radish
germination at the optimal germination temperatures of 23, 28, and 33℃ found in the blotter
paper trial. While the blotter paper method is a standard seed testing protocol (Association of
Official Seed Analysts, 2017) to estimate field germination, this trial allowed us to consider
germination rates in a soilless media that more closely replicates the seed-to-soil contact of field
soil conditions. When comparing germination results between the blotter paper and Jiffy tray
trial, there was not good correlation between the germination methods (R2 = 0.17 – 0.26).
However, within each trial method, the relative differences in germination between the cultivars
was consistent. This indicates that while numerically different and non-comparable germination
results were found in each trial, the general outcome of cultivar performance was consistent.
Overall, lower germination rates were observed in the soilless media trial, which may have been
influenced by the nature of the substate and the depth of the media. On the blotter paper the
seeds remained in contact with the wet paper, thus were better able to hydrate. Conversely, in the
soilless media, gravity pulled water away from the surface where seeds were sown. We observed
the bottom of the Jiffy trays were wetter than the top of the media, supporting this explanation.
Additionally, germination rates responded similarly to changes in water amount for both testing
62
trials. While some comparable trends were observed in air temperature between the testing trials,
testing a wider range of air temperature in the soilless media substate would allow for a stronger
comparison of that trend.
Application
Estimated germination rates of these cultivars has been identified by the producers and
distributers of these radish cultivars at optimal conditions within an agricultural cropping system.
The findings of this study have potential value in providing estimated germination rates of these
cultivars across a range of conditions, including sub-optimal air temperatures and water levels.
From these findings, management recommendations could be for using these radish cultivars in
sub-optimal conditions, such as landscape contractors looking to reduce site soil compaction by
using tillage radish. Understanding the expected germination rate of a radish cultivar at a certain
combination of air temperature and water amount can assist in management decisions to produce
an acceptable cover crop germination rate. Outside of temperatures ranging from 18-28℃,
higher water rates improved germination for Nitro, Smart, and Badger cultivars compared to
lower water amounts. Yet even with supplemental water, mean germination in these cultivars
was between 26 – 97.25%, showing that while water helped improve germination at low
temperatures, the air temperature had the stronger negative effect on germination. Nitro cultivar
had the best performance across a range of air temperatures, though the amount of water did
matter too. Higher water amounts improved Nitro radishes germination by 46% at 8℃ and by
34% at 38℃, suggesting that supplemental water may be necessary for adequate germination at
these temperature extremes. Our results show, for some tillage radish cultivars, temporary
supplemental irrigation during the germination and establishment may be beneficial during sub-
63
optimal temperature conditions. Under ideal temperatures (23-28℃) lower water amounts
achieved acceptable germination rates and additional water did not substantially improve
germination, suggesting that for the tillage radish cultivars tested, the need for higher soil
moisture was less under optimal conditions. Therefore, the potential need for supplemental
irrigation for adequate germination is reduced when air temperature is within the optimum range
for these cultivars. This study indicates that temperature and water differences in germination
exist among cultivars and that experimentation may be needed to identify cultivars that would
work best under local conditions and where supplemental management such as irrigation may be
needed to obtain desirable results.
Limitations and Future Research
This preliminary study of tillage radish cultivar germination at varying levels of air
temperature and water levels yielded promising results and identified some limitations that
should be addressed in future research. Our methods followed the standard procedure of counting
germination percentage after seven days (Association of Official Seed Analysts, 2017), but
higher germination percentages may have been observed if a longer germination window were
tested. Further research is needed to observe rate of germination of these cultivars, beyond
percent germination at seven days. Actual field conditions, including variations in temperature,
moisture, and other sources of variability that were not included in this controlled environment
experiment, may result in different tillage radish germination and growth. Such differences
would need to be field tested for future application. Furthermore, this study only observed
percent seed germination. While proper seed germination is necessary and a crucial component
in the development of a radish plant, a high germination percentage does not guarantee superior
growth and cover crop success. The threshold of desired germination rate and respective
64
management decisions may vary amongst growers of tillage radishes. Conclusion can be made
about the germination rates of these radish cultivars at certain air temperatures and water levels,
but further research is needed to better understand how the seedlings of these cultivars put on
growth and perform as a cover crop under these environmental conditions. Quantifying growth
and biomass production of these cover crop cultivars and making comparison to percent
germination could be another area of research helping to better understand the overall
performance of these cultivars at varying environmental conditions.
Conclusion
These studies utilized a germination box in growth chamber system and a soilless media
jiffy tray in growth chamber system to quantify percent germination of five cover crop radish
cultivars at varying combinations of air temperature and water amount. We found that air
temperature significantly affected germination percentage across all cultivars, and the optimal
range for germination was 23-33℃. Water amount significantly affected Nitro, Smart, and
Badger radish cultivars, suggesting that irrigation management during the germination of these
cultivars may be crucial to their success. In our study conditions, Nitro radish cultivar has the
highest overall germination percentage and the highest germination percentage at many of the air
temperature and water treatment combinations. Germination percentages were not particularly
comparable between the blotter paper and soilless media substrate at the optimal air temperature
ranges. Providing estimated germination rates of these cultivars across a range of conditions,
including sub-optimal air temperatures and water levels is a potential application of these
findings. More research is needed to better understand the effects of germination percentage on
tillage radish growth and cover crop success in reducing compaction of developed landscape
soils.
65
Acknowledgements
The authors gratefully acknowledge Elizabeth Hurley-Blewett, Abigail Enos, Connor
Evers, Cody McKune, and Pete Lawlor for greenhouse and laboratory assistance; Alex Lindsey
for statistical consultation; Dr. Ajay Nair and Dr. Marshall McDaniel for guidance and project
design; support from the Iowa State University Horticulture Research Station; and funding from
the Iowa State University Department of Horticulture. This research was also funded by NIFA
Hatch grant IOW03657, “In search of sustainable landscape and horticultural production
systems.” The authors appreciate the generosity of Albert Lea Seeds, Green Cover Seed, Hood
River Seed Company, and La Crosse Seed for donating seed for this research project. Data and
results are independent and not influenced by the material support from Albert Lea Seeds, Green
Cover Seed, Hood River Seed Company, and La Crosse Seed. Mention of trade names in this
publication does not imply endorsement by Iowa State University of products named, nor
criticism of similar products not named.
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CHAPTER 4. GENERAL CONCLUSIONS
General Discussion
Compaction continues to be a chief cause of soil degradation all over the world
(McGarry, 2003). This challenge has led to a substantial body of research targeted at
understanding the phenomenon of soil compaction, identifying the causes, and quantifying the
effects on plant growth and production (Unger and Kaspar, 1994; Batey, 2009; Hu et al., 2021).
Due to the major economic (Oskoui and Voorhees, 1991; Graves et al., 2015) and environmental
(O’Sullivan and Simota, 1995; Hu et al., 2021) costs of compacted soils on agricultural crop
production, much work has gone into identifying the major causes of soil compaction and
potential remediation practices for crop production systems (Hamza and Anderson, 2005; Kumar
et al., 2018). Cover crops have been widely utilized as a soil management practice in
conventional and organic agricultural systems for numerous benefits including soil stabilization,
weed suppression, and nutrient management (Fageria, Baligar and Bailey, 2005; Justes, 2017).
Cover crop species with extensive tap roots have shown to be effective at reducing soil
compaction, scavenging nutrients, and providing root channels for subsequent crops, such that
the term “bio-drillers” is sometimes applied to these tap rooted cover crops (Williams and Weil,
2004). Amongst these cover crop species, tillage radish (Raphanus sativus L.) and other cover
crop radish selections such as Daikon radish (Raphanus sativus L. var. longipinnatus) have
grown in popularity (Gruver, Weil and Lawley, 2016), valued for significantly higher soil
penetrating capacity (Chen and Weil, 2010) and nutrient accumulation ability (Toom et al.,
2019).
The causes of compaction and their detrimental effects in agricultural production are
comparable to soil compaction resulting from construction activities associated with urban
70
development and built landscapes. Soil compaction has shown to deter root exploration and
establishment of landscape plants since increased soil bulk density results in fewer, smaller, and
less well-connected soil pores for roots to inhabit (Kozlowski, 1999; Day, Seiler and Persaud,
2000). The negative impacts of soil disruption and the need for urban soil remediation to
improve plant growth have been identified (Sloan et al., 2012). Tillage (Layman et al., 2010),
soil fracturing (Somerville, May and Livesley, 2018), and compost incorporation (Sax et al.,
2017; McGrath et al., 2020; Somerville et al., 2020) are conventional remediation practices
utilized in the landscape management industry. While cover crops or temporary vegetative cover
has been utilized for erosion control or weed suppression on the exposed soil along roadsides or
construction sites (By and Busby, 2014), less published research has addressed the use of cover
crops for amending compacted landscape soils. Recognizing the successful reduction of
compaction from a tillage radish cover crop in agricultural cropping systems and the need for
compaction remediation in post-construction urban soils where landscape plants will be
established, we sought to test the potential of tillage radish for such uses.
The negative effects of soil compaction on agricultural crop production and landscape
plants establishment are both well documented. Management of a tillage radish cover crop in
agricultural cropping systems has been well studied and practiced, while management of a tillage
radish cover crop in landscape construction systems has yet to be defined. The difference in
cover crop implementation between these two systems is influenced by dissimilarity in the
seasonality of the practices and the nature of the subsequent “cash crop”. The period of sewing,
terminating, and incorporating of a tillage radish cover crop in an agricultural cropping system
falls in a relatively predictable pattern and can be implemented on an annual basis in rotation
with the annual cash crop. The timing of construction activites is more variable, therefore the
71
implementation of a tillage radish cover crop could be needed at any time during the growing
season and may only happen once before a semi-permanent perennial landscape is installed.
Recognizing the major difference between these two systems, we are beginning to explore
potential management practices of a tillage radish cover crop for compaction remediation on
landscape sites.
Our research found that ‘Nitro’ tillage radish produced comparable amounts of root and
shoot growth across a wide range of soil bulk densities (0.96 – 1.44g*cm-3) with significant
reduction in growth at a soil bulk density of (1.6g*cm-3). This result suggests that a tillage radish
cover crop can be effect in many compacted soils situations but may be limited by extremely
compacted soil which may require mechanical means of decompaction. Additionally, we
discovered instances where a tillage radish cover crop has the potential to reduce growth of the
following model landscape plant (coral bells) following certain cover crop termination methods.
Standard cover crop termination and incorporation practices prior to the planting of a perennial
landscape plant may be different than those prior to the planting of an agricultural crop, and we
have yet to determine those standards. Acknowledging the variety of environmental condition at
which a tillage radish cover crop would be sewn throughout the landscape construction and
installation season, we sought to identify the germination rates of five cover crop radish cultivars
across a range of air temperatures and water amounts. We found that air temperature
significantly affected germination percentage across all cultivars, and the optimal range for
germination was 23-33℃. Water amount significantly affected some of the top performing
cultivars, suggesting that irrigation management during the germination of these cultivars may be
crucial to their success. Amongst the cultivars, ‘Nitro’ radish had the highest overall germination
percentage and was the most successful across a range of air temperatures and water treatment
72
combinations. From these findings, we could potentially provide estimated germination rates of
these cultivars across a range of conditions, including sub-optimal air temperatures and water
levels. While our research has begun to explore the potential of a tillage radish cover crop and
the necessary management practices for alleviation of soil compaction on landscape construction
sites, continued work is needed to develop a holistic understanding of this soil amendment
practice and its applications for the landscape industry.
Recommendations for Future Research
Our preliminary study testing tillage radish at various compaction levels and subsequent
model ornamental plant establishment did allows us to draw some promising conclusions but
also exposed some limitations that should be addressed in future research. The use of a
controlled greenhouse environment for the study allowed for the simulation of the experimental
soil compaction treatments but imposed some challenges in mimicking field conditions and
management practices better suited to field research. There may be value in documenting case
studies with landscape contractors to observe the performance of a tillage radish cover crop in
field soils and the effect on perennial plants in an actual landscape. We saw first-hand the
negative effects that a cover crop can cause under certain management practices, so future
research is needed to determine effective cover crop termination and incorporation strategies for
landscape systems. There is a need to specifically address incorporating or not incorporating the
terminated tillage radish into the soil before adding landscape plants, the availability and timing
of nutrients with biomass decomposition, and residual effects of radish termination practices.
Coral bells (Heuchera micrantha var. diversifolia ‘Purple palace’) were used as a model plant in
this study and observed across eight weeks of growth, but landscapes are often comprised of a
diverse pallet of herbaceous and woody plant species and can be long-lived once well
73
established. Future studies are needed to observe the effects of a tillage radish cover crop on
other popular landscape species and the long-term effect of this practice on perennial plants in
order to establish more generalizable responses of plant growth and establishment. Using cover
crops in mixes has shown to be a valuable strategy in agricultural systems (Blesh, 2018), and
could be another area of research in developed landscape systems. Tillage radishes used in
conjunction with other cover crop species may result in multifunctional outcomes to aid in the
establishment of perennial ornamental landscapes.
We did find some promising results in a preliminary study quantifying the germination of
tillage radish cultivars at varying environmental conditions, but there is substantial opportunity
for further research to address the management practices need for the germination and successful
growth of a tillage radish cover crop at diverse conditions throughout the year. Our study
observed gemmation percentages of radish seeds at seven days, but in an applied landscape
setting seeds would continue to germinate past seven days with potentially higher germination
percentages. Further research is needed to observe rates of germination of these cultivars beyond
germination at seven days and to translate germination to actual radish growth and development.
Field conditions, including variations in temperature, moisture, and other sources of variability
that were not included in this controlled environment experiment, may result in different tillage
radish germination and growth. Such differences would need to be field tested for future
application. Our study was also limited by the fact that we only observed germination percentage
and did not regard the future growth and success of those radish plants. While we can make
generalized conclusions from our study about the germination rates of these radish cultivars at
certain air temperatures and water levels, further research is needed to better understand how the
seedlings of these cultivars put on growth and perform as a cover crop under these environmental
74
conditions. Developing a well-rounded understanding of cover crop radish seed germination and
subsequent landscape performance at varying environmental conditions will contribute to
outlining the practices to managing a tillage radish cover crop for compaction remediation.
The agriculture industry has served as a guiding example for how tillage radish and other
bio-drilling cover crops can be used to reduce the stress of soil compaction. There is potential to
also take advantage of this practice on landscape construction sites, and future research has the
capacity to help make it possible. Future research will also need to integrate the agricultural and
horticultural aspects of cover crops and perennial landscapes with the realities and timing of
construction activities for this to be widely adopted by the building and construction trade. This
research has taken some of the initial steps towards identifying the versatility of tillage radish
and addressing the manage practices needed to support practitioners in the landscape industry.
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