The Biology of Soil Comp Action

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4 Crops & Soils magazine | July–August 2011 American Society o Agronomy

Feature

SOILCOMPACTION

of

biology

The



agronomy.org/certifcations | soils.org/certifcations July–August 2011 | Crops & Soils magazine 5

By James J. Hoorman, Extension Educator, Cover Crops and Water Quality, Ohio

State University Extension, Columbus; João Carlos de Moraes Sá, Soil OrganicMatter and Fertility Specialist, University o Ponta Grossa, Ponta Grossa, Brazil;and Randall Reeder, Extension Agricultural Engineer, Food, Agricultural, andBiological Engineering, Ohio State University Extension, Columbus

Editor’s note: The ollowing article is being reprinted with permission in a slightly modifed ormat rom the Ohio StateUniversity Extension. The original document can be viewed here: http://ohioline.osu.edu/sag-act/pd/0010.pd.

Soil compaction is a common and constant prob-lem on most arms that till the soil. Heavy arm machinerycan create persistent subsoil compaction (Hakansson andReeder, 1994). Johnson et al. (1986) ound that compactedsoils resulted in: restricted root growth, poor root zone aera-tion and poor drainage that results in less soil aeration, lessoxygen in the root zone, and more losses o nitrogen romdenitrication.

Subsoil tillage has been used to alleviate compactionproblems. Subsoilers are typically operated at depths o 12to 18 inches to loosen the soil, alleviate compaction, andincrease water inltration and aeration. Subsoiling usually

increases crop yields, but the eects may only be tempo-rary as the soil re-compacts due to equipment trac. Someno-till elds never need to be subsoiled, but in other no-tillelds, deep tillage has increased yields especially i equip-ment trac is random. When subsoiling removes a hardpan, trac must be controlled or compaction will reoccur.I a hard pan does not exist, equipment trac generally willcreate one (Reeder and Westermann, 2006).

I the soil is subsoiled when the soil is wet, additionalcompaction may occur. In a loamy sand, Busscher et al.

(2002) ound that soil compaction increased with time,and cumulative rainall accounted or 70 to 90% o there-compaction due to water ltering through the soil andthe orce o gravity. The uel, labor, equipment, and time tosubsoil makes it an expensive operation. Subsoiling in dryconditions requires even more uel (Reeder and Wester-mann, 2006). Two other actors that aect soil compactionare rainall impact and gravity. In soils that have been tilled,both the velocity o the raindrop impact on bare soil andnatural gravity combine to compact soils.

Low organic matter levels make the soil more susceptibleto soil compaction. Organic residues on the soil suracehave been shown to cushion the eects o soil compaction.Surace organic residues have the ability to be compressed,but they also retain their shape and structure once the tra-c has passed. Like a sponge, the organic matter is com-pressed and then springs back to its normal shape. However,excessive trac will break up organic residues, and tillageaccelerates the decomposition o organic matter. Organicresidues in the soil prole may be even more important thansurace organic residues. Organic matter (plant debris andresidues) attached to soil particles (especially clay particles)keeps soil particles rom compacting. Organic matter bindsmicroaggregates and macroaggregates in the soil. Loworganic matter levels make the soil more susceptible to soilcompaction (Wortman and Jasa, 2003).

In the last hundred years, tillage has decreased soil or-ganic levels by 60%, which means that approximately 40%




o soil organic carbon stocks are remaining (InternationalPanel on Climate Change, 1996; Lal, 2004). Carbon pro-vides energy or soil microbes, is a storehouse or nutri-ents, and keeps nutrients recycling within the soil. Humus

or old carbon (>1,000 years old) is the most stable carbonand binds soil microparticles together to orm microaggre-gates. Humus is not water soluble, so it stabilizes micro-aggregates and is not readily consumed by microorgan-isms. Humus is more resistant to tillage and degradationthan active carbon.

Active carbon (plant sugars, polysaccharides, andglomalin) is consumed by microbes or energy. Activecarbon is reduced with tillage but is stabilized undernatural vegetation and no-till systems using a continuousliving cover. Active carbon is part o the glue that bindsmicroaggregates into macroaggregates and insulates themacroaggregate rom oxygen. Soil porosity, water inl-

tration, soil aeration, and soil structure increase undernatural vegetation and no-till systems with continuousliving cover. Increased soil macroaggregation improvessoil structure and lowers bulk density, keeping the soilparticles rom compacting.

Microaggregates and macroaggregate

formation

Microaggregates are 20–250 μm in size and are com-posed o clay microstructures, silt-size microaggregates,particulate organic matter, plant and ungus debris, andmycorrhizal ungus hyphae. Roots and microbes combinemicroaggregates in the soil to orm macroaggregates.Macroaggregates are linked mainly by ungi hyphae, rootbers, and polysaccharides and are less stable than mi-croaggregates. Macroaggregates are greater than 250 μmin size and give soil its structure and allow air and waterinltration. Compacted soils tend to have more microag-gregates than macroaggregates (Fig. 1 and 2).

Glomalin acts like a glue to cement microaggregatestogether to orm macroaggregates and improve soilstructure. It initially coats the plant roots and then coats

soil particles. Glomalin is an amino polysaccharide orglycoprotein created by combining a protein rom themycorrhizal ungus with sugar rom plant root exudates(Allison, 1968). The ungal “root-hyphae-net” holds theaggregates intact, and clay particles protect the roots andhyphae rom attack by microorganisms. Roots also createother polysaccharide exudates to coat soil particles (seeFig. 2 and 3).

The contribution o mycorrhizal ungi to aggregationis a simultaneous process involving three steps. First, theungus hyphae orm an entanglement with primary soilparticles, organizing and bringing them together. Second,ungi physically protect the clay particles and organic

debris that orm microaggregates. Third, the plant rootand ungus hyphae orm glomalin andglue microaggregates and some smallermacroaggregates together to orm largermacroaggregates (see Fig. 4).

In order or glomalin to be produced,plants and mycorrhizal ungi must existin the soil together. Glomalin needs tobe continually produced because it isreadily consumed by bacteria and othermicroorganisms in the soil. Bacteriathrive in tilled soils because they aremore hardy and smaller than ungi, so

bacteria numbers increase in tilled soils.Fungi live longer and need more stableconditions to survive. Fungi grow bet-ter under no-till soil conditions with acontinuous living cover and a constantsource o carbon. Since ungi do notgrow as well in tilled soils, less glomalinis produced and ewer macroaggregatesare ormed, which can result in poorsoil structure and compaction. Thus,soil compaction is a biological problem

Feature

Fig. 1. (a) Macroaggregate components—schematicillustration; (b) mechanical disturbance by tillage dis-rupts macroaggregates and exposes soil organic mat-ter (SOM) protected within the aggregate to microbialattack; (c) decrease o SOM within the aggregates dueto microbial attack causes dispersion o clay particles,clay microstructure, and silt+clay microaggregates.

Illustration courtesy of João Carlos de Moraes Sá.




related to decreased production o polysaccha-rides and glomalin in the soil. Soil compactionis due to a lack o living roots and mycorrhizalungi in the soil.

In a typical corn–soybean rotation, active roots arepresent only a third o the time. Adding cover crops be-

tween the corn and soybean crops increases the presenceo active living roots to 85 to 90% o the time. Active rootsproduce more amino polysaccharides and glomalin be-

cause mycorrhizal ungus populations

increase due to a stable ood supply.Surace and subsoil tillage may

physically break up hard pans and soilcompaction temporarily, but they arenot a permanent x. Tillage increasesthe oxygen content o soils and de-creases glomalin and amino polysac-

Fig. 2. Hierarchy o soil aggregates. Illustration republished with permission fromTe Nature and Properties of Soils, 14th ed., Brady and Weil (2008), Fig. 4.15 from p. 137.

Fig. 3 (below, left). Roots, ungi hyphae, and polysaccharides stabilizesoil macroaggregates and promote good soil structure. Photo by João Carlos de

Moraes Sá. Fig. 4 (below, right). A microscopic view o an arbuscular mycor-rhizal ungus growing on a corn root. Te round bodies are spores, and thethreadlike laments are hyphae. Te substance coating them is glomalin,revealed by a green dye tagged to an antibody against glomalin. Photo by Dr.

Sara Wright and Dr. Kristina Nichols (USDA-ARS).




charide production by reducing plant root exudates andmycorrhizal ungus populations. Soil compaction is a re-sult o the lack o active roots producing polysaccharidesand root exudates and a lack o mycorrhizal ungi produc-ing glomalin. In a typical undisturbed soil, ungal hyphaeare turned over every ve to seven days, and the glomalinin the ungal hyphae is decomposed and continually coatsthe soil particles. Disturbed soils have less ungi, morebacteria, and more microaggregates than macroaggre-gates. Heavy equipment loads push the microaggregatestogether so that they can chemically bind together, com-pacting the soil. Macroaggregate ormation improves soilstructure so that soil compaction may be minimized. Thus,soil compaction has a biological component (see Fig. 5).

Cultivation o soils, heavy rains, andoxygen promote thebreakdown o mac-roaggregates, whichgive soil structure andsoil tilth. Farmers who

excessively till their soils (or example, heavy disking orplowing) break down the soil structure by breaking up themacroaggregates, injecting oxygen into the soil, and de-pleting the soil o glomalin, polysaccharides, and carbon.Greater than 90% o the carbon in soil is associated withthe mineral raction (Jastrow and Miller, 1997). Glomalinand polysaccharides are consumed by fourishing bacteriapopulations that thrive on high oxygen levels in the soiland the release o nutrients in organic matter rom the till-age. The end result is a soil composed o mainly microag-gregates and cloddy compacted soils. Soils composedmainly o micro-aggregates prevent water inltration dueto the lack o macropores in the soil, so water tends topond on the soil surace. Farm elds that have been ex-cessively tilled tend to crust, seal, and compact more thanno-till elds with surace crop residues and a living cropwith active roots to promote ungal growth and glomalinproduction.

An agricultural system that combines a continuousliving cover (cover crops) with continuous long-termno-till is a system that closely mimics a natural systemand should restore soil structure and soil productivity. Acontinuous living cover plus continuous long-term no-tillprotects the soil rom compaction in ve major ways.First, the soil surace acts like a sponge to help adsorb theweight o heavy equipment trac. Second, plant roots

create voids and macropores in the soil so that air andwater can move through the soil. Roots act like a biologi-cal valve to control the amount o oxygen that enters thesoil. The soil needs oxygen or root respiration and to sup-port aerobic microbes in the soil. However, too much soiloxygen results in excessive carbon loss rom the aerobicmicrobes consuming the active carbon. Third, plant rootssupply ood or microorganisms (especially ungi) andburrowing soil auna that also keep the soil rom compact-ing. Fourth, organic residues let behind by the decayingplants, animals, and microbes are lighter and less dense

Feature

What is a clod?Many armers complain that their soil is cloddy and

hard to work. Clods are man-made and do not usually existin the natural world. Bricks and clay tile are ormed by tak-ing wet clay rom the soil and heating and drying the clay.When armers till the soil, they perorm the same processby exposing the clay to sunlight, heating and drying theclay until it gets hard and turns into a clod. illage alsooxidizes the soil and results in increased microbial decom-position o organic residues. Organic residues keep clay particles rom chemically binding. Clay soils that remainprotected by organic residues and stay moist resist turninginto clods because the moisture and organic residues keepthe clay particlesphysical-ly sepa-rated.

Organic residues act like sponges, absorb-ing water and soil nutrients and cushion-ing soil particles. Clods act like bricks, re-sisting water absorption and making soilshard and compacted. Photo by Jim Hoorman.

Fig. 5. illage disrupts themacroaggregates and breaksthem into microaggregates byletting in oxygen and releas-ing carbon dioxide. Photo by

João Carlos de Moraes Sá.




Building soil structureBuilding soil structure is like building a house.

Mother Nature is the architect, and plants and mi-crobes are the carpenters. Every house needs to startout with a good oundation like bricks (clay, sand, andsilt) and cement (cations like Ca++, K+). When a houseis ramed, various sized wood timbers, raters, andplanks are used to create rooms (represented by the various sized roots in the soil). Wood and roots givethe house and the soil structure, creating space wherethe inhabitants (plants, microbes, and soil auna) canlive.

Wood in a house is held together by various sizednails (humus) and lag screws (phosphate attachesorganic residues to clay particles). A house has braces

to add stability (nitrogen and sulur provide stability toorganic residues) and a roo to control the temperatureand moisture. In the soil, a deep layer o surace resi-dues controls oxygen and regulates water inltrationand runo. A roo insulates the house and regulatesthe temperature just like surace residue on the soilsurace keeps the soil temperature in a comortablerange or the inhabitants (microbes and plant roots).Houses need insulation and glue to keep them togeth-er. Root exudates orm polysaccharides and glomalin(ormed with mycorrhizal ungus) to insulate the soilparticles and keep thesoil macroaggregates

glued together. I theroo on a house isdestroyed, moistureand cold air can enterthe house and rot outthe wood and dissolvethe glues.

In the soil, organicmatter decomposes very quickly whentillage, excess oxygen,and moisture eitherbreak down the glues

(polysaccharides andglomalin) or are easily consumed by fourish-ing bacteria popula-tions. Excess oxygenin the soil (romtillage) stimulatesbacteria populationsto grow, and they consume the polysac-

charides as a ood source, destroying the soil struc-ture. With tillage, macroaggregates become microag-gregates, and the soil becomes compacted.

As every homeowner knows, houses need regularmaintenance. In the soil, the roots and the microbes(especially ungi) are the carpenters that maintain theirhouse, continually producing the glues (polysaccha-rides and glomalin) that hold the house together. Regu-lar tillage acts like a tornado or a hurricane, destroyingthe structural integrity o the house and killing o theinhabitants. illage oxidizes the organic matter in thesoil, destroying the roots and the active organic matter,causing the soil structure to crumble and compact. I you remove wood supports and glue in a house, thehouse becomes unstable just like the soil does when you remove the active living roots and active organicresidues (polysaccharides). Wood beams in a coalmine stabilize the coal mine tunnel like active livingroots and healthy microbial communities give the soilstructure to prevent soil compaction. Active roots andmacroaggregates give soil porosity to move air and wa-ter through the soil in macropores. In an ideal soil, 50to 60% o the soil volume is porous while in a degradedcompacted soil, soil porosity may be reduced to 30 to40% o the total soil volume. Compacted soil is like adecaying house turning into a pile o bricks, cement,and rubble.




than clay, silt, and sand particles, so adding organicresidues to the soil decreases the average soil density.Fith, soil compaction is reduced by combining microag-gregates into macroaggregates in the soil. Microaggregatesoil particles (clay, silt, and particulate organic matter) areheld together by humus or old organic matter residues,which are resistant to decomposition, but microaggregatestend to compact in the soil under heavy equipment loads.Polysaccharides and glomalin weakly combine microag-gregates into macroaggregates, but this process is brokendown once the soil is disturbed or tilled.

Summary

Soil compaction reduces crop yields and arm prots.For years, armers have been physically tilling and subsoil-ing the soil to reduce soil compaction. At best, tillage maytemporarily reduce soil compaction, but rain, gravity, andequipment trac compact the soil. Soil compaction has abiological component—it is caused by a lack o activelygrowing plants and active roots in the soil. A continuousliving cover plus long-term continuous no-till reducessoil compaction in ve ways. Organic residues on thesoil surace cushion the soil rom heavy equipment. Plantroots create voids and macropores in the soil or air andwater movement. Plant roots act like a biological valve tocontrol the amount o oxygen in the soil to preserve soilorganic matter. Plant roots supply ood or soil microbesand soil auna. Residual organic soil residues (plants,roots, and microbes) are lighter and less dense than soilparticles.

Soil compaction is reduced by the ormation o macro-aggregates in the soil. Microaggregate soil particles (clay,silt, and particulate organic matter) are held together byhumus or old organic matter residues and are resistant

to decomposition. Macroaggregates orm by combingmicroaggregates together with root exudates like polysac-charides and glomalin (sugars rom plants and proteinrom mycorrhizal ungi). Polysaccharides rom plants and

glomalin rom ungi weakly hold the microaggregatestogether but are consumed by bacteria, so they need to becontinually reproduced in the soil to improve soil struc-ture. Tillage and subsoiling increase the oxygen content insoils, increasing bacteria populations, which consume theactive carbon needed to stabilize macroaggregates, lead-ing to the destruction o soil structure. Soil compaction isa direct result o tillage, which destroys the active organicmatter, and a lack o living roots and microbes in thesoil. Heavy equipment loads push soil microaggregatestogether so that they chemically bind together, resulting insoil compaction.

AcknowledgmentsThis act sheet was produced in conjunction with the

Midwest Cover Crops Council (MCCC). The authors wishto thank Kim Wintringham (Technical Editor, Communi-cations and Technology, The Ohio State University) andDanita Lazenby (diagram illustrations).

ReferencesAllison, F.E. 1968. Soil aggregates—some acts and allacies as

seen by microbiologist. Soil Sci. 106:136–143.

Brady, N.C., and R. Weil. 2008. The nature and properties o soils. 14th ed. Prentice Hall, Upper Saddle River, NJ.

Busscher, W.J., P.J. Bauer, and J.R. Frederick. 2002. Recompac-tion o a coastal loamy sand ater deep tillage as a unc-tion o subsequent cumulative rainall. Soil Tillage Res.68:49–57.

Hakansson, I., and R.C. Reeder. 1994. Subsoil compaction byvehicles with high axle load-extent, persistence and cropresponse. Soil Tillage Res. 29(2–3):277–304.

Jastrow, J.D., and R.M. Miller. 1997. Soil aggregate stabilizationand carbon sequestration: Feedbacks through organomineralassociations. p. 207–223. In R. Lal et al. (ed.) Soil processesand the carbon cycle. CRC Press, Boca Raton, FL.

Johnson, B.S., A.E. Erickson, and A.J.M. Smucker. 1986. Allevia-tion o compaction on a ne textured soil. ASAE Paper No.86-1517. ASAE, St. Joseph, MI.

Lal, R. 2004. Soil carbon sequestration impacts on global climatechange and ood security. Science 304:1623–1627.

Reeder, R., and D. Westermann. 2006. Environmental benetso conservation on cropland: The status o our knowledge.p. 26–28. In M. Schnep and C. Cox (ed.) Soil managementpractices. Soil and Water Conservation Society, Ankeny, IA.

Wortman, C., and P. Jasa. 2003. Management to minimize andreduce soil compaction. NebGuide G896. University o Nebraska Extension, Lincoln.

Feature

Five ways soil organic matterresists soil compaction

1. Surace residue resists compaction. Acts like a spongeto absorb weight and water.

2. Organic residues are less dense (0.3–0.6 g/cm3) thansoil particles (1.4–1.6 g/cm3).

3. Roots create voids and spaces or air and water.

4. Roots act like a biological valve to control oxygen in thesoil.

5. Roots supply exudates to glue soil particles together toorm macroaggregates and supply ood or microbes.

Documents

The Biology of Soil Comp Action