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Basic Concepts in Soil Fertility
Jonathan DeenikAssistant Specialist, Soil Fertility
Department of Tropical Plant and Soil Sciences
Outline• Soil as a Nutrient Reservoir• Organic Matter• Soil Acidity• Liming• N and P• Soils of Hawaii
Soil Plant Relationships
Havlin et al., 2005. Soil Fertility and Fertilizers
Idealized Soil Composition
25% H2O
25% Air
45% Mineral
5% Organic Matter
50% Solids 50% Pores
Clay is Where the Action is!
•Clay Properties:Microscopic size (<0.002 mm)Extremely high surface area
- water retention- chemical reactions- biological activity
Clay surfaces carry charge (-/+)Site of chemical weathering
Brady & Weil, 2004. Elements of the Nature and Properties of Soils
In Hawaii Type of Clay is Critical2:1 layer silicates
1:1 layer silicates
oxides
Clay Minerals
2:1 Clays (high activity)
Montmorillonite (smectite)
Properties:
• Expanding• Permanent charge (-)• High CEC (80-120 cmolc kg-1)• High surface area• Sticky
http://webmineral.com/specimens/picshow.php?id=1285
Clay Minerals
Clays
Properties:
• Non-expanding• pH dependent charge• low CEC (1-10 cmolc kg-1)• Relatively low surface area• Non-sticky
Kaolinite
http://soil.gsfc.nasa.gov/forengeo/thnsec.htm
Al Oxides (Gibbsite)
Source:http://www.icmab.es/multimetox/docs_lectures/lectures_html/Gale_J/img033.JPG
Stable at low pHZero point of charge (pH = 5.9-6.7)Al only dissolves under very acidic conditions
Al and Clay Minerals
Clay Minerals
Al & Fe Oxides
Properties:• Non-expanding• pH dependent charge• Very little CEC • High anion retention• Non-sticky
Surface ChargePermanent Charge• Isomorphic substitution• Net negative• 2:1 clays
Brady & Weil, 2004. Elements of the Nature and Properties of Soils
Surface ChargepH-Dependent Charge• Net Negative or net positive• Charge determined by H+/OH-
• Oxide-rich soils have + charge at low pH
Protonation De-protonation
Weathering Intensity and Clay Type
Smectite Kaolinite Oxides
Fox et al., 1991
Organic Matter
Humus
http://www.sct.embrapa.br/diacampo/2004/releases.htm
Importance of Organic Matter in Soils
Physical- Improves aggregation (glue)- Improves water holding capacity (surface area)
Chemical- Increases nutrient availability (N cycling, P and
micronutrient solubility)- Increases CEC (200 cmolc kg-1)- Buffers the soil against pH changes
Biological- Increases microbial diversity- N fixation (rhizobia), P availability (mychoriza)- Assists in pathogen suppression
Clays and OM Buffer Soil System
Buffering Capacity:1.Resistance to change
2.The ability of the soil to re-supply nutrients the soil solution
Nutrientin
solution
Plant uptake
Solid phase nutrient(adsorbed/absorbed)
Total soil nutrient(unavailable)
Buffering capacity depends on:• clay content and type• CEC• organic matter
Soil Acidity
Natural Sources of Acidity:
Carbonic acid and organic acidsOrganic matterPrecipitation and cation leachingNitrificationN ImmobilizationAmmonium volatilizationCation uptakeDeprotonation of pH-dependent charge
Human Induced Acidity:
Acid rainUreaAmmonium fertilizersMono and diammonium phosphateElemental S
Soil Acidity40 years of N application
Source: Schwab et al., 1990 SSSAJ
(NH2)2CO + 4O2 2NO3- + 2H+ + CO2 + H2O
Adverse Effects of Soil AcidityAluminum toxicityManganese toxicityNutrient deficienciesDecreased microbial activity
Source: Hue et al., 1998
Source: Bohn et al., 2001
Al precipitated as Al(OH)3 down to pH 4.5
Aluminum Toxicity
At pH below 5.0 Al toxicity a problemUltisols most likely to have Al toxicity under acid conditionsSoils acidified by pineapple production may be problematic especially if Sicontent is high (>20%)Liming (CaCO3/CaSO4) and/or organic matter inputs alleviate Al toxicity
Soils with Potential Mn Toxicity
Average MnO2 content of soils = 0.1%Oxisols exisiting at low to moderate elevation (200-750 ft) with moderate rainfall (20-60 in/yr)Molokai, Lahaina, Wahiawa (1.5% MnO2) series
Kaolinitic Mollisols and Inceptisols in dry environmentsKeahua (0.4%), Ewa, Paia (1.7%), Hoolehua (1.5%),
kahana series
Manganese ToxicityMn toxicity increases as pH drops below 5.5.Reducing conditions (saturated soils) increase Mn toxicityFresh organic inputs increase Mn toxicityManage Mn toxicity with lime, water management, and careful attention to organic inputs
To raise pH- Reduce existing/potential toxicities- Increases P availability- pH range 5.5 - 7.0- Liming can be expensive because soils
are buffered (clay content and OM)
• To supply Ca- Highly weathered soils are almost always
deficient in Ca
Liming is Important
Correcting Soil Acidity is Costly
Brady & Weil, 2004
Correcting Soil Acidity is Costly
Al3+ + H2O Al(OH)2+ + H+
Liming Reactions:
CaCO3 + H2O Ca2+ + HCO3- + OH-
OH- + H+ H2O
HCO3- + H+ H2CO3
Al3+ + 3OH- Al(OH)3
Liming
Tons CaCO3 per AcreUchida & Hue, 2000
Nitrogen
http://www.bettersoils.com.au/module2/images/27.gif
N Mineralization
Mineralization: Decomposition of soil organic matter by soil microbes releasing inorganic N in the process.
Heterotrophs use organic molecules as source of energy• Bacteria – neutral to alkaline environments• Fungi - both neutral and acidic environments
Release of N from the organic matter• Soil Organic Matter ~5% N• 1 to 4% organic N mineralized each year• Added organic N sources
(C:N ratio < 20 = mineralization
ImmobilizationImmobilization• Conversion of mineral N to organic N by microbes
Organisms that decompose organic matter as an energy source require nitrogen
Organic materials with a low N content (C:N > 30) cannot supply the needs of these organisms thus they use soil N in competition with the crop.
Freshly immobilized N = 5-15% of soil N
Ammonium N
Ammonium N = NH4+
• Cation, therefore adsorbed on CEC
• Won't leach or denitrify
• Can be fixed in certain clay minerals – micaceous clay
• Plant uptake
• Rapidly converted to NO3-N under most conditions
• Volatilization at high pH
NH4+ + OH- → NH3↑ + H2O
High pH Gas
Nitrate N• Anion, therefore not adsorbed on CEC• Most common mineral form of N in most soils• Most common form taken up by plants• Very susceptible to leaching and denitrification
losses2NO3
- → N2O & N2 + 3O2Anaerobic gases
No oxygen - wet soil
Energy source for bacteria - organic matter
Warm temperatures
Favored by higher pH
Phosphorus
http://biology.kenyon.edu/courses/biol112/Biol112WebPage/Syllabus/Topics/Week%2013/PhosphorusCycle.jpg
P Fixation
Factors Affecting P Fixation
Soil typeAndisol>Oxisol≈Ultisol>Inceptisol>Mollisol≈Vertisol
Honokaa Kapaa MakaweliWaimea Wahiawa Waialua Kula Alaeloa Keahua
Lualualei
Soil pH- acid soils P fixed by Al/Fe oxides- alkaline soils P fixed by CaDecrease P fixation by liming and/or adding OM
Soil Fertility Depends on:
Clay contentClay mineralogy
- 2:1- 1:1- Oxides
Organic Matter
Fertile SoilsSlightly acid to neutral pHHigh CECHigh organic matterVertisols - LualualeiMollisols - WaialuaMedial Andisols -Waimea
Infertile SoilsAcid to strongly acid pHLow CECHigh P fixationToxicitiesOxisols - KapaaUltisols - HaikuHydrus Andisols - Hilo
Weathering Intensity and Fertility
Smectite Kaolinite Oxides
Category Series ClassificationHighly WeatheredVery Acid Kapaa Very fine, sesquic, isohyperthermic, Anionic Acrudox
Halii Fine, ferruginous, isothermic Anionic AcroperoxMakapili Very fine, sesquic, isohyperthermic, Anionic AcrudoxPooku Very fine, ferrihydritic, isohyperthermic, Anionic AcrudoxLawai Very fine, ferrihydritic, isohyperthermic, Typic HapludoxMahana Clayey, oxidic, isothermic, Typic AcrohumoxPuu Opae Fine, oxidic, isohyperthermic, Ustoxic PalehumultPuhi Very fine, ferruginous, isohyperthermic, Humic Kandiustox Kalapa Very fine, kaolinitic, isohyperthermic, Typic Palehumultkunuweia Clayey-skeletal, ferritic, isothermic Typic HapludoxHanamaulu Very-fine, ferruginous, isohyperthermic Humic KandiudoxLihue Very-fine, ferruginous, isohyperthermic Rhodic EutrustoxNiu Very fine, kaolinitic, isohyperthermic, Rhodic EustrustoxHihimanu Very fine, parasesquic, isohyperthermic, Oxic Dystrudepts
Moderately WeatheredSlightly acid
Limited Weathering Makaweli Fine, parasesquic, isohyperthermic, Torroxic HaplustollsNeutral to alkaline Pakala Fine loamy, parasesquic, isohyperthermic, Torroxic Haplustolls
Pohakupu Fine, parasesquic, isohyperthermic, Torroxic HaplustollsKoloa Fine, mixed, isohyperthermic, Andic Haplustollskaloko Fine, smectitic, calcareous, isohyperthermic Cumulic EndoaquollsKolokolo Fine, mixed, isohyperthermic, Cumulic haplustollsMokuleia Clayey over sandy sandy skeletal, mixed isohyperthermic, Entic HaplustollsMamala Clay, parasesquic, isohyperthermic, Lithic HaplocambidsKekaha Very-fine, parasesquic, isohyperthermic, Typic HaplocambidsIao Fine, mixed, active, isohyperthermic Cumulic HaplustollsWaikomo Clayey, mixed, isohyperthermic, Lithic HaplustollsNonopahu Fine, mixed, isohyperthermic, Chromic Haplotorrerts Hanalei Very fine, mixed, isohyperthermic, Typic EndoaqueptsKalihi Fine, halloysitic, isohyperthermic, Fluvaquentic Endoaquolls Nohili Very-fine, smectitic, isohyperthermic, Cumulic HaplaquollsKaena Very-fine, smectitic, isohyperthermic, Typic NatraquertsLualualei Fine, smectitic, isohyperthermic, Typic ChromustertWaiawa Clayey smectitic, isohyperthermic, Lithic Vertic Ustropepts
Kauai
Kahului
Paia
Lahaina
Kula
Hana
Kaupo
Kihei
Kaunakakai
Hoolehua
Kalaupapa
HalawaKaluakoi
Kamalu
Kona
Volcano
Pahoa
Summary
Clay mineralogy important determinant of fertilityOrganic matter fundamental to nutrient availabilitySoil fertility in Hawaii closely tied to weathering intensity
Resources
BooksHavlin, J.L., S.L. Tisdale, J.D. Beaton, and W.L. Nelson. 2005. Soil Fertility and Fertilizers. Pearson Education, Inc., Upper Saddle River, NJ
Foth, H. D. and B.G. Ellis. 1997. Soil Fertility. CRC Press, Inc., Boca Raton FL.Brady, N.C. and R.R. Weil. 2004. Elements of the Nature and Properties of Soils. Pearson Education, Inc., Upper Saddle River, NJ
Webhttp://www.extension.iastate.edu/pubs/so.htmhttp://www.montana.edu/wwwpb/pubs/mt4449.htmlExcellent short course in soil fertility