Presentation on theme: "Section J Soil Fertility and Plant Nutrition"— Presentation transcript:
1 Section J Soil Fertility and Plant Nutrition PhosphorusSection JSoil Fertility and Plant Nutrition
2 Phosphorus as a Plant Nutrient After N, it is the nutrient most likely to be deficient to plant growth.Plants use about _______ as much P as N or KFunctions:Component of amino acids, proteins, DNA, RNAEnergy transfer reactions ( ATP )Cell membranes (phospholipids)5 - 25%
4 Phosphorus Deficiencies P is a __________ nutrient, so deficiencies are first seen in ___________ leaves.Deficiency symptoms:stunted plantsdark green colorpurple streaks or spots on leavesmobileold
5 Nutrient Removal (kg/ha/yr) PKN/P RatioBroccoli (100 lb yield)0.440.070.356.3Celery (100 lb yield)0.190.050.423.8Corn (bushel of grain – 56 lb)0.750.244.0Alfalfa (ton)566.6508.5Oranges (ton)18.104.22.168.0Source: Plant Nutrient Use in North American Agri., PPI, 2002
6 Composition of organic fertilizers Here are compositions of some commonly used organic fertilizer materials.
7 Phosphorus vs. Nitrogen Similarities:Mineralization and immobilization of both N and P can be important for supplying available P to plantsBoth occur as oxyanions: nitrate (NO3-) and phosphate and (HPO4-, H2PO42-)Both can contribute to pollution
8 Phosphorus vs. Nitrogen Differences:Most (>95%) of soil N is organic in nature, usually 50% or less of soil P is organicPlants use about 4-10X more N than PPhosphate does not leach through soils as readily as nitrateNo gaseous forms of P; therefore no gaseous lossesThere is no P counterpart to N fixation
9 The Phosphorus Cycle Plant uptake Mineralization Solid Inorganic P compoundsDissolvedInorganic POrganic PMicrobialimmobilizationAdsorbed P
10 Phosphorus in Soils Soils may contain from 0.1 to 0.02% P N:P ratio in soils is about 8:1There is little relationship between total soil P and available P; only a tiny fraction of total P is available to plantsForms of soil P:Organic - various P forms associated with humusInorganic - mineral P, adsorbed PP in soil solution (ionic forms)
11 Mineralization-Immobilization of P Organic PInorganic PImmobilization and Mineralization of soil P are similar to that of N:If added organic materials have a C:P ratio of >300, there will be net immobilization Pif <200 there will be net mineralization of P
12 Mineral Forms of PIn neutral to alkaline soils, most mineral P will be as Ca-phosphates. Most of these are quite insoluble.In acid soils, most mineral P will be as Fe and Al-phosphates. Most of these are quite insoluble.The insolubility of most P minerals is one important reason that P availability to plants is usually low.
13 Adsorbed PPhosphate ions (HPO4-, H2PO42-) are strongly adsorbed to the surfaces of:Iron oxides, especially in acid soilsCaCO3, especially in alkaline soilsAdsorption is at a minimum in neutral (6-7) pHAdsorption reactions are another reason that P availability in soils is limited.
14 Phosphorus availability and pH Brady and Weil, Figure 13.10
17 Many tropical soils are depleted of P without phosphate, even weeds barely growCourtesy Potash and Phosphate Institute
18 P Availability Governed by: Mineralization-Immobilization of humus P But primarily by:Adsorption-desorption reactions of ionic P with Al and Fe oxides or CaCO3 andSolubility of various P minerals - Fe and Al phosphates in acid soils, and Ca phosphates in alkaline soils
19 Soil pH and Phosphorus Availability 6.54.55.05.57.58.59.08.07.06.0
20 Mole fraction of total P H3PO4H2PO4-HPO42-PO43-1.0 -0.8 -0.6 -0.4 -0.2 -0.0Mole fraction of total PpH
21 Phosphorus Availability in Soils Only H2PO4- and HPO42- in solution can be utilized by plants
22 Phosphorus “Fixation” Like N, much of the P applied in fertilizers is not recovered by plants in the first year. The reason is different:P reversion is the process wherein available, soluble P forms applied in fertilizers naturally transform back into less soluble forms over time.This is a non-biological process
23 Phosphorus FixationPhosphorus “fixation” (sometimes called “reversion”) refers to reactions of P in soils that cause P added in fertilizer to become less available with time:Reactions with Ca in calcareous soilsReactions with Al/Fe in acid soils
25 Factors Causing P fixation in Neutral or Calcareous Soils P forms relatively insoluble Ca phosphates in neutral to alkaline soilshydroxyapatiteoctacalcium phosphatePhosphate ions may be adsorbed to CaCO3 particles and on Ca-saturated clays
26 Phosphorus Reactions in Desert Soils CalcareoussoilsH2PO4-HPO4=Ca8H2 (PO4)6Inorganic PoctocalciumphosphateSodicsoilssodiumphosphateNa2HPO4
27 Phosphorus Reversion Alkaline soils MCP (fertilizer) over time transforms:MCP → DCP → TCP → OCP → ApatiteA similar process happens (with different forms) in acid soilsThis lowers the availability of PThe reversion process usually takes several months to years to be complete
28 Ca Phosphates Most soluble MCP DCP TCP OCP Least soluble Apatite Form added in fertilizerChemical transformationwith time in a calcareoussoil
29 Factors Causing P Fixation in Acid Soils Precipitation from soil solution with Al or Fe:vivianiteFe3(PO4)2.8H2OstrengiteFePO4.H2OvarisciteAlPO4.2H2OAdsorbed on surface of Fe and Al oxidesAdsorbed on clay particles (i.e. kaolinite)
31 Consumption of N, P2O5, and K2O in the U.S. N P2O K2OCurrent P consumption is similar to the late 1960sUS and Canadian commercial fertilizer consumption of N+P2O5+K2O products.From PPI
32 U.S. phosphate fertilizer consumption by crop in 2001 Total P2O5 consumption4.3 million short tonsCorn grain38.4%Other crops17.6%Alfalfa7.5%Soybeans7.7%Wheat16.5%Corn silage, 3.7%Cotton, 3.6%Potatoes, 2.5%Sorghum, 2.5%USDA-ERS, USDA-NASS, AAPFCO, PPI
33 Average P use on corn and soybeans relative to crop removal Gap isgrowingUseRemovalPotash and Phosphate Institute, 2001
35 Ratio of P removal by crops to fertilizer applied. DENDSKMBONBCABWAORMTIDSDMNPQNYPAOHINILIAWIMIWYUTNVCAAZNMNBNSPEIMENHVTMACTRINEKSMOKYWVVAMDNJNCTNAROKTXLAMSALGASCFLCO>1.50R/FPotash and Phosphate Institute, 2001
36 Ratio of P removal by crops to fertilizer applied plus recoverable manure. NDSKMBONBCABWAORMTIDSDMNPQNYPAOHINILIAWIMIWYUTNVCAAZNMNBNSPEIMENHVTMACTRINEKSMOKYWVVAMDDENJNCTNAROKTXLAMSALGASCFLCO>1.50R/(F+M)Potash and Phosphate Institute, 2001
37 Increasing concerns about P from fertilizers and animal manures entering surface water.
38 Unfertilized lakeCanadian lake fertilized with P
39 P Availability P availability to plants is limited because: soils often contain low amounts of Pmineral forms of P are insolubleadsorption of ionic PP does not move to the roots by mass flow because it is so insolubleP must move to roots by diffusion
40 P Availability P Availability is most likely to be limited in: Weathered soils: ___________________________Acid soils: ________________________________Alkaline soils: ______________________________Cold soils: ________________________________Soils high in Fe oxides: ______________________high Fe oxide content binds P ionshigh Fe oxide content binds P ionsP precipitates with Ca, lower solubilityP ions move slowlyP ions bind to Fe oxides
43 Improving P Availability Soil and tissue testingControl soil pH if possibleUse organic sources, i.e. manurePlacement - critical!!
44 Measuring P Availability Soil testsNeutral to alkaline soils - extraction of soil with 0.5 M NaHCO3, measure P in the extractAcid soils - extraction of soil with HCl and NH4F, measure P in the extractTissue testsNot as many P tissue tests as for N, fewer standards exist
50 Phosphorus Fertilizers Manufactured from mined apatite mineralsApatite is treated with H2SO4 or H3PO4 to form various inorganic P fertilizers:superphosphate (0-20-0) solidtriple superphosphate (0-45-0) solidmono ammonium phosphate ( ) soliddi ammonium phosphate ( ) solidammonium polyphosphate ( ) liquidPhosphoric Acid (0-52-0) liquidOrganic: manures contain 0.5 to 2.0% PP analysis in commercial fertilizers is expressed as %P2O5
51 Managing Soil P Managing soil P for maximum availability If possible, assure an optimum pH (6-7)Keep in mind that P is especially unavailable in cold soils.Apply P in bands in soilUse soil testing before planting each season, use appropriate guidelines.Band-apply NH4+ and P together--this usually increases P availability, particularly in alkaline soils. Why??
53 Nutrient Mobility in Soils Mobility in soils refers to the relative rate of movement of soluble nutrient forms in soils.Mobility is a function of soil texture and mineralogy (generally slower in clay soils)Usually, N (NO3-), S (SO42-), and Cl (Cl-) are considered mobile in soilsMost other elements are less mobile in soils.
54 Nutrient Mobility in Soil Soil volume exploitedfor mobile nutrients:N, S, ClSoil volume exploitedfor immobile nutrients:Most others
55 Apply immobile nutrients here (close to roots)Because P is immobile, we cannot relyon movement of irrigation water to transport P.
56 Take-Home Message for P Management P is less exciting, but no less important than N.Plants take up ______% as much P as NManures contain about ____% as much P as N.P is less subject to losses in soils compared to N, is usually immobile in soils.Timing of P applications to crops is less critical than for N.5-2550
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