Section J Soil Fertility and Plant Nutrition Phosphorus Section J Soil Fertility and Plant Nutrition

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 K Functions: Component of amino acids, proteins, DNA, RNA Energy transfer reactions ( ATP ) Cell membranes (phospholipids) 5 - 25%

Phosphorus Deficiency Corn Tomato

Phosphorus Deficiencies P is a __________ nutrient, so deficiencies are first seen in ___________ leaves. Deficiency symptoms: stunted plants dark green color purple streaks or spots on leaves mobile old

Nutrient Removal (kg/ha/yr) P K N/P Ratio Broccoli (100 lb yield) 0.44 0.07 0.35 6.3 Celery (100 lb yield) 0.19 0.05 0.42 3.8 Corn (bushel of grain – 56 lb) 0.75 0.24 4.0 Alfalfa (ton) 56 6.6 50 8.5 Oranges (ton) 8.8 0.8 9.1 11.0 Source: Plant Nutrient Use in North American Agri., PPI, 2002

Composition of organic fertilizers Here are compositions of some commonly used organic fertilizer materials.

Phosphorus vs. Nitrogen Similarities: Mineralization and immobilization of both N and P can be important for supplying available P to plants Both occur as oxyanions: nitrate (NO3-) and phosphate and (HPO4-, H2PO42-) Both can contribute to pollution

Phosphorus vs. Nitrogen Differences: Most (>95%) of soil N is organic in nature, usually 50% or less of soil P is organic Plants use about 4-10X more N than P Phosphate does not leach through soils as readily as nitrate No gaseous forms of P; therefore no gaseous losses There is no P counterpart to N fixation

The Phosphorus Cycle Plant uptake Mineralization Solid Inorganic P compounds Dissolved Inorganic P Organic P Microbial immobilization Adsorbed P

Phosphorus in Soils Soils may contain from 0.1 to 0.02% P N:P ratio in soils is about 8:1 There is little relationship between total soil P and available P; only a tiny fraction of total P is available to plants Forms of soil P: Organic - various P forms associated with humus Inorganic - mineral P, adsorbed P P in soil solution (ionic forms)

Mineralization-Immobilization of P Organic P Inorganic P Immobilization 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 P if <200 there will be net mineralization of P

Mineral Forms of P In 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.

Adsorbed P Phosphate ions (HPO4-, H2PO42-) are strongly adsorbed to the surfaces of: Iron oxides, especially in acid soils CaCO3, especially in alkaline soils Adsorption is at a minimum in neutral (6-7) pH Adsorption reactions are another reason that P availability in soils is limited.

Phosphorus availability and pH Brady and Weil, Figure 13.10

from Foth and Ellis, 1997

P Reactions with Soil Minerals

Many tropical soils are depleted of P without phosphate, even weeds barely grow Courtesy Potash and Phosphate Institute

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 and Solubility of various P minerals - Fe and Al phosphates in acid soils, and Ca phosphates in alkaline soils

Soil pH and Phosphorus Availability 6.5 4.5 5.0 5.5 7.5 8.5 9.0 8.0 7.0 6.0

Mole fraction of total P H3PO4 H2PO4- HPO42- PO43- 1.0 - 0.8 - 0.6 - 0.4 - 0.2 - 0.0 Mole fraction of total P 0 2 4 6 8 10 12 14 pH

Phosphorus Availability in Soils Only H2PO4- and HPO42- in solution can be utilized by plants

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

Phosphorus Fixation Phosphorus “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 soils Reactions with Al/Fe in acid soils

Soil Likely to “fix” P

Factors Causing P fixation in Neutral or Calcareous Soils P forms relatively insoluble Ca phosphates in neutral to alkaline soils hydroxyapatite octacalcium phosphate Phosphate ions may be adsorbed to CaCO3 particles and on Ca-saturated clays

Phosphorus Reactions in Desert Soils Calcareous soils H2PO4- HPO4= Ca8H2 (PO4)6 Inorganic P octocalcium phosphate Sodic soils sodium phosphate Na2HPO4

Phosphorus Reversion Alkaline soils MCP (fertilizer) over time transforms: MCP → DCP → TCP → OCP → Apatite A similar process happens (with different forms) in acid soils This lowers the availability of P The reversion process usually takes several months to years to be complete

Ca Phosphates Most soluble MCP DCP TCP OCP Least soluble Apatite Form added in fertilizer Chemical transformation with time in a calcareous soil

Factors Causing P Fixation in Acid Soils Precipitation from soil solution with Al or Fe: vivianite Fe3(PO4)2.8H2O strengite FePO4.H2O variscite AlPO4.2H2O Adsorbed on surface of Fe and Al oxides Adsorbed on clay particles (i.e. kaolinite)

Griffin is a highly-weathered clay soil

Consumption of N, P2O5, and K2O in the U.S. N P2O5 K2O Current P consumption is similar to the late 1960s US and Canadian commercial fertilizer consumption of N+P2O5+K2O products. From PPI

U.S. phosphate fertilizer consumption by crop in 2001 Total P2O5 consumption 4.3 million short tons Corn grain 38.4% Other crops 17.6% Alfalfa 7.5% Soybeans 7.7% Wheat 16.5% Corn silage, 3.7% Cotton, 3.6% Potatoes, 2.5% Sorghum, 2.5% USDA-ERS, USDA-NASS, AAPFCO, PPI

Average P use on corn and soybeans relative to crop removal Gap is growing Use Removal Potash and Phosphate Institute, 2001

Potash and Phosphate Institute, 2001

Ratio of P removal by crops to fertilizer applied. DE ND SK MB ON BC AB WA OR MT ID SD MN PQ NY PA OH IN IL IA WI MI WY UT NV CA AZ NM NB NS PEI ME NH VT MA CT RI NE KS MO KY WV VA MD NJ NC TN AR OK TX LA MS AL GA SC FL CO 0.00-0.49 0.50-0.89 0.90-1.09 1.10-1.49 >1.50 R/F Potash and Phosphate Institute, 2001

Ratio of P removal by crops to fertilizer applied plus recoverable manure. ND SK MB ON BC AB WA OR MT ID SD MN PQ NY PA OH IN IL IA WI MI WY UT NV CA AZ NM NB NS PEI ME NH VT MA CT RI NE KS MO KY WV VA MD DE NJ NC TN AR OK TX LA MS AL GA SC FL CO 1.10-1.49 0.50-0.89 0.00-0.49 0.90-1.09 >1.50 R/(F+M) Potash and Phosphate Institute, 2001

Increasing concerns about P from fertilizers and animal manures entering surface water.

Unfertilized lake Canadian lake fertilized with P

P Availability P availability to plants is limited because: soils often contain low amounts of P mineral forms of P are insoluble adsorption of ionic P P does not move to the roots by mass flow because it is so insoluble P must move to roots by diffusion

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 ions high Fe oxide content binds P ions P precipitates with Ca, lower solubility P ions move slowly P ions bind to Fe oxides

Sanchez 1980

Sanchez 1982

Improving P Availability Soil and tissue testing Control soil pH if possible Use organic sources, i.e. manure Placement - critical!!

Measuring P Availability Soil tests Neutral to alkaline soils - extraction of soil with 0.5 M NaHCO3, measure P in the extract Acid soils - extraction of soil with HCl and NH4F, measure P in the extract Tissue tests Not as many P tissue tests as for N, fewer standards exist

Sample of P Soil Test Guidelines

Phosphorus Fertilizers Manufactured from mined apatite minerals Apatite is treated with H2SO4 or H3PO4 to form various inorganic P fertilizers: superphosphate (0-20-0) solid triple superphosphate (0-45-0) solid mono ammonium phosphate (11-52-0) solid di ammonium phosphate (18-46-0) solid ammonium polyphosphate (10-34-0) liquid Phosphoric Acid (0-52-0) liquid Organic: manures contain 0.5 to 2.0% P P analysis in commercial fertilizers is expressed as %P2O5

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 soil Use 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??

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 soils Most other elements are less mobile in soils.

Nutrient Mobility in Soil Soil volume exploited for mobile nutrients: N, S, Cl Soil volume exploited for immobile nutrients: Most others

Apply immobile nutrients here (close to roots) Because P is immobile, we cannot rely on movement of irrigation water to transport P.

Take-Home Message for P Management P is less exciting, but no less important than N. Plants take up ______% as much P as N Manures 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. 5-25 50

Band Broadcast Sanchez, Swanson, and Porter 1990

Response of Celery to P Rate and Placement Espinoza, Sanchez, and Schueneman, 1993