Phosphorus Chemistry in Soils and Response to Fertilizer and Manures April Leytem USDA/ARS

The Phosphorus Cycle Animal manures and biosolids Mineral fertilizers Crop harvest Runoff and erosion Leaching Organic phosphorus Microbial Plant residue Humus Primary minerals (apatite) Plant residues Plant uptake Soil solution phosphorus HPO 4 -2 H 2 PO 4 -1 Secondary compounds (CaP, FeP, MnP, AlP) Mineral surfaces (clays, Fe and Al oxides, carbonates) Mineral surfaces (clays, Fe and Al oxides, carbonates) Inputs Component Outputs Atmospheric deposition PPI Precipitation Dissolution Desorption Adsorption Weathering Immobilization Mineralization

Estimates of 800 million hectares of calcareous soils worldwide Generally they are low in P due to fairly insoluble Ca-P minerals The concentration of available P continues to decrease over time Facts About Calcareous Soils

Phosphorus Sorption in Soils

0 ppm P Calcite Equilibrated for 10 Days Calcite Equilibrated for 10 Days

10 ppm P Calcite Equilibrated for 10 Days Calcite Equilibrated for 10 Days

50 ppm P Calcite Equilibrated for 10 Days Calcite Equilibrated for 10 Days

The quantity of P sorbed on calcite depends largely on its surface area Pure calcite has low surface area (1 to 2 m 2 /g) Soil calcite has surface area ranging from 16 to 200 m 2 /g (due to ongoing dissolution, reprecipitation, and incorporation of impurities) On pure calcite, sorption occurs on ~5% of the surface before adsorption clusters serve as nuclei for Ca-P precipitation

First few weeks, P initially precipitates as MCP…. Then DCPDFirst few weeks, P initially precipitates as MCP…. Then DCPD After 3 to 5 months, octacalcium phosphate precipitatesAfter 3 to 5 months, octacalcium phosphate precipitates After 8 to 10 months, tricalcium phosphate formsAfter 8 to 10 months, tricalcium phosphate forms Long periods (years) hydroxyapatite minerals form…Long periods (years) hydroxyapatite minerals form… (mineral with lowest solubility controls P concentration- while intermediates are unlikely to persist) Traditional Concepts Adding P Fertilizer to High pH/ High Calcium Soils

Incubation Period (days) Fertilizer P Availability (Fraction of Added P Still Extractable) (Sharpley et al., 1989) Phosphorus Availability Decreases Over Time Average of 44% P sorption between 30 and 180 days following application

Soil Test Calibration Studies 0 Lime 12% Lime P Rate, lbs P 2 O 5 /A UI, 2001

Soil Test Correlation Studies 0 Lime 12% Lime UI, 2001

University of Idaho: Potato P Recommendations take into account the free lime content of the soil

Leytem and Westermann, 2003 Majority of the soils had a break-point at approx. 150 ppm. low P (<150 ppm), P sorption with oxides, clay surfaces, and organically- complexed metals (Fe & Mn) high P (>150 ppm), precipitation reactions Sorbed P (mmol P kg -1 ) Equilibrium P (mg P L -1 )

Leytem and Westermann, 2003 Organically complexed Mn and Fe were the primary factors controlling P sorption

Phosphorus Sorption in Soils

How Can organically complexed metals affect P adsorption? P is complexed with OM through metal bridges? OM interferes with Ca-P and metal oxide precipitation by coating the calcite surfaces? PO 4

Robbins and Westermann previously showed that organic carbon enhanced the solubility and extractability of added P in calcareous soils- showing that there are many mechanisms regulating P sorption and solubility

Organic ligands affect P sorption- they compete with orthoP for similar sites on the surface of oxides. Organic matter may also chelate metals and prevent reactions between metals and P How does this relate to Humic Acid additions? Far West Fertilizer Meeting, 2004: “Never add P fertilizer to a Calcareous soil without adding Humic Acid with it” Organic Matter and P Interactions

University of Idaho experience with Humic Acid (2003 report)

If Organically Complexed Metals Control P Solubility, What Happens When We Add Manure?

Myths About P and Manures Manure P is organic PManure P is organic P Organic P is more soluble and more readily leached than inorganic POrganic P is more soluble and more readily leached than inorganic P Manure is a greater threat to water quality than fertilizer P because it is organic PManure is a greater threat to water quality than fertilizer P because it is organic P

HO P OH O OH HO P O O OH P OH O OH Organic P Inorganic P Phytic Acid ATP (monoester) Inorganic Orthophosphate Pyrophosphate OPO 3 H 2 H 2 O 3 PO OPO 3 H 2 H 2 O 3 PO OPO 3 H 2 Some Typical Organic P Compounds in Manures

P Sorption in Soils

HO P OH O OH HO P O O OH P OH O OH Organic P Inorganic P Phytic Acid ATP (monoester) Inorganic Orthophosphate Pyrophosphate OPO 3 H 2 H 2 O 3 PO OPO 3 H 2 H 2 O 3 PO OPO 3 H 2 Some Typical Organic P Compounds in Manures

Manure P Composition

How does manure behave as a fertilizer compared to inorganic fertilizer?

(Leytem & Westermann, 2005) Soil Test P Response in STP and Plant Growth

Plant P Uptake Plant P Uptake Although the soil test P increased less when we applied manures…. The amount of P taken up by the plants was greater for some manures than fertilizers

P Source RPEOlsenRPUptakeEfficiency Dairy Liquid Beef Solid Dairy Compost MCP MAP Phosphorus Uptake Efficiency Phosphorus Uptake Efficiency

Do these trends change in the field?

Olsen P with Depth in Field Plots a a b c a b b c a b ab Fertilizer and manure behave similarly in the topsoil, but fertilizer P enriched STP in the subsurface compared to manure and compost

Runoff P from Field Plots a c bc ab Fertilizer and manure had similar runoff P while compost and control were lower but similar Cumulative runoff P was closely correlated with the bicarbonate STP

P Uptake from Field Plots a b aa The addition of fertilizer, manure and compost increased the P uptake by the crop, but they did not differ significantly STP over 20 ppm does not require fertilizer addition

Is it really all just about the P in manures or is there something else going on?

13 C NMR of WS-Carbon Dairy Compost Dairy Manure Dairy Liquid Solid manures dominated by aromatic and aliphatic compounds (larger more stable) Liquid manures dominated by carbonyl functional groups (smaller more reactive)

The Carbon in Manures Influences the P solubility of the Soil

Microbial P mg kg -1 Olsen P mg kg -1 WSP mg kg -1 Effect of Microbial P on WSP and Olsen P Effect of Microbial P on WSP and Olsen P

P Release Over Growing Season From Manure, Compost and Fertilizer P Release Over Growing Season From Manure, Compost and Fertilizer C:P = 15 C:P = 45 C:P = 0

Summary Summary Manure with similar P composition applied at same total P rates Manure with similar P composition applied at same total P rates Differences in P solubility dominated by the amount of C added with the manure treatment Differences in P solubility dominated by the amount of C added with the manure treatment Manures increase STP less than fertilizer treatments but have a greater plant P uptake than fertilizer Manures increase STP less than fertilizer treatments but have a greater plant P uptake than fertilizer

Implications Implications We need to consider the amount of C in calcareous soils when determining P sorption/availability We need to consider the amount of C in calcareous soils when determining P sorption/availability When we make P risk assessments in calcareous soils with manure incorporation, we need to consider the effects of the C content and composition of manures When we make P risk assessments in calcareous soils with manure incorporation, we need to consider the effects of the C content and composition of manures Microbial P has a large influence on soluble P and this needs to be further explored Microbial P has a large influence on soluble P and this needs to be further explored

So Why the P Problems with Manures? N:P ratios don’t match up N:P ratios don’t match up Crops want a ratio of about 5:1 Crops want a ratio of about 5:1 Manure ratios are typically closer to 2:1 Manure ratios are typically closer to 2:1 Therefore when applying manure on an N based rate we are always over applying P Therefore when applying manure on an N based rate we are always over applying P Overfeeding of P in animal operations Overfeeding of P in animal operations Low P availability of feeds in monogastrics leads to P supplementation Low P availability of feeds in monogastrics leads to P supplementation Dairies traditionally overfeed P for insurance Dairies traditionally overfeed P for insurance Cost of transporting manures Cost of transporting manures Over application of manures on sites closest to manure generation/storage Over application of manures on sites closest to manure generation/storage

Northwest Irrigation and Soils Research Laboratory Kimberly, Idaho USA