Unlike nitrogen and sulfur, phosphorus (P) does not occur as a gas or disappear into the atmosphere. However, P can become strongly adsorbed to soil particles, and some plants may have trouble accessing it.
Phosphorus is prone to loss on soil particles via erosion, and in some rare cases, via leaching. Cover crops can help prevent the loss of P by preventing erosion and leaching.
Increasing P availability by improving soil structure
One of the most significant ways cover crops can improve P availability is by improving the soil’s physical conditions. When soil is compacted, plant roots have difficulty accessing water and P, and P does not diffuse through the soil. In these situations, adequate P may be in the soil, but it is not physically accessible.
Not all cover crops can alleviate soil compaction, but Brassica cover crops have taproots that are known for “biodrilling,” creating root channels that change the air and water dynamics of the soil.
Increasing P availability biologically and chemically
Some cover crops have a strong ability to access P pools in soil that most crops cannot access. As the cover crop biomass decomposes, the P then becomes available for subsequent cash crops. This cover crop P may not be available immediately, but is mostly present in organic pools instead of in mineral form adsorbed to soil particles. Buckwheat and lupine have been reported to access P pools unavailable to other crops. Using cover crops that have greater abilities to scavenge/access P can increase the P use efficiency of the farm and may reduce the amount of external inputs required to meet cash crops’ P demand. This will become increasingly important as global P reserves are depleted.
Buckwheat (Fagopyrum esculentum Moench) Potential to Contribute Solubilized Soil Phosphorus to Subsequent Crops by Jasper Teboh and David Franzen
Phosphorus Bioavailability following Incorporation of Green Manure Crops (2003). Michel Cavigeli and Steve Thien (USDA).
Forage radish also accumulates P and concentrates it in the soil surrounding the taproot hole:
Forage Radish Cover Crops Increase Soil Test Phosphorus Surrounding Radish Taproot Holes (2011). Charles White and Ray Weil.
One of the most important symbiotic relationships in soil, the mycorrhizal relationship between fungi and plants, can greatly affect P availability. This relationship has its own page.