Agricultural researchers in Alabama are studying the effect of phosphorus stratification on cotton yield.

Rishi Prasad, a nutrient management specialist with the Alabama Cooperative Extension System, is leading a research project that involves injecting P fertilizers six inches below the surface and comparing yields between broadcast, surface applications of dry P fertilizers and the injection of liquid fertilizer.

The appeal of injection of P is that it involves a single, high-rate application six inches below the soil surface with the possibility of replacing P fertilization for three to four years.

Phosphorus is the second most essential nutrient after nitrogen and is needed by any crop to complete its lifecycle and achieve significant yield. Plants rarely show visible deficiency symptoms unless P is completely unavailable, symptoms which result in purple leaves. An undersupply of P can reduce yields.

Phosphorus is commonly applied through surface applications of commercial fertilizers as DAP (diammonium phosphate) or monoammonium phosphate (MAP) or manure such as poultry litter.

Phosphorus is an immobile nutrient and its movement within the soil layer is very slow, which is why P tends to accumulate in the soil surface. Soil sampling below the routine 6-inch depth in several Alabama farms has revealed the presence of P stratification, a situation in which surface soil (top 6 inches) tests “high” to “extremely high” in P but subsurface soils (below the top 6 inches) remain low to very low in P.

Phosphorus stratification is a non-uniform accumulation of P in the surface profile, with a sharp decline of P in the lower soil profile. It represents oversupply of P in surface layers and undersupply in the subsurface profile. During drought conditions, plant roots run deeper to obtain water and nutrients. An undersupply of P in bottom soil layers can cause hidden hunger in the plant and can penalize yield.