Influence of Tillage Practices on Phosphorus Distribution and Forms in Three Ultisols

Abstract

Abstract Many studies have indicated that organic matter levels in soils tend to rise when no‐till (NT) management replaces plowing. It was hypothesized that organic P may thus play a more important role in the plant availability of P in NT soils. A laboratory study was conducted to investigate the organic and inorganic forms of P and their cycling in NT and conventionally tilled (CT) silt loams. Soils from three long‐term field experiments were sampled in the fall of 1984, obtaining core segments from 0 to 2, 2 to 4, 4 to 8, 8 to 12, 16 to 20, and 24 to 28 cm. Soil analyses indicated significant differences in the distribution with depth of macro‐nutrients between tillage systems. Total P, dilute acid‐extractable P (DA‐P), total N, and organic matter were more concentrated in the 0‐ to 2‐cm layer under NT compared to CT management. Organic P did not accumulate to the same extent as did organic N and C, resulting in a lower organic P/organic C ratio under NT than under CT management. With increased rates of P fertilization on one soil, total P and DA‐P increased dramatically in the upper soil layers, but a similar increase and stratification of organic P was not observed. Samples taken in the spring of 1985 indicated that, despite significantly higher microbial respiration in NT cores, there was no consistent change in the solubility of P over a 75‐d incubation period. Thus, it was concluded that the incubation procedure inadequately measured P mineralization in these soils. However, water‐soluble P was significantly greater in the upper 5 cm under NT, even where no P fertilizer had been applied. This may have been a result of interference with P‐adsorption reactions by accumulated organic matter. This study provided no evidence that organic P is of more importance to plant P availability under NT.