Changes of Soil Phosphorous Fractions under Long‐Term Corn Monoculture

Abstract Corn ( Zea mays L.) monoculture is a common management practice due to high production levels with economic and agronomic advantages. Phosphorus reactions in soil affect sustainable, economic corn management and environmental quality. Our objectives were to quantify changes in soil P fractions as related to additions of manure and inorganic P with time using a modified Hedley fractionation scheme. A field experiment was conducted on a Chicot sandy clay loam soil (fine‐loamy, mixed, frigid Typic Hapludalf) from 1984 to 1993. With an annual rate of 60 kg P ha −1 manure P plus 44 kg P ha −1 inorganic P, labile inorganic P (Pi; Bicarb‐Pi) increased and moderately labile Pi (NaOH‐Pi) decreased. A higher annual P rate of 132 kg P ha −1 plus manure P increased soil Pi through Bicarb‐Pi, NaOH‐Pi, and stable Ca‐bound Pi (HCl‐Pi). Inorganic P at either rate combined with manure P decreased labile Bicarb‐Po fractions and increased moderately labile NaOH‐Po, with the result that total extractable soil organic P (Po) increased. Without manure, 44 kg P ha −1 , approximately equal to P removal by the crop, resulted in no change in soil P fractions except for residual P (Res‐P), which decreased. Soil Po and Pi levels increased continuously in soil fertilized with 132 kg P ha −1 yr −1 inorganic P only. With the manure‐inorganic system, 86% of the Bicarb‐Pi was supplied from added inorganic P, indicating reduced P fixation compared with the inorganic P system. The NaOH‐Pi and Res‐P were soil P sinks, with Res‐P being supplied from both the NaOH‐Po and HCl‐Pi fractions. In contrast, NaOH‐Pi in the inorganic P system was the major source for Bicarb‐Pi.