Transformation of Phosphorus in Agricultural Soils.

Commercial phosphorus (P) fertilizers are applied to agricultural crops in order to increase crop yields.  Unlike other nutrients, crops cannot take up all applied or soil P.  As a result, residual P accumulates in soil over time and may remain as recalcitrant P phase for plant uptake.  To understand the bioavailability and fractionation of P in different soil phases, we performed sequential extraction of P phases in soil samples collected during three growth stages of a corn crop. Our results showed that the residual P concentrated in top 35 cm depth in clay- and organic-rich soil layer. NaHCO₃-P and NaOH-P were dominant P phases in the soil due to the continuous application of fertilizer over years. An increase in NaOH-P and HCl-P over time both in P fertilizer and manure amended soils suggested an active transformation of originally bioavailable P into moderately or non-available P phases. Transformation of P was found to be active in a single crop cycle. This transformation was in significant in control site (received no P fertilizer or manure) where NaHCO₃-P decreased significantly due to continuous removal of P by plant. It is less likely that the decrease in soil pH (by ~0.5 and 1.0 unit in fertilizer and manure amended soils, respectively) alone could have promoted this change in P phases. Further research is needed to identify enrichment of particular P phases and factors that influence P transformation in soil.