Phosphorus availability and speciation in long-term no-till and disk-till soil

Conservation tillage results in the concentration of plant-available P near the soil surface. We studied the effects of conservation tillage on P speciation by examining the distribution of P in inorganic and organic chemical pools. Depth-incremented soil samples were collected from long-term (9- and 10-yr) no-tilt (NT) and disk tillage (DT) systems cropped in corn (Zea mays L.) with a wheat (Triticum aestivum L.) cover crop. Rates of P were 0, 20, and 60 kg P ha(-1) yr(-1). Total P (P-T), organic P (P-o), and available P (Mehlich-3, M3-P; Olsen NaHCO3-pH 8.5, Olsen-P) were determined. P was also extracted from the following chemical pools: non-occluded Al-bound (AI-P), non-occluded Fe-bound (Fe-P), occluded-reductant-soluble (CBD-P), and Ca-bound (Ca-P). Total P did not vary with depth, but was greater in NT than in DT and increased with P rate. Organic P increased with P rate in the 0- to 8-cm depth. Organic P was greater in NT plots in the 8- to 60-cm depths, averaging 75 mg kg(-1) for NT and 48 mg kg(-1) for DT plots. Mehlich 3-P and Olsen-P were greatest in the surface 4 cm and in the 60-kg P ha(-1) plots, with higher levels observed in NT plots. On average, the forms of P (as a % of total P) in NT soil was 6.2% Al-P, 33.9% Fe-P, 33.9% CBD-P, and 4.7% Ca-P. Average P distribution in DT soils was 5.4% AI-P, 35.6% Fe-P, 31.3% CBD-P, and 5.1% Ca-P. The influence of tillage on P distribution was primarily limited to the soil surface, with the exception of Al-P, which was greater in the 8- to 30-cm depths of the NT plots. Because the impact of tillage was limited to a thin, soil surface layer (<4 cm), soil P-test rating would not be affected by tillage practice. However, the improper collection of soil samples from NT (i.e., too shallow) for P-testing may provide erroneous P-test results and fertilizer recommendations.