Phosphorus Dynamics in Amended Soils during the Growing Season: II. Ligand Exchange and Mineralization.

A field study was conducted near Bushland TX to evaluate changes in phosphorus pools in soils amended with cattle manure and monoammonium phosphate (MAP) throughout a single growing season. Unfertilized checks were included for P extractability comparisons. Grain sorghum (Sorghum bicolor (L.) Moench) was planted after fertilizer applications and received supplemental irrigation (180 mm). Soil samples (0-150 mm) were collected prior to fertilizer applications and periodically throughout the growing season. Various forms of bioactive P present in soil were determined peridically during the two growing seasons of 2005 and 2006. Three P fractions were differentiated, i.e., manure water-extractable phosphate-P (WEP), ligand-exchangeable inorganic phosphate-P (EEPi), and the all-inclusive total bioactive P (WEP + EEPi + EDTA-PHP) to evaluate the in-season changes in these pools for commercial fertilizer and cattle manure applications. There were distinct seasonal fluctuations in soil concentrations of EEPi and total bioactive P, which all peaked during the warmest month of the season. Current P fertilization practices may need to be adjusted for mitigating potential environmental impact of a temporaryly elevated level of soluble P; such a buildup was predicted by in-season mineralization of organic P of soil or manure or other forms of organic amendement.