Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/29/2011
Publication Date: 11/1/2011
Citation: Schwartz, R.C., Dao, T.H., Bell, J.M. 2011. Manure and mineral fertilizer effects on seasonal dynamics of bioactive soil phosphorus fractions. Agronomy Journal. 103(6):1724-1733. Interpretive Summary: Applications of animal manure and fertilizer can elevate soil phosphorus (P) levels and can contribute to contamination of surface water. A two-year field study evaluated changes in soil P levels following applications of cattle manure and synthetic fertilizer. Fertilizers were applied prior to planting grain sorghum. Soil samples were collected prior to fertilizer applications and throughout the growing season and analyzed for Mehlich soil test P, water-soluble P, and several other assays to evaluate the availability of P. Distinct seasonal fluctuations of phosphorus levels were observed in manure and fertilizer amended plots during both years of this study. In contrast, fluctuations in extractable soil P in unamended plots were absent with the exception of one P assay. Phosphorus levels in water extracts depended on both pH and calcium levels. This result suggests that precipitation and dissolution of calcium phosphates in the soil contributed to observed seasonal fluctuations of phosphorus. Current soil sampling and fertilization practices may need to be adjusted to properly manage soils with elevated levels of P.
Technical Abstract: Seasonal fluctuations in bioavailable soil phosphorus can influence soil test results and associated assessment of off-site transport risk. Our objective was to evaluate changes in soil P speciation and availability with time following applications of grain fed cattle (Bos taurus) manure or monoammonium phosphate (MAP). Beef cattle manure or MAP was applied at a targeted rate of 200 kg P ha**-1 9 on a Pullman clay loam in 2005 and 2006 and planted to grain sorghum (Sorghum bicolor (L.) Moench). Soil samples (0-0.15 m) were collected prior to and throughout the growing season and analyzed for Mehlich-3 P (Me3P), 1:10 water extractable P (WEP10), water extractable cations, pH, and fractions of bioactive soil P(TBIOP), which comprised 1:100 water extractable P, EDTA exchangeable inorganic phosphate (EEPi) and the EDTA-exchangeable phosphohydrolase-labile P (EPHP). Levels of soil Me3P, WEP10, and allfractions of TBIOP in MAP-amended plots fluctuated significantly (p less than 0.05) during both seasons. Except for Me3P, manure amended plots also exhibited significant (p less than 0.05) seasonal variations in soil extractable P and a delayed release of P that extended well into the growing season. In contrast, fluctuations in extractable soil P in unamended plots were not significant except EPHP. In water extracts, a significant (p less than 0.05) dependence of solution P on pH and calcium suggested that precipitation-dissolution reactions contributed to observed seasonal fluctuations in P. Fluctuations in total bioactive soil P were two to four times greater than above ground biomass P highlighting the importance of accounting for seasonal dynamics in assessing offsite P transport risks.