Soil phosphorus dynamics in response to dairy manure and inorganic fertilizer applications

In modem agriculture practices, the application of plant nutrients to soil is usually required for sustained high crop yields. Manure is generally applied to meet crop N needs. The impact of N-based manure application rates on soil P transformations should therefore be evaluated for efficient utilization of both manure N and P. In this study, 11 dairy manure and inorganic fertilizers were added to two Maine soils to supply 100 mg organic N kg(-1) soil. Our data showed that the general distribution patterns in manure-amended soils for three P species, inorganic P (Pi), enzymatically hydrolyzable organic P (Peo), and nonhydrolyzable organic P, basically followed the trend of changes for inorganic fertilizer amended (control) soils, indicating that soil properties played a major role in controlling P dynamics. In H2O and NaHCO3 fractions, soil bioAvailable P (Pi and P-eo) was linearly related to added P. However, NaOH-extractable soil P was not closely related to the P applied in the manure. This work confirmed that a single application of manure impacted soil bioavailable P in a manner similar to fertilizer and did not significantly impact soil P properties and environment. This means that growers converting from conventional to organic practices (i.e., where manure is the sole source of N and P) may estimate manure bioavailable P to meet their short-term P needs. Further studies need to be conducted to determine if similar results can be found under field conditions.