The effect of green manuring and rock phosphate application on soil phosphorus in a North Carolina ultisol

The objective of this research was to test a combination of fertilizer and management technologies on soil phosphorus (P) and maize yield on a site with Cecil sandy loam (clayey, kaolinitic, thermic, Typic Kanhapludult) soils with low soil P in North Carolina. Fertilization was with a sedimentary North Carolina rock phosphate (rock P). Management technologies were (1) green manuring with summer grown common buckwheat (Fagopyrum esculentum) and a winter crimson clover (Trifolium incarnatum) cover crop, (2) green manuring with the natural vegetation (3) leaving the natural vegetation undisturbed.;The addition of rock P significantly increasedthe yield of maize grain from 1573 kg grain/ha to 2143 kg grain/ha. Grain yield was not affected by management, or the interaction between fertilization and management. Rock P increased the concentration of soil P extracted using a modified Hedley P fractionation procedure, in all fractions except the residual fraction. Green manuring reduced P availability in unfertilized treatments by increasing the mineralization of the readily mineralizable organic compounds associated with the Hedley HCO3 organic fraction. In both fertilized and unfertilized treatments green manuring reduced the amount of moderately labile organic P associated with the Hedley OH organic fraction. Total P decreased from 220.5 to 179.0 mg P/kg soil under buckwheat and crimson clover, green manure management, from 220.9 to 183.0 mg P/kg soil under natural vegetation, green manure management and from 216.6 to 197.5 mg P/kg soil under the natural vegetation, undisturbed management during the 22 month research period. This decrease was due to a reduction in the size of the OH organic fraction. It is hypothesized that the reduction in the size of the OH organic fraction was caused by movement of organo-metal P complexes associated with the OH organic fraction to the more acidic lower horizons.