The effect of phosphate buffering capacity and other soil properties on North Carolina phosphate rock dissolution, availability of dissolved phosphorus and relative agronomic effectiveness

The dissolution of North Carolina phosphate rock (NCPR) in soil was investigated in a laboratory study using surface soils sampled from 28 permanent pasture sites. The relationships between phosphorus (P) dissolved, P availability and various soil properties were investigated using simple and multiple linear regression and the findings related to the relative effectiveness of NCPR for pasture production at the sites. The extent of dissolution of NCPR was positively correlated to P buffering capacity (r(2) = 0.42). Phosphorus buffering capacity and titratable acidity together accounted for 72% of the variance in dissolution. Bicarbonate-extractable P ('available' P) generally increased as dissolution increased. However, the increase in available P was consistently lower for soils with higher P buffering capacity. The proportion of dissolved P that was available also decreased with increasing P buffering capacity (r(2) = 0.63). Consequently, the increase in available P was highest for soils with very low to low P buffering capacity. This suggests that the effectiveness of NCPR as a fertiliser may be more closely related to the availability of dissolved P, than to the amount of NCPR dissolved in a soil. Consistent with this laboratory finding, the agronomic effectiveness of NCPR relative to superphosphate measured in the field tended to decrease with increasing P buffering capacity. The agronomic effectiveness of NCPR was comparable with superphosphate only at certain sites, and with some noted exceptions, most of these had surface soils with very low to low P buffering capacity. The high relative effectiveness of NCPR at these sites was mostly attributed to the loss of superphosphate by leaching. Since NCPR dissolves much more slowly than superphosphate, only a small amount of the P applied as NCPR would be lost during leaching events. Slow dissolution of the remaining NCPR probably supplied a small amount of dissolved P over an extended period of time, and due to the low P buffering capacity, much of this was available to plants.