The dissolution in 2 acid soils (Weston [Florida, USA], pH 4.5; Davidson [Alabama, USA], pH 5.2) of P from North Carolina [USA] P rock, a highly reactive rock, was decreased by additions of (NH4)2SO4 or KCl or their mixtures, but increased by the addition of urea. The effect of urea was decreased by addition of KCl. Urea hydrolyzes the organic matter of the soil and the products of hydrolysis chelate Ca ions and so increase the dissolution of P from the rock. (NH4)2SO4 and KCl increase the P-sorption capacity of the soil and so decrease the water-soluble P in the soil. The beneficial effects of N and K fertilizer salts on the uptake of P by plants may result from stimulation of plant growth and not from solubilization of P rock.