The effect of rock phosphate properties on the extent of fertilizer dissolution in soils

Abstract

Two soils, a West Australian Petroferric Udipsamment (WA4) and a Colombian Typic Haplustox (SA21) were incubated for up to 31 days with 23 phosphate fertilizers. These comprised 18 apatitic rock phosphates, three calcined rock phosphates, monocalcium phosphate and dicalcium phosphate. The fertilizers were mixed through the soils at rates of addition between 0.37 and 37 mg g-' soil. The extent and rate of dissolution of fertilizers were measured by determining changes in Ca using the ACa technique. Most dissolution generally occurred within one day for all fertilizers. The proportion of fertilizer that dissolved decreased with increasing rate of addition, and this proportion differed greatly between fertilizers (1-43% in WA4, 2-72% in SA21). All fertilizers dissolved to a greater extent in the Colombian soil. Calcined (1500°C) Queensland rock phosphate was the most soluble of the rock phosphates, which is consistent with its high availability to plants. The calcined iron-aluminium rock phosphates (Calciphos, Phospal) dissolved to a smaller extent than most of the apatitic rocks. The CaO/P2O5 ratio of the apatitic rock phosphate fertilizers was a slightly better predictor of the extent of dissolution than the apatite unit cell a dimension, although neither explained more than 60% of the variance. The present results are consistent with the accepted reactivity scale for the apatitic fertilizers, but the amount of each fertilizer which dissolved depended greatly on the soil to which it was applied.