EFFECT OF MANURE ON SOLUBILITY OF PHOSPHORUS IN CALCAREOUS SOILS

Abstract

We studied the effects of farmyard manure, with and without concentrated superphosphate on the solubility of phosphorus in three calcareous soils. Concentrated superphosphate, at rates of 0, 10, 20, 40, and 60 parts per million of phosphorus, and air-dried manure, at rates of 0,11, 22, and 44 metric tons per hectare, were mixed with the soils, and the soils were wetted and dried four times in the greenhouse. Adding manure and phosphorus together (mixed treatment) to Pierre and Apishapa soils increased soluble phosphorus in 0.01 M CaCl2 more than each treatment when added separately (unmixed treatment). However, adding farmyard manure and concentrated superphosphate together to the Tripp soil, at the higher rates of fertilizer, did not increase the soluble phosphorus in CaCl2 as much as each treatment separately. However, the mixed treatments had higher HCO3-extractable phosphorus for all three soils than the sum of separate effects from the unmixed treatments. Adding 44-metric tons of farmyard manure per hectare to Pierre, Apishapa, and Tripp soils increased soluble phosphorus in NaHCO3 equivalent to 27, 21, and 21 parts per million of phosphorus as concentrated superphosphate, respectively. Solubility data suggested that octocalcium phosphate could be present in the Tripp soil, since most of the experimental points for mixed treatments plotted near the stability isotherm of this phosphate mineral.