Effects of animal effluents on the phosphorus sorption characteristics of soils

Abstract

Two groups of soils were examined to determine the effects of dairy, pig, or sewage effluent and other materials containing phosphorus (P) on their P sorption characteristics, using the Langmuir equation to estimate values of both sorption capacity and sorption strength. There were 19 soils (0-15 cm) from 6 sites in the Williams River catchment and 3 soils (0-100 cm) from Bermagui, all from coastal New South Wales.

Effluent usually decreased P sorption capacities of the Williams River soils, and in 3 soils the capacities were reduced to zero. Sorption strength was reduced substantially by effluent treatment in all soils except one, which had received effluent for only 3 years. Sorption strength, but not necessarily capacity, was also lower after treatment with poultry manure or chicken litter than after treatment with superphosphate only. Where effluent did not decrease sorption capacity there was a substantial increase in total carbon and iron, both of which could increase sorption capacities.

After 3 years of effluent treatment of the Bermagui soil, sorption capacities had been reduced in the top 70 cm depth, the extent of the reduction varying from 17% at 0-7 · 5 cm depth to 38% at 40-70 cm depth. Sorption strength was reduced in the top 40 cm depth only. After 12 years of effluent treatment, sorption capacities and strength had also decreased at the deeper sampling depths (to 100 cm), and the average reduction in capacity was about 40%. These results suggest that P leaching will begin well before the total sorption capacity has been saturated. There was a direct and significant correlation between the sorption strength of the untreated soil and the percentage saturation reached before leaching began. Further saturation of the sorption complex appears to be slow after this degree of saturation has been reached, and it seems that P leaching exceeds adsorption during this phase. There was also a negative correlation between sorption strength and KCl-soluble P in all soils, suggesting that soil P solubility and potential saturation are both controlled by this characteristic.