Long-term effects of grazing on the fertility of soils

Abstract

The long-term effect of excreta return was studied on a bull-beef system that had been under stable management for 23 years. Excreta return did not greatly affect organic matter properties. Mineralisable nitrogen showed differences with depth where excreta had or had not been returned, but was not different in the 0-75 mm soil depth. Excreta return positively affected Olsen P, sulphate-sulphur and organic-sulphur soil test values. Single-core sampling studies showed that very high Olsen-P values occur in some cores and can weight the paddock mean upward, resulting in the phosphorus status being overestimated for much of the area. Excreta return strongly influenced potassium cycling, by maintaining a large pool of available potassium. A relatively small amount was lost to drainage water, however, as the cycling potassium displaced calcium and magnesium. Magnesium loss from the system was large compared with the quantity cycling, and appears to be one of the major long-term negative effects of grazing on nutrient reserves. Keywords: excreta return, grazing, soil fertility, soil nutrient reserves