Effects of composted feedlot manure on the chemical characteristics of duplex soils in north-eastern Victoria

Abstract

The beef feedlot industry in Australia produces a large amount of solid organic by-product each year that is currently applied to agricultural land as a fertiliser supplement. Manure is known to be a valuable source of organic matter and some plant nutrients, especially nitrogen and phosphorus. In addition, manure contains excessive quantities of cations such as sodium (Na) and potassium (K), which may result in long-term sustainability problems for the soil, particularly when large amounts are applied over short time periods. The aim of this study was to determine the effects of composted beef feedlot manure when applied to agricultural soils. Two sites were selected, one a brown Dermosol and the other a red Kurosol, both in north-eastern Victoria near the Rutherglen Research Institute. Both sites received rates of manure up to 109 t/ha in 1996. In 1997 soil samples were collected and compared with untreated control soils. The Dermosol site was sown to an oat and clover mixture in 1996 and 1997 and the red Kurosol was sown to lupin in 1996 and wheat in 1997. The application of composted bovine manure resulted in a 1% increase in soil organic carbon, an increase in soil pH by 1.5 units, increased levels of magnesium, calcium, nitrogen and K in the surface 10 cm soil layer at both sites and an increase in extractable phosphorus levels in the subsoil. There was no detectable increase in surface Na, although there was a small but significant decrease in Na in the 40–80 cm soil layer. It is suggested that soluble organic compounds, migrating down through the soil profile are able to complex with Na and effectively remove some of this cation from the exchange sites of the clay surfaces. In addition, the high porosity of these soils coupled with the high degree of Na mobility ensures that most of this cation is transported deeper into the soil profile. The beneficial effects of applying composted manure are promising as a means of reducing sodicity although these results will require further validation. In addition, the long-term effects of saturating subsurface soil with Na are also a cause for concern and need to be further investigated.