An investigation of factors contributing to soil degradation under dairy farming in the Tsitsikamma.

Abstract

Pasture-based dairy farming is the major land use in the Tsitsikamma region of the Eastern Cape. Permanent kikuyu grass (Pennisetum clandestinum) dominates pastures in the region. Kikuyu pastures do not, however, provide adequate year-round quality feed for dairy cows. This has led to the use of annually sown pastures with perennial ryegrass (Lolium perenne) to provide winter forage. Soil degradation under this management has, however, become recognised as a major limitation. Soil quality and degradation under annual and permanent pasture in the region were evaluated in three separate studies. These were (i) an investigation of the extent of loss of soil organic matter and related soil microbial properties and aggregate stability under annual pastures, (ii) a comparison of soil physical properties under annual and permanent pastures and (iii) a survey of the nutrient status of soils and pasture herbage in the region. In the first study, four commercial dairy farms, situated on sites which represented the three main soil groups in the region were sampled, were taken from under permanent kikuyu pastures, annual ryegrass pastures and undisturbed native vegetation nearby. In comparison with undisturbed, native vegetation, soils under both annually cultivated and permanent pasture had gained soil organic matter on the sandy, low rainfall eastern end of the Tsitsikamma. By contrast, at the higher rainfall, finer-textured, western end, where the native vegetation consists of coastal forest, there was a loss of soil organic matter under both types of pasture. Despite this, soil organic C content was lower under annual ryegrass than permanent kikuyu pasture at all the sites reflecting the degrading effect of annual cultivation on soil organic matter. As a consequence, labile, K(2)S0(4) - extractable C, microbial biomass C, basal respiration, arginine ammonification, flourescein diacetate hydrolysis rates and aggregate stability were all less under annual ryegrass than permanent kikuyu pastures at all the sites. The effects of annual ryegrass and permanent kikuyu pastures on soil physical properties and root length density were compared with those of undisturbed native vegetation on the four experimental sites. Root density and the depth of rooting were much less under annual ryegrass than under kikuyu pastures or native vegetation. There was no consistent effect of improved pastures or pasture type on bulk density and total porosity or penetrometer resistance, although annual pasture soils generally had higher bulk densities and lower total porosities than those under native vegetation. There was a tendency for smaller saturated hydraulic conductivity and air permeability under ryegrass than kikuyu pastures, regardless of whether total porosity was higher or lower under ryegrass. This was attributed to annual cultivation and subsequent natural consolidation causing a decrease in pore continuity under ryegrass pastures. Penetrometer resistance values confirmed the presence of subsoil compacted layers at two annual ryegrass pasture sites. At one such site, subsoil tillage was effective in reducing penetrometer resistance and bulk density, increasing pore continuity (as evaluated by hydraulic conductivity and air permeability) and greatly increasing root density and rooting depth. The nutrient status of soil and herbage from annual ryegrass and permanent kikuyu pastures sampled from 40 dairy farms in the Tsitsikamma region were evaluated. Along with the decreased organic matter content, there was a decrease in soil pH and a loss of exchangeable cations under annual pastures. Large concentrations of extractable P and sometimes exchangeable K were measured in soils under both ryegrass and kikuyu pastures and it was concluded that the rates of applied P, and sometimes K, were often excessive (particularly under kikuyu). Various nutritional problems were also identified. These included the need for Ca supplementation, particularly under kikuyu, due to the low herbage Ca concentrations. The low Ca : P ratio measured in annual ryegrass pastures, and more particularly in kikuyu herbage, highlighted the low Ca content of herbage and also the tendency of kikuyu grass to accumulate large concentrations of P. The large K concentrations and high K : Ca +Mg ratios identified in pasture herbage suggest the potential for animal nutritional problems such as hypomagnesaemia. It was concluded that although kikuyu is an excellent pasture in terms of dry matter production it tends to be deficient in Ca (and sometimes Na) and can contain prohibitively high K levels, which are likely to induce Mg deficiencies in grazing animals. The micronutrient concentrations in herbage were generally adequate, although copper concentrations tended to be low suggesting that fertilizer applications and/or feed supplementation is required. It was concluded that annual conventional tillage results in a substantial loss of soil organic matter, soil microbial activity and aggregate stability under annual ryegrass pastures when compared to those under permanent kikuyu grass. This loss of soil organic matter can result in natural consolidation of the soil in the cultivated layer and exasperated through treading by the grazing cows. The annual cultivation can also lead to the formation of a subsoil compacted layer. Nonetheless, compaction can also occur under permanent pasture presumably due to treading damage. Careful management to avoid treading damage to pastures should be practised. In order to protect the organic matter status of annual pastures, direct drilling of such pastures should be seriously considered. In some cases, annual fertilizer P rates (and to lesser extent those of K) could be reduced considerably since the levels accumulated in the soils are excessive.