History: Cultivated pastures increased the production potential of dairy operations in the coastal belt of the southern Cape. From the 1960s until the late 1980s, the cultivated pastures were conventionally tilled with various implements, which caused severe soil disturbance and inversion. During this period, fertiliser guidelines were developed according to the needs of the cultivated pasture crops within complex fodder flow systems comprising oats and annual ryegrass as winter pasture, forage sorghum hybrids, and hybrid millets as annual summer pasture. The perennial pastures were based on rain-fed lucerne with supplementary irrigation or mixtures of perennial ryegrass, tall fescue, cocksfoot, white clover and red clover under permanent irrigation systems. Over time, soil compaction became a problem and soil structure was broken down due to loss of soil organic matter. Wind and water erosion led to a loss of topsoil and the production potential was decreased. As a result, soil degradation caused these systems to become unsustainable. In an attempt to protect soil from further degradation, irrigated pastures were converted to minimum-tillage regimes with kikuyu as the pasture-base during the 1990s.
Essentials of the minimum-tillage pasture management are as follows: During autumn the kikuyu growth rate and herbage quality decrease and the ground-cover consists of stolons and rhizomes, which are mulched to prevent the kikuyu from building up an unpalatable matt of poor-quality forage. The mulched material is used as a seedbed into which ryegrass species are sown by using minimum-tillage seed drills. The objective of the use of temperate and high-quality ryegrass species is to reinforce the kikuyu during the colder seasons when kikuyu is dormant, and thereby to secure a steady fodder flow for the cow.
The results thus far showed that conversion to minimum-tillage systems prevented wind and water erosion, supported aggregate stability and restored good soil structure with high plant productivities. This promoted increased soil biological quality by enhancing soil organic matter components, nitrogen, chemical qualities and microbial functionality.
Study: A potential problem noticed by the authors cited below was that in the minimum-tillage systems the same lime and fertiliser guidelines that were developed for conventional tillage systems are being followed. They suspect that neither guideline may be agronomically or environmentally sound for minimum-tillage. The aim of their study, therefore, was to survey the chemical status of the irrigated minimum-tillage kikuyu–ryegrass pasture soils in the total area of the southern Cape where dairy production is practiced.
The results showed that the soil fertility status of these pastures was affected, in many instances severely, one reason possibly being irresponsible sales-driven fertiliser advice to farmers, but otherwise the continued adherence to the lime and fertiliser guidelines that were originally developed for conventionally tilled pasture systems. Effects of lime application were noticeable in the higher levels of exchangeable calcium and magnesium. The soil pH was therefore higher than in virgin soil. The total nitrogen increased in association with organic carbon and organic matter accumulated in the surface soil layers. Exchangeable potassium and sodium and extractable copper and manganese showed higher levels in the cultivated pasture soil than in virgin soil. The higher levels of these minerals are satisfactory from an agricultural and environmental viewpoint. Boron was not affected by the management practices.
The concern expressed was that extractable phosphorous and zinc were drastically increased. This could cause deleterious effects on ecosystem health and sustainability of pasture production. Thus far, no attempt has been made to prevent or mitigate the loading of these minerals in the topsoil of the area. The authors therefore stressed the need for remedial and preventative strategies to lower phosphorous and zinc build-up to toxic levels , with a concomitant monitoring of nutrient loading with time, and a nutrient budgeting system at farm level. In addition, fertiliser guidelines should be applied strictly and should fit the farming system.
PA Swanepoel, C C du Preez, P R Botha & H A Snyman, 2015. A critical view on the soil fertility status of minimum-till kikuyu–ryegrass pastures in South Africa. African Journal of Range & Forage Science 32, 113–124.