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HISTORY OF MINIMUM TILLAGE AND SOIL IMPROVEMENT IN KIKUYU-RYEGRASS PASTURES AND THE CONSEQUENCES OF POOR ADVICE.

Discipline: grazing/pastures; Key words: cultivated pastures, fertilisation, nutrient cycling, phosphorus, soil fertility

Virgin rangelands in the southern Cape region have been converted to cultivated pastures since the sixties because of the inherently low dairy production potential. Initially, the cultivated pastures were conventionally tilled with various implements, which caused severe soil disturbance and inversion. Fertiliser guidelines were developed according to the needs of the 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 problematic, because the dominant soil forms are well-sorted sands characterised by very high fine-sand fractions (>90%). Soil structure was broken down due to a loss of soil organic matter. Wind and water erosion led to a loss of topsoil and, consequently, the system production potential was decreased.  

In an attempt to protect the soil from further degradation, irrigated pastures were converted to minimum-tillage regimes with kikuyu as the pasture base since the nineties. During autumn (March to May) the kikuyu growth rate and herbage quality decrease and the ground-cover consists of stolons and rhizomes, which are mulched to prevent accumulation of an unpalatable matt of poor-quality material. The mulched material is used as a seedbed into which temperate and high-quality ryegrass species are sown by using minimum-tillage seed drills. The ryegrass species is intended to reinforce the kikuyu during the colder seasons when kikuyu is dormant, and thereby to secure a steady fodder flow for dairy production. Conversion to minimum-tillage systems prevents wind and water erosion, supports aggregate stability and restores good soil structure with high plant productivity. This has led to increased soil biological quality by enhancing soil organic matter components, mineralisable nitrogen (N) and microbial functionality. The biological properties are highly stratified, decreasing sharply from the surface layer to a depth of 300 mm, which usually contrasts with that of conventional tilled systems. It is therefore evident that the physical and biological properties of the soils in the region changed, and since soil chemical processes are closely related to physical and biological processes, the chemical quality of the soils in the region are also likely to be altered. It therefore seems inappropriate that following changeover to minimum-tillage systems, the same lime and fertiliser guidelines of conventional tillage systems were still followed. The impact of the cultivated pastures on soil quality and fertility in comparison to virgin soils, in addition to the continuing lime and fertilizer recommendation of previous cultivation practices were the basis of the study by the authors referenced below.

The results show that the effects of lime application were visible through the higher levels of exchangeable Ca and Mg. Cultivated pasture soil pH was higher than in virgin soil. Total N increased in association with organic C, and CEC was improved in cultivated pasture soil as a result of organic matter build-up in the surface soil layers. Exchangeable K and Na and extractable Cu and Mn registered higher levels in the cultivated pasture soil than in virgin soil. The higher levels of exchangeable K and Na, and extractable Cu and Mn were viewed as satisfactory from an agricultural and environmental viewpoint. Boron was not affected by the management practices. Extractable P and Zn were drastically increased and could cause deleterious effects on ecosystem health and sustainability of pasture production. As yet, no attempt has been made in the southern Cape region to prevent or mitigate the loading of soil with P and Zn in topsoil. 

This study stresses the necessity for remedial and preventative strategies to lower P and Zn build-up to toxic levels in cultivated pasture soil, with a concomitant monitoring of nutrient loading through time, and a nutrient budgeting system at farm level. Fertiliser guidelines should be adapted to those recommended for the minimum tillage kikuyu-ryegrass practices and applied strictly. In doing so, the dual goals of economic and ecological sustainability should be achieved. 

Reference:

P. A. Swanepoel, C. C. du Preez, P. R. Botha and 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 2015, 32(2): 113–124.