SOIL PREPARATION

8.2 Organic Material An old practice of incorporating organic material when preparing soils has not been adequately researched in South Africa. This concept was, however, tested in two divergent viticulture regions of South Africa namely Stellenbosch and Robertson (Saayman & Van Huyssteen, 1980). The Robertson investigation was conducted on Chenin blanc/101-14 Mgt planted on a Hutton/Sterkspruit soil. Two tillage depths of 75 cm and 120 cm were combined with different levels of compost and fresh straw application. The organic material was placed in the open furrow left by the delve plough after each traverse. At Robertson, no benefit was obtained from applying either straw (maximum 28 t/ha), compost (maximum 28 t/ha) or a combination of the two (38 t/ha). In Stellenbosch, similar research was carried out on a Colombar/143B on a Hutton/Clovelly soil (Saayman & Van Huyssteen, 1982). The organic material applications were lower than at Robertson and amounted to a maximum of 16.6 t of straw/ha, 18 t compost/ha and 23 t of organic material mix/ha (12 t compost and 11 t straw). None of the organic material treatments had any effect on shoot mass, but an application of 18 t compost/ha increased cumulative grape yield significantly. This positive effect could, however, not be repeated in a second experiment that ran concurrently on the same soil, despite an increase in the quantities of organic material used. Based on their Robertson results and the inconsistent results obtained in Stellenbosch, the authors (Saayman & Van Huyssteen, 1982) came to the conclusion that application of organic material during soil preparation is of doubtful value, both from a technical and an economic point of view. It may be time, however, to revisit this matter in field experiments using different organic material sources, larger volumes and better techniques of incorporating the organic matter into the soil. In the Stellenbosch experiments, profile wall root studies showed a “sandwich” effect of organic material present in thin layers and consequently it could not improve the soil physical conditions of a large enough volume of soil. Fine roots exploited these organic matter layers in abundant numbers. Deeper placement and better mixing of the organic matter with the soil can therefore probably stimulate much more root growth and produce a more positive effect than what was achieved in the field trials of the 1980s. Saayman & Van Huyssteen (1981a) suggested the growing of a legume, e.g. lucerne or vetch, before soil preparation to ensure a good reserve of N and organic matter during establishment of the young grapevines rather than incorporating manure, compost or straw into the soil. They mentioned that the localised application of manure or compost on the planned row position may be beneficial on sandy soils (< 10 % clay). No experimental evidence to support these suggestions, however, exists in South Africa.

138 | APPLICATION OF AMELIORANTS DURING SOIL PREPARATION