FERTILISATION GUIDELINES FOR THE TABLE GRAPE INDUSTRY

– sedimentary material, weathered granite or dolomite, which have given rise to deep, fertile yellow to red soils. Almost without exception, these soils require physical rectification, with South Africa being unique in this regard, on account of drastic deep cultivation actions being used. At the same time, this also offers an opportunity for rectification of any possible chemical deficiencies, through the addition of suitable ameliorants in the right quantities. Except for the Orange River, most of these soils are characterised by low phosphorus (P) contents, and in many cases (except for the Lower Orange River area) also by excessive soil acidity. In exceptional cases soil acidity may be the result of hydrogen (H + ) ions, derived from high organic matter content of the specific soils, typical for those with fynbos origin. These H-ions are not detrimental, but may disturb the accessibility of specific nutrients, and the general nutrition of non-acid loving plants, like the grapevine. More serious limiting soil acidity is caused by the loss of basic cations, due to advanced weathering of clay minerals and leaching, with the vacated positions then being taken by the omnipresent H-ions, thus disintegrating the clay mineral on which it is adsorbed, giving rise to the release of aluminium (Al) ions. These Al-ions generate more H-ions, resulting in a further reduction of soil pH. At high concentrations aluminium is also toxic to root growth and functioning, thus disturbing general nutrition. CORRECTI ON OF SO I L AC I D ITY Since lime moves very slowly in most soils, soil preparation is the only opportunity for adequate application. In contrast to P (to be discussed), the ultimate aim of lime application is to rectify soil pH, as homogeneously as possible to a depth of at least 1 000 mm, thus making it possible for roots to utilise a large soil volume unhindered, to buffer the grapevine against climatic shocks. The coarse fraction of the soil must be considered, in order to prevent excessive lime applications. The pH does not give an indication of the lime requirement. Rather, lime requirement is dictated by the exchange capacity of the soil and the extent up to which it is occupied by hydrogen – and aluminium ions. For table grapes, the well proven Eksteen-method (Eksteen, 1969) is proposed for calculation of lime requirements. This method utilises the general exponential relationship between soil pH and the H saturation level of the soil, as represented by the so-called R-value , i.e. the Ca+Mg/H relationship. For vineyards the desirable pH of 5.5, measured in KCl, is targeted (Conradie, 1983), which corresponds an R-value of 10. The following formula may be used to calculate the lime requirement of of a soil, down to a depth of 150 mm:

Lime requirement (ton ha –1 /150 mm) = [(H x 10) – (Ca + Mg)] x 4/11 where H, Ca and Mg are imported as cmol kg –1

Experience has however shown that for soils with high magnesium (Mg) content, this formula under calculates the lime requirement because in comparison to Ca, the Mg

30 | CHEMICAL CORRECTION OF SOILS DURING SOIL PREPARATION