FERTILISER GUIDELINES FOR THE WINE INDUSTRY

INTRODUCTION

The characteristics of South African soils is such that in most cases vines should not be planted without thorough soil preparation. During soil prepara tion a single opportunity is presented to correct the nutrient status of the soil, especially the subsoil. In this chapter emphasis is placed on the application of lime and gypsum in particular, while phosphate is discussed in Chapter 3 and also in Chapter 5.

APPLICATION OF LIMING MATERIALS:

Liming is done to neutralise excessive soil acidity. In acidic soils an excess hydrogen ions is present, which as such is not detrimental for plants, but does give rise to high aluminium contents in particular, which is toxic for root growth and therefore could be detrimental for nutrient uptake. CALCULATION OF LIME REQUIREMENT In the past reference was always made to the lime requirement of a soil, i.e. maintaining a certain pH value for the specific soil, but currently sufficient sci entific proof exists to rather refer to the lime requirement of a specific crop in terms of the acid saturation approach. It is thus not enough to merely know the pH value, because this is not an indication of the amount of ameliorant required for correction. An accurate determination of the lime requirement can only be made if soil analyses indicate the exchangeable acidity (H + ). The Eksteen method has to date been found to be the best for determining the lime requirement for agronomic, fruit and vineyard soils in the Western Cape. According to this method the so-called R-value is used, which indicates the Ca+Mg/H ratio in soil and has an exponential relationship with the pH values of the soil. To obtain a desired pH KCl of 5,5, a R-value of 10 is required. A R-value of 5 will only correct the pH to 5, which is acceptable for most sandy soils. By making use of the exchangeable acidity (H) and the exchangeable calcium (Ca) and magnesium (Mg) values of a soil, expressed in cmol/kg, the lime requirement for grapevines can thus be calculated using the Eksteen formula.

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