FERTILISATION GUIDELINES FOR THE TABLE GRAPE INDUSTRY

In the fruit industry gypsum applications are also advised in order to ensure sufficient Ca levels in soil, without increasing the soil pH. Lime applications may increase the soil pH too much leading to problems with trace element nutrition, a scenario for which fruit is sensitive. The grapevine, however, is not so pH sensitive and does well even in soil with free lime and pH KCl of as high as 7.2. Soil for table grape cultivation may thus be confidently limed to such an extent that the pH in KCl is pushed to 5.5 or higher – this will ensure adequate Ca nutrition for the lifetime of the vines. In this instance, gypsum application will not be required. In areas with high rainfall, and far from the industries, Sulphur (S) deficiencies are increasingly being experienced, because currently the high-grade fertiliser mixtures contain little to no S. Consequently, S is not applied to the soil as “by product” any more. It should therefore be standard practice to analyse the soil for S content as well. In the case of deficiencies, gypsum will be the obvious ameliorant and a single application of 1 ton ha –1 can be considered. Agricultural gypsum contains 90% to 95% calcium sulphate and contains on average 22% calcium and 17.5% Sulphur. CORRECTI ON OF PHOSPHATE CONTENT Upon contact with the soil, water soluble monocalcium- or ammonium phosphates (P) are fixed almost immediately and converted to less accessible di- and tri-calcium phosphate, so that it does not move within the soil, with the exception of grey sands which do not contain iron or clay minerals, as found in the Hex River Valley. However, the grapevine can utilise these less accessible forms of P to the extent that even organo-chemical rock phosphate may be used as a source of P. Due to the immobility of P, deep soil preparation provides an excellent opportunity for the supplementation thereof beyond the soil surface. The norms for the P content of soil for table grape cultivation are given for different extraction methods in Table 1 of Chapter 3. The P requirement is calculated only for the upper 600 mm soil, where most feeding roots occur. The effectiveness of mixing is also of less importance, because the grapevine roots will “find” the P. Furthermore, the coarse fraction of the soil irrelevant and it is thus unnecessary to compensate for it. Here the objective is to obtain enough P in the soil, with the P content of the upper soil layers as guideline and not to get the overall P content of the soil to a specific level. The grapevine is therefore fertilised, not the soil. For this reason, P may be placed in a band close to the vine row in existing vineyards where adequate P fertilisation was neglected during soil preparation. An example for calculation of the amount of P required is shown below: The P content of a soil layer of 300 mm thickness is 15 mg kg –1 and the P soil norm is 30 mg kg –1 , thus the P deficiency is 15 mg kg –1 . For a soil depth of 300 mm and an area of one hectare, 4.5 kg P is required for every 1 g kg –1 that the soil P must be increased. Therefore 15 mg kg –1 x 4.5 kg ha –1 = 67.5 kg P ha –1 is required for the soil layer of 300 mm. The P content of double superphosphate is approximately 20%, thus 67.5 kg x 100/20 = 340 kg must be applied per hectare. A similar calculation is done for the second soil layer of 300 mm thickness and the amounts for the two soil layers are summated.

34 | CHEMICAL CORRECTION OF SOILS DURING SOIL PREPARATION