FERTILISATION GUIDELINES FOR THE TABLE GRAPE INDUSTRY

CHAPTER 4

When the pH KCl of a soil is adjusted to 5.5 by means of calcitic lime, a Ca saturation of approximately 80% of the soil’s exchange-complex is obtained. If the correct amount of dolomitic lime is applied separately or together with the calcitic lime, a Mg saturation of approximately 15 – 20% will be obtained, indicating that no additional Ca or Mg nutrition should be necessary. In the case of Ca, it eliminates the necessity for gypsum applications. APPLI CATI ON OF LIME Lime is broadcast over the entire surface of the soil that must be limed, and application time is naturally determined by the time of soil preparation. The water content of the soil is of vital importance during soil preparation. If the soil is too wet it will smear and diagonal sods, or the “sandwich” effect will be obtained, resulting in poor mixing. If the soil is too dry large clods are formed, causing the broadcast lime to be poorly mixed with the subsoil. This can be circumvented if the topsoil is ploughed finely beforehand or if the soil is ripped to a depth of 30 – 45 cm, in order to form clods intentionally. During deep soil preparation the width of preparation must not be wider than the tracks of the caterpillar, to ensure all clods on the surface are trampled. Soil should rather be prepared when too dry than when too wet. The ideal water content during soil preparation is when the soil material is not plastic. The water content of the soil must be below the threshold at which the soil can no longer be rolled into a stable sausage between the palm of the hands. Make profile pits to check the water content of the subsoil. If it is too wet, stay clear of being kept “hostage” by a contractor who may fortuitously be working in the vicinity. GYPSUM APPLI CATI ONS Gypsum (CaSO 4 ) is generally applied on saline soils, wherein gypsum is not present naturally. The objective is replacement of the excess sodium (Na) on the exchange- complexes, by the Ca in the gypsum, so that the Na may be leached, together with the excess dissolved salts by means of irrigation. For this, efficient drainage is naturally essential. It is important that the leaching water has a high electrolyte content, to prevent dispersion and compaction of the soil. This is best accomplished by saturating the irrigation water with gypsum before it comes into contact with the soil. In practice, however, the gypsum is mixed as thoroughly as possible with the soil, while care is also taken to ensure that there is a layer of gypsum on the surface of the soil. For each cmol kg –1 exchangeable Na that has to be eliminated from the clay complex, approximately 3.4 ton ha –1 of gypsum is required for every soil depth of 300mm. However, analytical values obtained from the laboratory for the Na content of saline soils, do not reflect only the exchangeable Na, but also include the Na present in the soluble salt component of the soil, mostly resulting in over estimation. To compensate for this, gypsum applications should be limited to a maximum of 10 ton ha –1 . In cases where the calculated gypsum requirement exceeds 10 ton, the maximum application can be made and after a year, the soil should be analysed again, in order to ascertain whether a lowering in the soil’s salt content and Na levels have occurred.

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