FERTILISER GUIDELINES FOR THE WINE INDUSTRY

SOIL pH The pH of soil is determined in either potassium chloride (KCl) or water (H 2 O). Most laboratories in the Western Cape use the KCl method, while water pH is mostly reported in European and American laboratories. Although the difference is not always constant, soil pH KCl is roughly one pH unit lower than pH water . The reason for this is that the K + ions in the solution displace the H + on the clay lattices (the exchangeable H + ), which is then measured along with the active H + -ions in the soil solution. A solution with a pH KCl below 5.5 (pH water < 6.5) is regarded as suboptimal for grapevines. The lower the pH, the more acidic the soil, e.g. there is a higher concentration of active hydrogen ions (H + ). The more acidic the soil, the higher the solubility of aluminium (Al 3+ ); until it reaches a toxic concentration that negatively affects root growth. Grapevines underperform in acidic soils due to poor root functioning, leading to reduced water and nutrient uptake as well as possible pathogen and nematode infection. Lime should therefore be applied to make a correction. Various methods for determining the lime requirement have been developed. The well-known Eksteen method has proven to be reliable for South African soil conditions and vineyards (Eksteen, 1969). Calculation of the lime requirement is fully discussed in Chapter 4. The optimal soil pH (pH KCl ) for grapevines varies between 5.5 and 6.5. Below this range root growth is increasingly impeded and both phosphorus (P) and molybdenum (Mo) becomes gradually less available for uptake. Acid soils also often are highly leached and depleted of nutrients, e.g. nitrogen (N), potassium (K), calcium (Ca) and magnesium (Mg). Above this pH range, both P and the other micro-nutrients (except Mo) become less available for plant uptake, as indicated in Figure 1. This is due to immobilisation when P reacts with Ca and micronutrients with hydroxides and carbonates. To prevent nutrient deficien cies on soils with high pHs, regular P fertilisation and annual foliar applications of micro-nutrients is required.

3

16