FERTILISATION GUIDELINES FOR THE TABLE GRAPE INDUSTRY

The “finger method” can be used to assist in classifying the soil’s texture as either sandy, loamy or clayey. Alternatively, texture can be determined in the laboratory. Unfortunately, not all laboratories routinely report soil texture. Most laboratories will only report texture upon request. A full textural analysis can be carried out, to specify the exact percentages of sand, silt and clay. SO I L AC I D ITY OR ALKALI N ITY The pH of soil is determined in either potassium chloride (KCl) or water. Most laboratories in South Africa use the KCl method (pH KCl ), while most European and American laboratories determine a pH in water and report it as such (pH water ). Even though the difference between the two methods is not 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 together 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 under perform in acidic soils due to poor root functioning, leading to reduced water and nutrient uptake as well as the possibility of pathogen and nematode infection. The optimal soil pH (pH KCl ) for grapevines varies between 5.5 and 6.5. For soils with lower pH values, lime should be applied to correct the situation. Various methods for determining the lime requirement have been developed. The Eksteen method has proven to be reliable for South African soil conditions and for vineyards (Eksteen, 1969). Calculation of the lime requirement is fully discussed in Chapter 4. Apart from root growth being increasingly impeded in acid soils, both P and molybdenum (Mo) become gradually less available for uptake. Acid soils are often also highly leached and depleted of nutrients like nitrogen (N), potassium (K), calcium (Ca) and magnesium (Mg). Where pH levels are higher than the above-mentioned optimal pH range, both P and the other micro-nutrients (except Mo) become less available for plant uptake. This is attributed to immobilisation when P reacts with Ca, and micro-nutrients with hydroxides and carbonates. To prevent nutrient deficiencies in soils with high pH values, regular P fertilisation and annual foliar applications of micro-nutrients is required. PLANT-AVAI LABLE NUTR I ENTS In South Africa plant-available nutrients are normally extracted by means one of two extraction agents – mostly ammonium acetate (NH 4 Ac), but some laboratories also use Mehlich III. Although laboratories will often refer to “exchangeable” nutrients in their reports, their figures normally indicate “plant-available” or “extractable” nutrients, which include “soluble” as well as “exchangeable” nutrients. In practical terms this means that the nutrients that could be leached out with water (soluble), are determined together

20 | INTERPRETATION OF SOIL ANALYSIS REPORTS FOR VINEYARDS