FERTILISATION GUIDELINES FOR THE TABLE GRAPE INDUSTRY

The primary sources of K in the soil are K-feldspars (orthoclase, microcline), micas (biotite, muscovite, vermiculite) and micaceous clays like illite. Usually the K content of soil exhibits an increase with depth due to erosion in the top layers and leaching in towards the deeper layers. In material with low K content it is, however, often the A horizon that has the highest K content due to cation circulation and fertilisation practices. Table 8 show that approximately 96 kg K ha –1 is required for a production of 30 ton ha –1 (3.2 kg K per ton). The largest fraction of this (62 kg) is required by the grapes (Table 9), thus being permanently removed from the vineyard. Because K leaches easily from sandy soils, a vineyard’s K fertilisation must be largely based on soil texture (Conradie, 1998). On sandy soils (< 10% clay) annual maintenance fertilisation must be based on expected production, divided according to the guidelines in Table 10, thus coming down to 40% between bud break and fruit set, 45% between fruit set and harvest and 15% in the post- harvest period. On such sandy soils with a K content of higher than 50 – 80 mg kg –1 , the soil should contain sufficient K for that season, implying that maintenance fertilisation will be unnecessary during that year. Depending on the mineralogy, heavier soils may often be naturally well supplied with K, while little leaching will occur, implying that excessive K levels will build up in the soil if K fertilisation is applied according to production. On such soils (> 10% clay) soil analyses must be used as an indication of K availability, and the “sufficient” level, that will be determined by the mineralogy of the region (Table 16), can vary between 90 and 120 mg kg –1 . If a soil contains more than 140 mg kg –1 K, it will never be necessary to apply K fertilisation. At sub-optimal levels two approaches may be followed, depending on the K content of the soil: 1: If the K content of the soil is higher than 50 mg kg –1 , a single K application is necessary to increase the K level to the required level. 2: For soils with K contents lower than 50 mg kg –1 , a single application can be applied on the basis of expected production (3.2 kg K per ton), combined with an additional 50% of this amount. After this, soil analyses must be done every three years to monitor K levels. FIGURE 10: Symptoms of leafroll infection (left) must not be confused with potassium deficiency symptoms – the photo on the right show the difference between K deficiency (top) and leafroll symptoms (bottom) (Photos: P. Raath).

54 | MAINTENANCE FERTILISATION