FERTILISATION GUIDELINES FOR THE TABLE GRAPE INDUSTRY

The grapevine requires small quantities of micro-nutrients and the availability thereof is directly dependent on the pH of the soil solution. Where the pH is high, manganese (Mn) and zinc (Zn) are inaccessible to the plant since these elements do not remain in the solution. Sometimes these elements may therefore be present in the soil, without being accessible to the plant. Consequently, soil analysis is not always a reliable means of determining the availability of micro-elements. To ascertain whether the metals are excessive or insufficient, leaf analyses should be done. For soils with low pH, boron (B) and (Zn) deficiencies may be expected. On the other hand, Mn can be so soluble in low pH soils that it may become toxic to the vine. Lime applications will solve these problems, by making B and Zn more available to the plant and Mn less soluble (Van Schoor, 2001). In Table 5 the norms for optimal micro-nutrient concentrations in soil are provided. In cases where the concentrations of the nutrients in the soil are below these norms, deficiencies may occur. In such cases the vineyard should be monitored visually for deficiency symptoms. If there is still any doubt, leaf analyses should be done. In the past, Cu deficiencies very rarely occurred in vineyards, due to the use of fungicides that contained Cu. With the decline in use of these products, attention should also be given to this micro-nutrient.

TABLE 5: Minimum micro-nutrient concentrations in soils with pH KCl values of 5.0 to 6.5 (mg kg –1 ). B Mn Zn Cu 0.3 2.0 0.5 0.5

If deficiencies of micro-nutrients do occur, foliar application of nutrients is a simple solution. Table 6 may be used as a guideline to determine if the micro-nutrient status is optimal.

TABLE 6: Micro-nutrient concentrations in petioles, sampled at fruit set and veraison, for assessment of the nutritional status of vineyards. (mg kg –1 ).

Fruit set

Veraison

High to excessive tot excess

High to excessive tot excess

Deficient Sufficient

Deficient Sufficient

Micro-

nutrient

Fe *NB 30 – 180

*NA

< 25 30 – 200

*NB *NB *NB

Cu < 3 5 – 10

25 – 50

< 2.5 3 – 20

Zn < 15 20 – 150 Mn < 20 30 – 60 B < 25 30 – 70 Mo *NB 0.2-0.4

*NA

< 15 20 – 150 < 20 30 – 200 < 25 30 – 90 *NA 0.2 – 0.5

>300 >100

> 1500

> 150

*NA

*NA

*NA – not available.

26 | INTERPRETATION OF SOIL ANALYSIS REPORTS FOR VINEYARDS