FERTILISATION GUIDELINES FOR THE TABLE GRAPE INDUSTRY

Fertilisation according to soil analyses In general, there is a direct correlation between the organic N content and carbon (C) content of a soil. In Table 11 guidelines for the expected N requirements of soils according to the carbon (C) content thereof are indicated.

TABLE 11: Potential N application according to C content of soils (adapted from Conradie, 2001).

Nitrogend fertilisation (kg ha –1 year –1 )

Carbon (C) – content (%)

After bud break After fruit set

After harvest

Total

Sandy soil (<6% clay)

> 0.7

0

0 0

0 – 30

0 – 30

0.5 – 0.7 0.3 – 0.5

0 – 15

20 – 35

20 – 50 50 – 90 90 – 120

15 – 35 30 – 40

0 – 20

35 40

< 0.3

20 – 40

Heavier textured soil (>6% clay)

> 0.9

0

0 0

0 – 20

0 – 20

0.6 – 0.9 0.3 – 0.6

0 – 10

20 – 30 30 – 35

20 – 40 40 – 70 70 – 100

10 – 25 20 – 35

0 – 10

< 0.3

10 – 25

40

Under South African conditions, soils are normally still cold at the time of bud break, which means that the organic material is mineralised at a slow rate. During this time the vines are moreover also largely dependent on reserves accumulated during the previous post-harvest period. Normally N fertilisation on soils with a low C content is required approximately four weeks after bud break, which means that three N instalments might be required on such soils (Table 11). On soils with higher C contents the amount of N, as well as the number of instalments, are gradually decreased. For light textured soils with C contents > 0.7% and for soils with a heavier texture and with C contents >0.9%, natural N supplying should be sufficient to meet the vine’s needs. In South Africa the inorganic N content (normally predominantly NO 3 -N) of soils is seldom determined. However, where this content is more than 10 mg kg –1 three to four weeks after bud break (end of September), it means that the plant available N in the topsoil (0 – 300 mm) will amount to 45 kg/ha –1 . Nitrogen fertilisation during the bud break to fruit set period will be unnecessary under these conditions. During the rest of the growing season the release of inorganic N will largely be affected by factors such as leaching, soil temperature and the water status of the soil. In cases where the inorganic nitrogen content after flowering is >15 mg kg –1 , it will not be necessary to apply any N fertilisers during the rest of the pre-harvest period.

46 | MAINTENANCE FERTILISATION