FERTILISATION GUIDELINES FOR THE TABLE GRAPE INDUSTRY

diphosphate (ADP) and adenosine triphosphate (ATP), and is required for cell division and meristematic growth. However, P is only required in small quantities by the grapevine, while the vine is also able to utilise less accessible sources of P due to specific root properties and mycorrhiza associations, as well as the long growing season over which it is possible to do so. Reaction to P fertilisation is consequently seldom observed in grapevines, despite the generally low P contents of South African vineyard soils. As already mentioned, the P requirement of the grapevine is relatively low (Table 8), with only 0.7 kg ha –1 P being required annually for a production of 1 ton ha –1 . Approximately 35% of the annual requirement is absorbed between bud break and fruit set, followed by a further 40% between fruit set and harvest and 25% in the post-harvest period (Table 10). Soluble P in soil is derived from erosion of minerals, mineralisation of organic material or from fertilisation. Most soils fix added soluble P so quickly that little P movement occurs in practice, but it does happen in bleached sands and in peat, where there is an absence of sesquioxides. The P concentration in the soil solution is pH dependent, with the highest concentrations between pH KCl 3.5 and 6.0 (Follet et al ., 1981). When the pH is lower than this, less soluble Fe- and Al-phosphate compounds form and at higher pH values the Ca phosphates that form is also relatively insoluble. Due to the immobility of P in most soils, the risk of losses is irrelevant, consequently P maintenance fertilisation is only applied when P levels in the soil fall below the optimum levels, as indicated in Table 14 for soils with pH KCl values of between 5.0 en 6.0. Because P is fixed by clay, P norms increase with an increase in clay content. Due to the complex relationships between the different forms of P in the soil and the concentrations thereof in the soil solution, as discussed in Chapter 3, different empirical extractors are used in practice to try to imitate the ability of a specific group of plants to acquire P. For grapevines the stronger P extractors like Bray II, Mehlich III or 1% citric acid are used.

TABLE 14: Norms for phosphate content of vineyard soils with pH (KCl) 5.0 – 6.0.

Phosphate norm (mg kg –1 )

Clay content (%)

Citric acid* 1 Mehlich III* 2

Bray II* 3

Bray I* 4

Olsen* 5

0 – 6

25 30 35

20 25 30

20 25 30

15 20 25

6 – 15

14 16

> 15

* 1 Citric acid (1%). * 2 0.2M acetic acid (CH

3 COOH) + 0.25M NH 4 NO 3 + 0.015M NH 4 F + 0.13M HNO 3 +

0.001M EDTA, comparable with citric acid (1%). * 3 Stronger acid & ammonium fluoride mixture for strong P feeders and acidic soils. Citric acid (1%) is comparable with this. * 4 Weaker acid & ammonium fluoride mixture for weak P feeders and acidic soils. * 5 Sodium bicarbonate & ammonium fluoride & sodium EDTA mixture for high pH soils.

50 | MAINTENANCE FERTILISATION