FERTILISATION GUIDELINES FOR THE TABLE GRAPE INDUSTRY

Without cultivation, lime moves very slowly in the soil and where lime is applied on the surface only, the efficacy of the neutralisation of acidity is thus doubtful. The use of micro-fine lime may possibly solve this problem, because the suspended lime particles can infiltrate the soil together with the water, and percolate down to the root zone. However, the distribution pattern and effectiveness of this type of lime must still be quantified. Increase in salt load Temporary increases in salt load may be obtained within the root zone, following fertiliser applications. The data mentioned below (Table 29) from a laboratory experiment emphasises the effect of fertiliser on the salt balance in the root zone. The size of the fertiliser instalments should thus be carefully managed.

TABLE 29: The effect of K 2 SO 4 application on soil, in a laboratory experiment by Bemlab

EC – saturated paste (mS m –1 )

Resistance (ohm)

Period following incorporation of product

100 kg K 2 SO 4 per ha

100 kg K 2 SO 4 per ha

Control

Control

Directly after incorporation and wetting 24 h after incorporation and wetting

8.99

16.72

5 680

2 420

8.53

18.79

6 000

1 880

Notes: An equivalent of 40 kg K per ha is applied as 100 kg per ha K 2 SO 4 . It is accepted that a salt effect will be obtained immediately for the upper 75 mm of soil. Therefore, the application was equivalent to 0.1 g K 2 SO 4 per kg soil (100 kg per ha for full surface application). The test was done on a highly leached soil. Texture and % stone will influence the intensity of the soil’s reaction. Soil biology Up to now, research to determine the impact of fertilisation on soil biology, yielded divergent results. However, it is accepted that fertiliser will lower the diversity of soil biology, due to the impact on soil pH and the short term “salt effects”. In reference to the criticism, voiced in certain ranks, on the use of KCl as fertiliser, it is necessary to point out that there is an enormous difference between a chloride ion (Cl – ) that occurs free in water, or in the soil in association with K or Na, and chlorine that occurs as the hypochlorite (OCl – ) -ion. The latter is the form that has a sterilising effect. The oxidation status of chlorine in Cl – and in OCl – differs dramatically. The former contains an extra electron, in the Cl-atom while the latter as ceded an electron and is thus positively charged. Conversion of Cl – (in the reduced state) to OCl – (where the Cl + is in the oxidised state) cannot occur in the soil, due to the large amount of energy required for this (therefore the necessity for an electrolysis electrode in swimming pools to convert

88 | PRACTICAL GUIDELINES FOR IMPLEMENTATION OF THE FERTILISATION PROGRAMME