FERTILISER GUIDELINES FOR THE WINE INDUSTRY

If the conductivity of a saturated soil extract exceeds 400 mS m -1 and the ESP is less than 15%, the soil is classified as saline. In the case of soil with an ESP > 15 %, containing free gypsum or lime, it is classified as a “salinesodic soil”. In both these cases the salts may simply be washed out using good quality irrigation water – providing that free gypsum is present, otherwise the soil colloids will disperse making the soil impermeable for water. In cases where gypsum is not present in the soil, the leaching water should be saturated with gypsum before it comes into contact with the soil. PHOSPHORUS (P) In soil analysis reports phosphorus (P) is usually indicated in mg/kg. The opti mal plant-available concentration depends on the soil texture and soil pH. It is therefore important that laboratories also report the soil texture. Depending on the extraction method being used, the norms for optimal P-concentration also differ because the extractants differ in pH and aggressiveness by which the P is extracted. A comparative list of norms are supplied in Table 1, indicating the applicable values for the most commonly used extractants. Due to the fact that P extracted with Bray I, Bray II and Mehlich III reduces as the soil pH increases, distinction needs to be made between the norms used for soils of different pH. Furthermore, the P that is required to raise the concentration to the minimum level might not necessarily be reflected when the soil is extracted at high pH. For soil with a pH regime that is regarded as optimal for grapevine production (e.g. pH KCl 5.5 to 6.0), Bray II and Mehlich III extraction provides similar values and reflects most accurately the available concentration of P in the soil. Bray I extractions is consistently lower and for vineyard soils the accuracy thereof with regard to P availability has not yet been confirmed.

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