FERTILISER GUIDELINES FOR THE WINE INDUSTRY

PLANT-AVAILABLE NUTRIENTS In South Africa plant-available nutrients are normally extracted by means of two extractants – mostly ammonium acetate (NH 4 Ac), but some laboratories also use Mehlich III. Although laboratories will often refer to “exchangeable” nutrients, their figures normally indicate “plantavailable” or “extractable” nutrients, which include “soluble” as well as “exchangeable” nutrients. In practical terms this means that the nutrients that could be leached out with water (soluble), is determined together with those that are retained on the clay lattice (exchangeable). As indicated below, it is especially important to keep the above-mentioned in mind for saline soils. RESISTANCE A saturated paste extract of the soil is prepared using distilled water, and its resistance to allow the flow of an electrical current through it is measured. The unit in which resistance is expressed is “ohm”, and it is reciprocal to electrical conductivity (mS/m). Salts, e.g. potassium, sodium and chloride, conduct electricity and therefore reduce the resistance of the soil solution. The lower the resistance measurement the larger the quantities of salts in the soil, i.e. the more saline the soil. A resistance below 300 ohm indicates that an excessive amount of salts are present in the soil – to the level that vine performance is negatively affected. If the resistance is 200 ohm and less, the soil is classified as saline. The lower the resistance, the larger the negative impact on the vine will be. Different salt fractions are encountered in soils. Both the exchangeable sodium percentage (ESP), i.e. the percentage constituted by Na, as a fraction of the total amount of exchangeable cations (Svalue), and the specific resistance serve as criteria for classifying the type of soil salinity.

3

18