FERTILISATION GUIDELINES FOR THE TABLE GRAPE INDUSTRY

CHAPTER 3

with those that are retained on the clay lattice (exchangeable). As indicated later, it is especially important to keep the above-mentioned in mind for saline soils. RES I STANCE A saturated paste extract of the soil is prepared using distilled water, and its resistance (ability to allow an electrical current to flow through it) is measured. The unit in which resistance is expressed is “ohm” and is reciprocal to electrical conductivity (mS m –1 ). 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 quantity of salt in the soil, i.e. the soil is more saline. A resistance below 300 ohm indicates that excessive quantities of salts are present in the soil. At this level vine performance is negatively affected. The lower the resistance, the larger the negative impact on the vine will be. If the resistance is 200 ohm and less, the soil is classified as saline. Different salt fractions are encountered in soils. Both the soluble sodium percentage and the exchangeable sodium percentage (ESP), i.e. the percentage that Na constitutes as a fraction of the total amount of exchangeable cations (S-value), and the specific resistance serve as criteria for classifying the type of soil salinity. If the resistance of a saturated soil extract drops below 300 ohms (or conductivity of a saturated soil extract exceeds 400 mS m –1 ), with the ESP lower than 15%, the soil is classified as saline. Soil with an ESP > 15%, while containing free gypsum or lime, is classified as a “saline-sodic soil”. In both cases the salts may simply be washed out, using good quality irrigation water – provided that free gypsum is present, otherwise the soil colloids will disperse, making the soil impermeable for water. Where gypsum is not present in soils with ESP > 15%, the water that will be used for leaching should be saturated with gypsum before it comes into contact with the soil, or the gypsum requirement must be determined, for application of gypsum to the soil. PHOSPHORUS In soil analysis reports, phosphorus (P) is usually indicated in mg kg –1 . The optimal plant- available concentration depends on soil texture and soil pH. It is therefore important that laboratories also report the soil texture. Depending on the extraction method that was used, the norms for optimal P concentration will differ, because the extraction agents differ in pH and aggressiveness with which the P is extracted. A comparable list of norms is supplied in Table 1, indicating the applicable values for the most commonly used extraction agents. Because the amount of 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 with different pH values. Furthermore, the P that is required to raise the concentration to the minimum required 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 most accurately reflects the available concentration of P in the soil.

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