SOIL PREPARATION

CHAPTER 8

8.3 Phosphorous Phosphorous is an essential macro-element, but is only required in small quantities by the grapevine. Local research revealed that 0.7 kg/ha of P is annually required by a vineyard for each tonne of grapes produced. Phosphorous is furthermore immobile in most soils with the exception of peat soils and leached sands. The quantity of P to be applied will depend on the adsorption capacity of the soil which correlates with its clay content. The higher the clay content, the more P must therefore be applied to reach the norm for vineyards (Raath, 2016a). The P availability in soil is also pH dependant with the highest concentration of P available between pH KCl 3.5-6.0. At a lower pH insoluble Fe- and Al-phosphate compounds are formed while Ca-phosphates are formed at higher pH levels (Saayman, 2016). At soil preparation, the P content of the soil is determined to a depth of 60 cm. In order to increase the P content of the soil by 1 mg/kg to a depth of 600 mm, an amount of 9 kg/ha should be applied. This macro-element, usually in the form of superphosphate, should therefore be broadcast on the soil surface and incorporated deeply into the soil during soil preparation as stock fertilisation to rectify the P content of the soil. Subsequently, after the vineyard has been planted, maintenance fertilisation needs to be applied only when the P concentration of the soil drops to suboptimal levels again. In special cases, the calculated amount of P that has to be applied during soil preparation should be adapted. On high pH soils (pH KCl > 7), the recommended quantity of P to be applied during soil preparation can be reduced and the annual amount of maintenance fertilisation can be increased (Raath, 2016a). In soils with a high clay and a low P content, the required amount of P fertilizer to be added as stock fertiliser may become so great as to be uneconomical. In such cases, P must preferably be placed in a band close to the vine row at a rate of approximately 30 kg P/year (Saayman, 1981). ) is applied to soil for the reclamation of saline soils. Such soils form due to the accumulation of excess salts as a result of salt-containing rock ( e.g. Malmesbury shale), inadequate drainage, poor water quality and poor management practices ( e.g. over-irrigation). Soil salinity can follow a certain progression from white or salt brack to acid salt sodium brack (Saayman, 1981). In general, the stage of excess salts is followed by a stage when there is too much Na + relative to Ca ++ and Mg ++ , the soil colloids disperse and water infiltration deteriorates. In addition to water infiltration problems on saline soils, large concentrations of Na + and Cl - are toxic to grapevines, while they are also negatively affected by the high osmotic potential of the soil solution due to the excess salts. 8.4 Gypsum Gypsum (CaSO 4

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