FERTILISATION GUIDELINES FOR THE TABLE GRAPE INDUSTRY

CHAPTER 4

on the exchange-complex seems to contribute less to the reduction of soil acidity. Consequently, the contribution of Mg is down-sized, by replacing the (Ca + Mg) – component in the formula with (Ca x 1.25), for soil with a Ca/Mg relationship of < 6. The calculated lime requirement must be corrected/reduced according to the volume percentage of the coarse fraction. Finally it must be increased if the thickness of the specific soil layer is > 150 mm. The lime requirements of the relevant soil layers are then summated to a total depth of 1 000 – 1 200 mm. The Ca/Mg relationship of the soil is also an indication of the type of lime that will be required and as a first approach calcitic lime (CaCO 3 ) can be used if this relationship is < 6, while dolomitic lime (CaMgCO 3 ) is the choice where relationship is >6. However, it may happen that a soil with a Ca/Mg relationship of < 6 will require such a large amount of calcitic lime that it will lead to an excess Ca or too little Mg. In such cases (calcitic lime > 20 ton ha –1 ) the formula provided below, developed by JJN Lambrechts, may be used. In this formula the amount of dolomitic lime, that must be applied together with the calcitic lime, is calculated as follows: Dolomitic lime (kg ha –1 /150 mm) = [(8Ca+2Lime)-40Mg]/6 x Soil Depth/150 x (1-Stone%/100) With Ca and Mg in cmol kg –1 and lime the calculated lime requirement for a depth of 150 mm. This will ensure a Mg saturation of 15 – 20% of the exchange complex. In soils which contain high quantities organic material, acidity is largely due to excess H-ions, and to a lesser degree by high Al contents, often resulting in the calculated lime requirement being excessive. WJ Conradie (Conradie, 1994), suggested that the following empirical adjustments could be made:

Carbon content of soil (%)

Lime requirement adjustment

< 1

None

1 – 2 2 – 3 3 – 4

80% of calculated lime requirement 60% of calculated lime requirement 40% of calculated lime requirement

The above-mentioned empirical formula should normally be completely acceptable. If more exact calculations are required, the formula provided below, developed through research by Smuts (2001), which comes down to a correction of the H content of an organic rich soil, may be used:

Corrected H = Measured H – [(7- pH) x %C x 0.202] Where pH refers to the pH KCl of the soil sample, with 0.202 the correction factor for organic material.

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