FERTILISATION GUIDELINES FOR THE TABLE GRAPE INDUSTRY

In practice it has also been learnt that the lime requirement of bleached sandy soils may be overestimated, due to the relative absence of Al in this predominantly quartzite material. For this Smuts (2001) suggested that the measured H content of sands with Ca + Mg > 1.5 cmol kg –1 , must be corrected as follows:

Corrected H = H – [Ca + Mg) – 1.5] With H, Ca and Mg in cmol kg –1 units

The lime requirement of sand is now calculated on equivalent basis at the hand of measured exchangeable H content, or the H corrected as shown above.

Lime Requirement (ton ha –1 /150 mm) = H x 1.6

In sands, with their low exchange-capacity, cations leach easily, inter alia also the highly hydrated and thus very mobile Mg. It is thus advisable to use dolomitic lime principally for sandy soils. It is known that ammonium containing N fertilisers, as well as urea, acidify soils and specifically in the fruit industry, periodic lime applications is general practice, in order to compensate for this. However, this lime moves very slowly, and the upper soil layer may sometimes be over limed, while the pH of the root zone remains unchanged. Consequently, application of sufficient lime during soil preparation is essential and slight over liming will counteract the above-mentioned acidification. LIMI NG MATER IALS The purity and fineness of a liming material determines the effectiveness thereof. According to law the purity of calcitic lime should be at least 70%. Coarse lime is often less pure, and the reaction speed of liming materials is inversely correlated with the degree of fineness degree. Finer lime should thus enjoy preference. However, experience indicates that it is unnecessary to grind lime finer than 60 mesh (0.25 mm in diameter) The actual analysis and calcium carbonate equivalent value (CCE) of a specific product (Table 7) is necessary in order to be able to decide between different lime sources, while transport costs may also be a decisive factor. The neutralization value of dolomitic lime is slightly higher than that of calcitic lime.

TABLE 7: Calcium carbonate equivalent values (CCE) of different liming materials.

Composition in pure form

Liming material

CCE (1)

Formula

% Ca

% Mg

Calcite

CaCO 3

40 20 54

–

100 109 136

Dolomite

CaMg(CO 3 )

12

Slaked lime

Ca(OH) 2

–

(1) CCE = Amount of calcitic lime equivalent to 100 kg of specific liming material.

32 | CHEMICAL CORRECTION OF SOILS DURING SOIL PREPARATION