SOIL PREPARATION

the soil to a depth of 15 cm. Two implements were used, namely a delve plough and a wing plough, to deep-plough the soil. The lime was mixed beforehand with a fluorescent mixture that could be observed under ultraviolet lighting (Figure 8.2). TABLE 8.1: Implements and methods of lime applications used for investigating the effectiveness of lime application on an Avalon soil (adapted from Saayman & Van Huyssteen, 1981b).

Method of lime application (% of total requirement)

Implement

Treatment No.

On surface Blown-in Against slanted wall of furrow

Delve plough (share & ripper)

D1 D2 D3 W1 W2

50 100 33

– – 67

50 – –

Wing plough

0 33

100 67

– –

The surface application of lime before deep ploughing was more effective than expected and compared well with conventional lime application or with lime blown in. None of the methods however, yielded the desired effect namely, even and thorough mixing of lime with the soil. As expected, the wing plough gave unsatisfactory mixing of lime with the soil since this implement was developed to loosen the soil without bringing subsoil to the surface. An unevenly tongued mixing pattern was obtained with the delve plough. These authors came to the conclusion that combining blown-in lime together with applying lime on the surface or conventionally would be the best option. They also proposed that the best mixing of lime and soil would be achieved through deep ploughing a second time at a 30° angle to the original ploughing direction. The use of a lime blower has been discarded since, due to technical problems. Lime compacted inside the bin mounted on top of the blower and the only way to keep the lime flowing was to continuously stir it manually. Furthermore, lime distribution obtained with the blower, particularly behind the wing plough, was no better than the mixing achieved with surface-applied lime. In fact, the lime blower tended to place the lime in a band without mixing it with the soil. The current practice of lime application in South Africa entails applying the lime on the soil surface (Figure 8.3) and mixing it with the topsoil through ploughing to a depth of 30 cm followed by deep-tillage to a depth of 0.8 m-1.0 m. It is further advisable to apply the lime at least six months before planting to allow it to neutralise the H + and Al 3+ that are responsible for the low soil pH.

136 | APPLICATION OF AMELIORANTS DURING SOIL PREPARATION