SOIL PREPARATION

RE-COMPACTION A key question regarding deep soil preparation has always been, “will the soil re-compact and how long will it take?” The positive effects of deep soil preparation can become undone through re-compaction which can either be man- made or due to natural processes. Man-made processes include vehicle traffic such as tractors and other machinery while natural processes vary in nature, as discussed below. The grapevine root however, does not make a distinction between natural and man-made soil compaction. Both types are equally harmful to the vine (Figure 9.1). Very often re- compaction is not recognised as such because it happens gradually, is not always visible, and on densely-packed sandy soils the physical indications of re-compaction are subtle, e.g. a gradual increase in penetrometer soil resistance with depth. Successive wetting and drying cause compaction of the soil and also hard-setting in some unstable soils (Van Huyssteen, 1989). Re-compaction can also be of a chemical nature. Aluminium, silica and iron compounds were found to act as cementing agents in soils. Age-hardening of soil (Dexter et al ., 1988) is another natural process that causes soil to regain most or all of its original strength after it has been loosened. The two most important processes contributing to age-hardening of soils are chemical cementation of bonds between soil particles or rearrangement of particles. Organic matter is reported to decrease or even stop age-hardening (Dexter et al. , 1988).

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PHOTO: J.E. HOFFMAN, STELLENBOSCH UNIVERSITY.

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