SOIL PREPARATION

CHAPTER 1

The majority of soils used for wine and table grape production in South Africa are notoriously shallow in their natural state, i.e. they are restricting root penetration due to various factors such as compaction, soil acidity and abrupt textural transitions. The results of such shallow soils are uneven and poor vineyard performance that eventually lead to unprofitable vineyards. Research has proved the relationship between available rooting volume and aboveground grapevine performance and consequently also the necessity for modifying the soil profile before planting. An optimum available rooting volume exists, exceeding which, the grapevine vegetative growth becomes excessive. Deep tillage before planting of a vineyard can therefore either be inadequate or overdone and wasteful. Soil preparation is not only necessary for establishing first generation vineyards but, due to re-compaction, is often also required for the replanting of existing vineyards. Soils can re-compact naturally with lapse of time, but can also be re-compacted through man-made actions e.g. by driving over deep- ploughed soil or by repeated mechanical tillage at the same depth. Man-made soil compaction in the form of traffic pans and plough pans occurs frequently in vineyards and orchards, and can in fact be found in most intensively mechanised crop production systems. Traffic pans occur mostly on sandy soils and are more common than plough pans which occur on clayey soils where tillage implements smear and compact soil just below the ploughing depth. Re-compaction can even have such unlikely causes as farmers allowing livestock to graze in their vineyards and orchards, without realising that severe compaction is caused by the treading of livestock (Mitchell & Berry, 2001). The worst compaction is by “pugging” i.e. plastic flow around animal hooves in wet soil (Singleton & Addison, 1999). Already in the first century after Christ, in his book De Re Rustica, the Roman agriculturist, Columella (Thayer, n.d.) also warned against re-compaction of loose soil when a new vineyard is planted. In a translation of his writings it is said: “For it is of no little importance that trenched ground be in a very loose state and, if possible, not violated even by a footprint; so that the earth, being evenly stirred, may give way generally to the roots of the young plant in whatever direction they creep out, not repelling their growth by its harshness….” Deep preparation of vineyard soils has a long history in South Africa. The well-known viticulturist, A.I. Perold (1926), had already discussed the pros and cons of “delving versus ploughing” and based his recommendations on soil type. He also recommended deep delving especially for table grapes. He did, however, differentiate between soils and mentioned that delving is not suitable for heavy clayey and loamy soils since such soils will quickly re-settle again after heavy rains. In the early parts of the 20th century, equipment for deep soil preparation was lacking, and farmers used hand delving to a depth of 60 cm and even 75 cm.

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