SOIL PREPARATION

CHAPTER 2

2.2 Soil Acidity Soils having a pH KCl

below 5.5 are commonly found in the coastal areas of the Western Cape and even in the more arid Breede River and Olifants River areas (Saayman, 1981; Conradie, 1983). In the coastal region, these acid soils originate from certain types of parent rock, particularly granite, Malmesbury shale, Table Mountain sandstone and local colluvium, all of which are highly weathered due to age and high rainfall. Consequently the basic cations were leached and replaced by hydrogen ions (H + ) and aluminium ions (Al 3+ ). The latter element not only contributes to soil acidity, but it is also toxic to plant roots. Soil acidity can also be caused by high rainfall, continuous use of NH 4 + containing N fertilisers and even the application of large amounts of organic matter (Saayman, 1981). Normally the pH decreases with soil depth, i.e. it becomes more acid, which complicates the remediation of acidity. Soil acidity is determined by measuring its pH in either water (H 2 O) or potassium chloride (KCl). Most laboratories in South Africa use the KCl method, but it is useful to remember that pH KCl is roughly one pH-unit lower than pH H 2 O (Raath, 2016a). At low pH, aluminium (Al) in the clay becomes soluble and replaces cations such as calcium (Ca) and magnesium (Mg) on the clay complex. It also reacts with water to release more H + which leads to further acidification (Saayman, 1981). The optimum soil-pH (pH KCl ) for grapevines varies from 5.5-6.5 (Raath, 2016a). A pH KCl below 5.5 is not optimal for grapevines since root growth is inhibited and the availability of P and molybdenum (Mo) for uptake decreases. Acid soils are often also highly leached and contain low concentrations of nutrients such as nitrogen (N), Ca, Mg, and potassium (K). In addition, the solubility of aluminium (Al) increases with decreasing pH until Al ions become toxic and negatively affect root growth. Conradie (1983) demonstrated this impeding effect of soil acidity on the root growth of grapevine rootstocks. Interestingly, he mentioned that, in addition to Al toxicity, this result might have partly been due to an unfavourable physical structure of the acid soil. Soil amelioration with lime is the tested way to increase the pH, but lime has to be well-mixed with the soil, especially in the zone where the acidity problem occurs. Depending on the outcome of soil analysis, either calcitic or dolomitic lime must be used. Most important, however, is to remember that the alkalinisation effect of lime does not move downward in soil. Consequently lime application must be done during soil preparation, an opportunity when this ameliorant can be placed all through the entire soil profile, and especially into the subsoil (see also “8.1 Lime”).

SOIL PREPARATION | 23