For optimal growth and yield, the soil pH has to be correct.
Soil acidity may easily be a restrictive factor in terms of yield, unless fertilisation is applied correctly. Two main restraining factors of acidified soil on the plant’s productivity are nutritional deficiencies and metal toxicities, such as soluble aluminium and manganese. Furthermore phosphate additions in acidified soil also give rise to problems as a result of strong solidification of aluminium and iron. The toxic effect of Al3+ on plant roots is well known and typical symptoms of aluminium damage include stunted root tips, causing reduced accumulation of nutrients and water uptake.
Chemical adjustments, such as the application of the correct amount and kind of lime to adjust the soil pH, as well as the phosphorus status, are part and parcel of soil cultivation. It is important to adjust the soil pH for optimal root development of the grapevines/rootstocks. In most instances liming of soil to a pH of six will significantly increase the root mass of rootstocks.
Before plans are made to establish a vineyard, broadcast chemical adjustments are required using a specific source of lime, phosphate, potassium and even gypsum to the correct depth. The adjustments are determined by the soil type and effective root depth (Photo 1).
PHOTO 1. A precision applicator is used for liming before soil preparation.
Because lime has a relatively low solubility, the correct implement must be used for mechanical application of the ameliorants, so that it may be mixed with deeper horizons. Remember that low soil pH can only be adjusted by means of a carbonate and that gypsum will not cause any changes in pH. The wetting area of a dripper is usually in a strip alongside/in the rows of grapevine trunks. When soil chemistry is not adjusted during soil preparation, the roots tend to grow in the wetting area only and in some instances the chemical limitations result in permanent root restriction (Photos 2 and 3). Although drip irrigation is an excellent aid to keep the soil status balanced, traditional liming cannot be effectively replaced by applying liquid lime through the drip system.
PHOTO 2. Good root development obtained through correct liming and phosphate adjustment before soil preparation, as opposed to incorrect soil preparation that did not reach the correct depth, with subsequent weaker root distribution and root limiting layers.
PHOTO 3. Obvious excessive liming visible as a result of incorrect soil preparation techniques using ameliorants. Even after 10 years the lime remains visible.
The management of soil acidity below the dripper remains limited, even in the new era of technology, in view of the fact that the soil below the dripper is not cultivated and the lime does not actually move down into the soil horizons. A constant addition of the basic cations is essential to retain optimal balance in the soil. Fertilisers that contain calcium are therefore a better option to counter the acidifying effect of ammoniac fertilisers.
Irrigation by means of drippers is the most beneficial option for producers based on financial and water savings considerations, because nutrient deficiencies can be addressed easily and quickly and water-soluble fertiliser may be applied throughout the growing season (Photo 4).
PHOTO 4. Drippers commonly used in the Western Cape.
However, fertilisers that contain ammonium have a detrimental effect on sandy soils especially, due to soil acidity. If nitrate uptake by the plant is insufficient, the soil may acidify as a result of drip irrigation in the volume of soil that has been wetted, which restricts yield when managed incorrectly.
Under certain conditions and if applied to specific soil types liquid gypsum may be used to good effect, because gypsum is more soluble than lime and the excess salts (sodium and magnesium) are taken from the root zones, and the calcium can therefore move into deeper soil horizons. Gypsum is not always the correct answer, however, and should be carefully considered in conjunction with a soil scientist.
Gypsum dissolves more quickly than lime and only requires 25 mm/ha water to dissolve. It is applied either as a plant nutrient (calcium and sulphur) or to remove excess salts, such as sodium and magnesium, from the topsoil. With sandy soils there is a bigger risk that magnesium levels will decrease once the gypsum has dissolved. On heavier soils it has to be ascertained first that the soil enjoys good internal drainage before gypsum is applied. Soil samples should be taken regularly to control the soil status, in view of the fact that each district, farm and soil type is unique and reacts differently.
Talk to a soil scientist to decide whether to apply gypsum, calcitic lime or dolomitic lime for your specific blocks. Also remember that calcium and magnesium in lime improve not only the pH, but also the accessibility of other elements, such as potassium.