Magnesium deficiency can lead to premature fruit drop at harvest and while foliar sprays can correct in-season deficiencies – particularly as a result of a poor K:Mg balance - soil applied magnesium is the best long term strategy to ensure this element is not limiting. Peak magnesium uptake is during shoot development and at the beginning of berry development.
Nitrogen is needed at high levels during phases of rapid growth. However, too much nitrogen can encourage excessive growth and vigor. It can also result in shading and reduced fruit quality at later stages of growth. Nitrogen also needs to be available within the plant at the beginning of ripening for maximum accumulation in the grapes.
Where fertigation is not used, nitrogen is normally broadcast as early as possible in the spring, unless rainfall patterns are likely to lead to leaching. Nitrogen is used during berry development, and applied just prior to leaf senescence to provide reserves in the vine in order to boost early season growth in the following season. Fertigation is used to supply low rates of nitrogen throughout the season.
Minimal phosphorus maintenance programs, to ensure good root and vine development, are usually all that are required.
Potassium is generally needed in greater quantities than nitrogen – removal in the fruit of a relatively high yielding table grape crop can be nearly double that of nitrogen. South African experience is that Potassium-uptake can be over 200kg/ha, significantly more than that for nitrogen (Figure 5). However, care is needed as excessive use of potassium can lower magnesium and/or calcium availability.
Greatest potassium need is during fruit development and maturation, with the potassium important for carbohydrate synthesis. In dry growing seasons and where soil moisture levels are low, potassium may not readily be available to the vine, therefore foliar applications, particularly around veraison, are common practice.
Calcium is also needed in relatively large quantities of up to 150kg/ha. In many situations, it is equally important to nitrogen. Nearly 40% of calcium uptake is for leaf and branch growth and takes place between leaf emergence and the fruit set stage.
Early season use of calcium nitrate can supply both these needed nutrients for strengthening root development and producing new leaves. However, calcium sprays directly on the fruit, especially late in the season, are important to improve skin strength, and storage and handling characteristics.
While much lower levels of micronutrients are needed to satisfy yield and quality grape crop production, the correct balance of these micronutrient is essential. Leaf tissue analysis to assess micronutrient need, will enable deficiencies to be corrected or the effects of over-dosing to be minimized.
Boron deficiencies can seriously affect table grape production, by causing poor berry set, with large and small berries in the bunch (Hen and Chickens). Boron toxicity must be managed when irrigating in high boron soils by leaching pre-season. Copper is rarely needed as a result of its use as a foliar fungicide.
Lack of iron reduces leaf growth and resultant berry size and yield.
Zinc deficiencies can be a serious problem causing poor fruit set and stunted shoots with small, misshapen leaves. Foliar application or fertigation helps minimize in-season micronutrient deficiencies by quickly correcting the problem. Dry applied manganese and zinc, for example, can become complexed in the soil and be more slowly taken up.