It is far better to start off having things as correctly in place as possible before planting, rather than trying to rectify mistakes or imbalances later on. It’s essential to know the soil’s potassium, calcium and magnesium status and its condition. Cations need to be sufficient and in balance with one another. Plants remove these cations from the soil in proportion to their availability. Too much of one means more of the others have to be added to achieve a balance.
One can get a calcium deficiency, for example, even if the soil level of calcium is adequate but the other cations are in excess. Calcium is a cation that needs to be put in balance before planting. That can be done by adding lime or gypsum or both, depending on the advice you get from the company supplying the fertiliser. When potassium is on the low side, it can be corrected by applications of nitrogen during growth. Add potassium nitrate if it’s expedient to do so. Very often a farmer would start off with an adequate balance and be surprised to have a calcium deficiency develop at a later stage.
The problem is that the balance can change as the crop develops and the plants make use of the available elements. It’s customary to push up potassium levels as the fruits start reaching a fair size. That can increase fruit quality and size, but the extra potassium often throws the balance out. hat is especially true in crops that are drip-irrigated. Only a restricted area ends up wet and that’s generally where all the action takes place. The roots will concentrated there if the plants are growing in dry conditions. The wet area represents a relatively small area of the total land area resulting in an ability, by a relatively small amount of fertiliser, to upset the balance.
An imbalance can also result from other irrigation water. If borehole water is being used, there’s a good chance of a high calcium load if the borehole is located in a dolomite area. Again, if drippers are used, changes would manifest more rapidly and the extra calcium could raise the soil pH in the dripper area, causing an iron deficiency. Iron becomes less available as the pH rises. On many occasions I have seen how such a deficiency results in young leaves developing a yellowy-white colour. part from calcium, a magnesium deficiency can also develop later in the crop’s growth cycle when potassium is added. You could anticipate that and look for symptoms . Chlorosis would often start at the base of the plants. Fertiliser on a tomato crop needs monitoring throughout the crop’s development. It requires that you know what to look for and anticipate possible deficiencies. – Bill Kerr ([email protected] or call (016) 355 0616).