Our latest trial results indicate that autumn applications are best for tree health and fruit quality. We need to measure which nutrients are needed, decide which commercial fertilisers are most suitable, how much of each to apply and when to apply it. In this article, we discuss the research we have carried out during the past three years on nitrogen fertilising of citrus. Although nitrogen is only one of the elements affecting tree and fruit growth, its misuse can have huge effects and we considered it the most important starting point for our studies. We present some interesting data from trials on satsuma mandarins and oranges which can assist in making nitrogen fertilising as efficient as possible.
Most growers can recognise the overall pale green colour of a nitrogen deficient citrus tree.
Very often, it is hard to avoid this symptom in the rows closest to the shelter belt, but when a complete block begins to look short of nitrogen, then yield losses will already have been incurred. This principle is true for all nutrients - if you can visually detect a deficiency, then the tree will have lost some production. A sample of leaves is picked in late February and sent to a laboratory where the level of each nutrient is determined. These results are then compared to a set of standards which have been determined over many years and confirmed in many countries as being ideal for tree performance. Each important element is described as being deficient, adequate or excessive, and a fertiliser programme to correct the imbalances can then be planned.