Soil and Leaf Samples

Although soil samples can be taken at any time, except after an application of fertilizers, it has little value in diagnosing or confirming a nutrient deficiency symptom. In commercial citrus production, for instance, fertilizers are applied from July to December and, in certain cases, as late as February. Soil sampling is therefore only reliable between February and June, which is the time to take soil samples to formulate the fertilization program for the coming season.

In annual crops, the timing of soil sampling depends on whether double cropping is practised or not. Double cropping implies the cultivation of one crop in the autumn/winter and one in spring/summer in succession to each other on the same field. In this case, soil sampling can only be done after the previous crop has been harvested. The time before planting the next crop is often less than two months and sampling the soil in good time is of the essence. If only one crop is being produced on the field, then there is more time, but soil sampling should commence shortly after the existing crop has been harvested, in preparation for the following crop.

There are specific prescriptions for leaf sampling for individual crops. Consult with a fertilizer consultant and the analytical laboratory on the procedure for the specific crop you are working with. A specific example is that of leaf sampling in sunflowers to detect a B deficiency. According to the guidelines, the uppermost mature leaf, without the petiole, has to be taken. If the leaf sample was taken a month after planting, it should contain at least 60 ppm B, while a sample taken at the onset of flowering should contain at least 40 ppm B.

Norms to evaluate the results of leaf analyses are developed for a specific leaf type, at a specific position on the plant, taken at a specific physiological stage. This is called the diagnostic leaf. Deficiency symptoms seldom develop on these leaves. Deficiency symptoms mostly develop on young or old leaves and reference norms are usually not available for the symptomatic leaves. Leaf analyses therefore sometimes fail to detect the deficiency and taking leaf samples have limited value. Other factors such as total biomass produced can complicate the interpretation of the results.