Crop nutrient requirements
The amount of fertilizer required depends foremost on the total quantity of nutrients removed by the crop from the soil or substrate. In order to achieve optimum yields, each crop should have at its disposal the pre- determined amount of nutrients. These conform the basis of all fertilizer recommendations.
Soil fertility analysis
The soil is the source from which the crop retrieves the mineral nutrients it requires. Therefore it is necessary to check the availability of these nutrients in the soil on which the crop is to be grown. This is done by a soil fertility analysis.
It is recommended to perform the analysis prior to seeding / planting the crop, and, if required along the season as well. The analysis is performed on soil samples that truly represent the plot on which the crop is to be grown. Soil samples are 1.5 – 2.0 kg in weight and are formed by mixing samples retrieved with a special soil- sampler while walking the field in zigzag. For most purposes, the upper soil layer (0-30 cm.) is sampled.
In greenhouses and detached media the soil solution is sampled (by means of special suction devices). Alternatively, the drainage water may be sampled.
Soil fertility analysis is performed to determine the availability of nitrogen, phosphorus and potassium to the crop. Also the pH and the electrical conductivity (EC) of the extract from the saturation paste are determined. (This is a measure of soil- salinity). Sometimes an additional analysis for micro- elements is required.
Plant tissue analysis
Plant tissue analysis is recommended for perennial crops, mainly orchards and vineyards and for some annuals. The leaf blade, the petiole, or another organ, is analysed for its nutrient content. This analysis indicates the quantity of nutrients the plant has been able to retrieve from the substrate under the growing conditions provided by the grower.
The grower may also resort to visual observation of his crops in order to discover any nutrient deficiency or disequilibrium along the season. Each deficiency manifests itself in a manner characteristic of each crop. Whenever such symptoms are identified, remedial fertilization measures should be undertaken.