With foliar fertilisers, the nutrients are applied directly to the leaves of the crops. This type of fertiliser is particularly suited to quickly alleviating nutrient deficiencies that occur for a short time. A temporary lack of nutrients arises principally due to:
Plants principally absorb the nutrients they need through their roots. In the case of the main nutrients nitrogen (N), phosphorus (P), potash (K), magnesium (Mg) and sulphur (S), a foliar fertiliser can only be used to cover temporary peaks in demand. Unlike in the case of the trace nutrients boron (B), iron (Fe), manganese (Mn), molybdenum (Mo), copper (Cu) and zinc (Zn). Here the plants are supplied solely through the leaves and therefore foliar fertilisers can be used.
The absorption of nutrients via the leaves takes place through diffusion. This assumes that the nutrients are available in a dissolved form. In the case of water-soluble salts, like those contained in EPSO products and non-polar substances (chelates, urea, etc.), this is so. Suspensions are relatively unsuited to a rapid absorption via the leaf, as the name suggests their production elements are suspended rather than dissolved, such as magnesium hydroxide or elemental sulphur which are finely ground and mixed with water. The nutrients contained in suspensions are therefore not water-soluble or insufficiently so, and consequently barely usable for the plant.
The prerequisite for the diffusion of nutrients into the plant is a sufficiently moist leaf surface. To prevent the drying out of the sprayed film on the leaf surface, spraying should preferably be carried out in the evening. The nocturnal formation of dew keeps the plants moist and enables a good absorption of nutrients and a high level of effectiveness. Already dried-out spray is again dissolved by the formation of dew and diffusion into the leaf continues.
Wetting agents can be used to optimally wet the leaf surface. These are often included in plant protection agents as a formulation aid. As the application of foliar fertiliser is normally carried out together with the application of fungicides or insecticides, the producer's recommendations regarding the concentration and miscibility of the components should be followed. Small mixing tests are also to be recommended.
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