RATIONAL METHODS OF MINERAL FERTILIZING OF VEGETABLE CROPS GROWN IN GREENHOUSES
The fertilizer quantities, which ought to be introduced into the soil during the vegetation period, depend upon the uptake of nutrients by the plants and their needs of such. With greenhouse growing, the soil nutrient reserves are subjected to greater fluctuations, hence, they are to be checked at least once in four weeks (according to Göhler and Drews, 1967).
Greater, though not exceedingly high nutrient stock should be maintained in the soil. Excess quantities of nutrients can lead to plant damages. Investigations carried out over a number of years by Geissler, Kaufmann and Drews in Grossbeeren have resulted in the determination of the most favourable limits of application of the three basic nutrients, such as N, P and K, shown in table 1. These limits ensure sufficient nutrient quantities for the plants. Dressing must only recover the quantities already used by the plants. This rate of uptake of some of the more important vegetables is presented in table 2. Naturally, the mean values for certain yields are concerned here, which may greatly vary in accordance to the term of growth and the yields obtained.
Therefore, the rational dressing ought to contribute towards meeting the nutrient requirements for the time being as well as towards ensuring a continuous stock of nutrients which should be maintained within the admissible range throughout the whole vegetation period.
With nitrogen, these quantities amount to 5g, with potassium to 8–10g per square metre weekly. With phosphorus, the introduction of 11–13g per sq. m every four weeks has proved most effective.
Therefore, even with the continuous dressing with mineral fertilizers, a regular checking of the soil mineral content ought to be carried out, in order that proper fertilizer rates might be applied. Only by conducting such a control optimal yields and most rational utilization of the fertilizers can be attained.
Observing the proper dates of introducing the nutrients in accordance with the needs is also of great importance. These dates must be coordinated with the developmental phases and water requirements of the plants. Hence, the combination of dressing with irrigation results in an optimum supply of nutrients, provided it is properly carried out. The irrigation requirements for a defined period of time can be determined by moisture measurements by means of a tensiometer, or by measuring