The soil exhaustion in the glasshouses is considered to be the consequence of a complex of several causes.
The decrease of fertility is said to be caused with the permanent roofing when the soil is isolated from the natural environment and so the changes of soil microflora occur.
The soil freezing is regarded as very important and it is considered better than any disinfection.
It is not possible, nevertheless, to regenerate the soil by means of these processes.
Inhibiting substances contribute to this soil exhaustion; these substances are produced by rhizosphere microflora of some actinomycetes and bacteria.
Such substances can also be secreted as root excrets or released from plant debris.
Changes of the physical properties of the soil are closely connected with microbiological processes in soil and with its exhaustion, and with unfavourable chemical changes as well.
Intensive cropping requires rich nutrient storage.
Our present stocks of organic fertilizers are restricted.
Crop nutrition may be influenced more actively and supplemented by mineral fertilizers.
Moreover, easily soluble salts and their nutrients are advantageous on the one hand and unfavourable too on the other hand.
Soluble nutrients within the reach of the root system can be easily washed off.
The individual nutrients are bound with unequal force within the soil and so their harmonic ratio in the nutri-culture medium is disturbed (Soukup, 1966).
The unfavourable properties of mineral fertilizers are improved by the production of more concentrated fertilizers and fertilizers without ballast.
It occurs very often in the practice that salt concentration and osmotic pressure of the soil solution are increased up to the toxic limit when higher rates of mineral fertilizers were applied.
This increased concentration is caused not only by nutrients, salts involving the "ballast" in fertilizers are also responsible for it (Drews, 1964). They can accumulate in soil as they are of lower availability to plants than the nutrient ions.
It is suggested that just these ballast substances in fertilizers, mostly represented by sodium, chlorides, sulphates and other substances, strongly influence the soil salinization which occurs as the most frequent evident cause of the soil exhaustion (Geissler, 1963, Balvoll 1965). Duchon (1948) notes that all mineral fertilizer components cannot be considered as a useless ballast.
Our trial was placed in a hangar glasshouse.
The trial equipment consisted of eight 150 x 100 cm of 100 cm depth concrete soil containers.
The firm concrete slanting ground leads to a shaft provided with a sieve.
The shaft is connected by a tube with the outer side of the container and leads into the room under the ground.
Thus the water surplus which went through the whole layer of soil could be taken for further analyses.
The following test plants were used: head lettuce - var. "Lednicky",