THE EFFECT OF POTASSIUM-IONS ON BORON FIXATION

As a result of intensive fertilization with K-fertilizers in last 15 - 20 years an amount of potassium in soil considerably increased. At th e same time structure changes of ploughed layer were observed - a fine -lumped and a lumped structure started to disintegrate and to change i nto powdered one. Owing to this the topsoil of our most fertile cherno zem became a castive and it had a negative influence upon the plant gr owth. The disintegration of topsoil structure was mostly ascribed to t he negative effect of heavy mechanisms, especially wheeled, replacing work of animals and light wheel tractors in a large-scale production. M i k l o s , C i c e l (1992) contrived that this effect is secondary . Owing to the potassium ion fixation and also in a lesser degree to t he ammonium ion fixation, mineralogical changes in soil proceeds - mon tmorillonite is gradually changed into illite. In this way physical pr operties of the topsoil changed and the effect of heavy mechanisms jus t accelerates the lumped structure disintegration. It was demonstrated by laboratory experiments and practical observations that the boron a mount available to plants is reduced paralelly with the montmorillonit e structure change - arising illite is boron fixing. We found out that the highest abundance of heart rot was observed in regions with high and very high contents of potassium available to plants. There is the negative correlation between the amount of water-soluble boron (by Ber ger and Troug) and the amount of available potassium (by Schachtschabe l). In laboratory conditions we followed up the potassium influence up on the boron fixation by bentonite (with the content of more than 60 % of montmorillonite) and by the natural soils collected in the Nove Za mky district. We found out that the fixation is highly dependent upon the sequence of components added to the system (bentonite - water, soi l - water). We confirmed the data presented in literature that boron i s adsorbed not only on the mineral surface, but it is also entering th e crystal lattice. After addition of potassium (ammonium) the interlay er space contraction appeared. Owing to this the extraction of boron w as prevented - boron is fixed and becomes unavailable for the plants. This process was intensified by repeated drying and wetting of montmor illonite (soil). Our experiments confirmed the negative influence of p H increase upon the extractibility of added boron. In practice it is n ecessary to expect that 10 - 25 % of boron amount taken into soil can be fixed and will not be available for the plants according to the act ual soil and climatic conditions.