THE INFLUENCE OF POTASSIUM FERTILIZATION ON MICROORGANISMS IN THE SOIL UNDER SPRING BARLEY

The content of potassium (K) in soil in relation to the content of oth er nutrients is a very important factor for plant nutrition. The avail ability of K for plants is influenced not only by various physical and chemical properties of soil, but also by soil microorganisms. On the other hand, the amount of K and its proportion to other nutrients rece ived influences the amount and the quality of plant root exudates, whi ch can affect the composition and activity of microbial communities. I n the current study, the effect of two different K-doses on soil and r hizosphere microorganisms in spring barley was studied in a field expe riment. The experiment was performed in a site near Vodnany, South Boh emia, on a cambisol soil type. Two levels of K were applied - 80 kg (w hich represents an usual dose) and 200 kg K/ha per year, in the form o f 60 % KCl. To investigate the reaction of microorganisms, soil and pl ant, the samples were repeatedly taken and the content of nutrients, p hysiological groups of microorganisms, microbial biomass and nitrogena se activity were determined. A higher dose of K-fertilization resulted in a higher level of the available K in soil and in a higher K-uptake by barley plants. This was reflected in the elemental composition of the above-ground plant biomass containing more K, N and also P in comp arison with that from a less fertilized plot. The amount of microorgan isms of various physiological groups increased in the expected sequenc e: soil between the rows of plants - soil in the rows - rhizosphere so il, without an unambiguous relation to the K-dose. The microbial bioma ss significantly decreased as influenced with higher K-dose. There wer e, however, no significant differences between the soil taken in the r ows and that taken between the rows of plants. This shows a direct neg ative influence of K on soil microbial biomass in given soils. The amo unt of N2-fixers insignificantly increased in the soil taken in the ro ws as well as in the rhizosphere soil from the plot with a higher K in put. The opposite effect of K on nitrogenase activity was observed: so il (taken in the rows of plants) fertilized with more K had a much low er nitrogenase activity than that fertilized with a lower K-dose. This negative effect of K was very much pronounced mainly during May and J une, that is during the period of the intensive growth of plants. The effect of a higher K-fertilization on some soil microbial parameters, e.g. repression of nitrogenase activity, decrease of microbial biomass and the changes in the amount of microorganisms of various physiologi cal groups indicate that K can influence soil microorganisms directly and/or indirectly through plants.