A field experiment was conducted during 1992-2004 on Luvisols (formed from silty light loam) at the Experimental Station at Bacyny to investigate the response of the most important plants cultivated in Poland to monocultural cultivation. Twelve plant species were cultivated in crop rotations: (A) potato-oats-flax-winter rye-faba bean-winter triticale; (B) sugarbeet-maize-spring barley-pea-winter rape-winter wheat, and (C) each of the species in monocultures. The organic matter content in the soil plough layer (0-250 mm) in 1992, 1998 and 2004 is presented. Fractionation of humus was carried out with the Duchaufour and Jacquin method. Under the conditions of the crop rotations, organic matter content slightly increased; similar tendencies were observed in most monoculture fields. The lowest content of organic C was recorded under the conditions of continuous cultivation of pea, maize and faba bean. Humins and humus compounds (extraction II and III) had a dominant share among the determined fractions. Humic acids prevailed over fulvic acids in the crop rotation cultivations and in the most monoculture fields. However, the prevalence of fulvic acids was found under the conditions of continuous cultivation of sugarbeet, potato and winter rye.
