The main competitors of cotton during the crop production are annual late-spring weeds with some perennial species representing the second most important group. During the 1999-2001 period the effect of 3 main weed species – namely Solanum nigrum L., Amaranthus retroflexus L. and Convolvulus arvensis L. on the soil moisture in 3 layers (0-10, 20-20 and 20-30 cm) was studied at the Cotton and Durum Wheat Research Institute in Chirpan. Seven variants of increasing population density of different species were evaluated and the moisture content of the soil was calculated by weighing samples in the phases of square formation, flowering and maturity of the crop. Two-factor dispersion analysis (ANOVA) was used to determine the significance of the differences between treatments. Soil moisture content was found to be affected at two stages (flowering and maturity) by both weed density and soil layer. In the phase of square formation no differences were observed between moisture content of the three studied soil layers (0-10, 10-20 and 20-30 cm).
