Phytotoxicity of sunflower residues against some summer season crops

The growth of four summer season crops. namely Cyamopsis tetragonoloba, Sorghum vulgare, Pennisetum americanum and Zea mays, in fields with or without residues of the preceding sunflower crop was poor. Crop density, weight of seed or grain and total yield were significantly lower in sunflower fields than in the control fields (i.e. those without previous sunflower crops). Growth in terms or plant height and biomass was drastically reduced after 60 days. The effect was more pronounced in the fields where sunflower residues were allowed to decompose than in those where residues were completely removed. The soil collected from sunflower fields (both with and without residues) was found to be rich in phenolics, which in a laboratory bioassay were found to be phytotoxic. The reduced growth and yield or crops can be attributed to the release of phytotoxic phenolics from decomposing sunflower residues.