Chemical transformation, phytotoxicity and nutrient availability in progressive composting stages of wheat straw

Chemical maturity parameters in addition to plant growth limiting factors have been monitored in the course of a 2-month composting experiment. Wheat straw with 5% dry w horse manure was adjusted to C/N = 45 with urea. The pile was rotated and homogeneous samples were taken every four days. The most intense changes in straw fractions occurred in the first 20 days of composting, as suggested by wet chemical analyses, thermogravimetry and C-13 NMR spectrometry. Nevertheless, plant response to compost application gave significant changes at between 20-60 days that were not clearly reflected by the above techniques. Glasshouse experiments with a soil treated with compost samples taken at the successive transformation stages suggested no linear correlation between composting time and the potential of compost in improving plant yield. In the samples taken after 20 days in the conditions studied, referred to as postmature composts, the ryegrass yield did not depend on most of the organic matter characteristics, but closely paralleled the concentration of available nitrogen and - to lesser extent - phosphorous in the compost. The probable immobilization of these elements in the course of composting was also suggested by plant response experiments with different doses of compost and the addition or not of mineral solution.