CHEMICAL TRANSFORMATION, PHYTOTOXICITY AND NUTRIENT AVAILABILITY IN PROGRESSIVE COMPOSTING STAGES OF WHEAT-STRAW

Chemical maturity parameters in addition to plant growth limiting fact ors have been monitored in the course of a 2-month composting experime nt. Wheat straw with 5% dry w horse manure was adjusted to C/N = 45 wi th urea. The pile was rotated and homogeneous samples were taken every four days. The most intense changes in straw fractions occurred in th e first 20 days of composting, as suggested by wet chemical analyses, thermogravimetry and C-13 NMR spectrometry. Nevertheless, plant respon se to compost application gave significant changes at between 20-60 da ys that were not clearly reflected by the above techniques. Glasshouse experiments with a soil treated with compost samples taken at the suc cessive 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, referre d to as postmature composts, the ryegrass yield did not depend on most of the organic matter characteristics, but closely paralleled the con centration of available nitrogen and - to lesser extent - phosphorous in the compost. The probable immobilization of these elements in the c ourse of composting was also suggested by plant response experiments w ith different doses of compost and the addition or not of mineral solu tion.