AEROBIC DECOMPOSITION OF ORGANIC WASTES .2. VALUE OF COMPOST AS A FERTILIZER

Pot experiments in 20-1 cylindrical pots and in rectangular plastic containers 40x60 cm and a soil depth of 40 cm were used to evaluate different kinds of compost. The results are related to the chemical reaction occurring by decomposition of the organic compounds and to chemical analyses of the compost. A good aeration during decomposition of the organic compounds promotes the conversion of carbohydrates into carbon dioxide and water. An incomplete aeration may result in accumulation of acetic acid, thus giving trouble to plant growth if the compost is incorporated into the soil. Surface application of the compost minimized the trouble. Proteins and amino acid are converted to ammonia and microbial protein. When no action is taken to collect the ammonia it will be lost through volatilization. The microbial protein is found in the soil fraction, biomass. The plants cannot utilize the nitrogen in the biomass until it is mineralized into ammonium and nitrate nitrogen. For that process a propagation of microbes in the soil is necessary and caused a delay in the utilization of nitrogen. A proper relation between available carbon and nitrogen enables a mineralization. If the available content of carbon relative to nitrogen is high, the microbes absorb mineral nitrogen from the soil solution promoting nitrogen deficiency in the plants. That biological nitrogen fixation is probable if acetic acid and other water soluble compounds are accumulated in the compost. A surface application of compost minimizes the risk of biological nitrogen fixation. The risk of ammonia volatilization in the field is very little as ammonia is converted into biomass during decomposition. The fertilizing value of the phosphorus and potassium content in compost is normally high as that of fertilizers. However, both the water solubility of phosphate and the fertilizing value decrease it the storage time of the compost continues until the nitrate accumulates. Due to a high solubility of most nutrients in compost the fertilizing value of the compost is nearly as good by surface application as by incorporation into the soil. By using compost in moderate quantities on the same soil for 8 years the soil structure was markedly improved, especially when the compost was applied on the soil surface and after sowing of the crop. The surface was thereby protected against the compressing effect of rain drops and a rapid drying up afterwards. Water infiltrated into the soil much faster where the compost protected the soil. This was true even with a thin layer of compost.