Agronomic and Economic Effects of Compost on Maize: The Importance of Compost Composition, Price and Rate.

Application of organic manure in crop production has been strongly encouraged in many places including Thailand but often without consideration on its quality, soil properties and economic evaluation. A field experiment, employing a randomized complete block design with three replications, was carried out in Thailand with maize grown on a Red clay soil (Rhodic Kandiustox) with the objectives: (1) to examine effects of 5 successive annual applications of mineral fertilizers, compost and stubble removals on soil properties and cumulative yields; and (2) to evaluate the economic viability of different treatment combinations. The treatments were: 4x4 factorial combinations of two factors [(1) mineral fertilizers at 0-0, 31.25-31.25, 62.5-62.5 and 125-125 kg N-P₂O₅ ha^-1yr^-1; and (2) compost (0.53-0.59%N, 0.20-0.31%P, 0.55-1.05%K and 3.55-7.33% organic carbon) prepared from bagasse at 0, 1875, 3750 and 7500 kg ha^-1 yr^-1].

Stubble management, compost and mineral fertilizer and their interactions did not have significant effects on soil bulk density, available water capacity, total N and extractable N. However, without compost application but with stubble return, the highest rate mineral fertilizer increased soil organic matter (SOM) whereas with compost application or stubble removal it did not affect SOM. Extractable P in soil was increased by mineral fertilizer regardless of stubble management and by compost but was not affected by stubble management. Increased mineral fertilizer up to the highest mineral fertilizer rate increased cumulative grain yields and cumulative shoot dry matter regardless of stubble management whereas compost did not affect them. Economic evaluation showed that mineral fertilizer at the rate 125-125 kg N-P₂O₅ ha^-1 yr^-1 with stubble return gave the highest net profit whereas the highest rate compost without mineral fertilizer gave the biggest loss. DSSAT simulation of yield indicated that compost would not only be as profitable as mineral fertilizer if the nutrient concentrations are 3-4 times higher than the present compost or if the compost price is greatly reduced. The suitability of compost is a fertilizer replacement for maize depends on composition, rate of application and price.

Keywords: compost, DSSAT, maize, mineral fertilizer, profitability, soil properties, yields