The optimum amount of manure for long term application and adjustment of fertilizer use in thick high-humic andosols in the south-west warm region of Japan

The present study was to determine the optimum amount of organic manure to be applied to upland crop fields with thick high-humic andosols in the southwest warm region of Japan. The experiment was conducted for 8 years (1977-1985) in the forage crop fields in which corn was grown in summer and Italian ryegrass in winter. The highly processed manure was applied annually to each crop at a rate of 3 ton10a with or without chemical fertilizer of standard dose. The effect of successive applications of manure, enhanced plant growth and significantly increased dry matter yield, was first evident during the 6th crop in the 3rd year, that is, the growth of the manured plants was enhanced and consequently the dry matter yield increased significantly. From then on, the nitrogen dose of chemical fertilizer was reduced to two-thirds of standard, but no reduction in yield was observed. It was revealed that forage crop N-uptake originating from manure increased gradually each year and reached 8kg/10a in the 10th crop. This amount coincided well with the value estimated from the nitrogen-decomposition rate of manure which had been wrapped with cheese-cloth and kept in soil. Taking into account the nitrogen-decomposition rate of manure and the nitrogen requirement of forage crops, the amount of manure applied was reduced to 2 ton/10a for each crop. Judging from the nutrient balance in 2 ton/10a, fertilizer-N of two-thirds the standard dose seemed to be sufficient to maintain a desirable utilization rate and uptake of nitrogen. Fertilizer-K was also reduced, because the amount of potassium uptake from manure by the crops was essentially equivalent to the total amount of potassium in applied manure. Phosphorous fertilizer free cultivation after the 15th crop suggested that uptake efficiency of phosphorus released from manure was higher than that of phosphorous of fertilizer origin