Improving Yield and Nitrogen Fixation on Soybean in Thailand

Abstract

Attempts to improve the efficiency of N2-fixation by soybean-rhizobium symbiosis in Thailand has been made under specific agronomic conditions through assessment using the N-15 isotope dilution technique. Specific objectives were to evaluate the adaptable soybean cultivars together with effective strains of Bradurhizobium japonicum, develop management practices and minimize environmental constraints. The series of field experiments were conducted at Chiang Mai, Khon Khan, Kampang Saen and seven farmer fields in the north, central and north-east of Thailand to evaluate (1) the appropriate non-N2-fixing crop to quantity nitrogen fixation by soybean (2) the ability of Thai soybean cultivars to support symbiotic N2 -fixation (3) selection of the effective of B. japonicum under different conditions (4)effect of herbicide (5) water application (6) ritrogen application (7) phosphorus application on N2 fixation and (8) testing of selected B.japonicum strains in farmer's field. The results showed that : (1) The two Japanese non-nodulating soybean lines Tol-o and A62-2 appeared to be the most appropriate non-N2-fixing control plants for 15N-isotope dilution method. Maize proved to be another appropriate non-N2 -fixing crop at some sites. Sorghum, wheat and the US non-nodulating line were not appropriate non-N2-fixing control plants. (2) The recommended Thai soybean cultivars showed considerable potential in their ability to support N2 fixation average 44% or 10 kg N/rai of the soybean plant's nitrogen. (3) The best strains for combination of yield and N2 fixed are Niftal (TAL) 411, 431 61A148, THA2 and THA7. The latter two strains were isolated from Thai soil. (4) Evaluation of the application herbicides showed no detrimental effects on seed yield or benefits of N2-fixation except paraquat at some sitedecrease N2-fixation. (5) Similar yield and N2-fixation was obtained by watering soybeans at weekly or biweekly intervals but these were significantly reduced when water was applied only when wilting symptom occurred:(6) Addition of N fertilizer to soybean in the soil which contained indigenous rhizobium or soil which primary grown soybean must applied at low rate otherwise nitrogen applied might decrease the amount of N2 fixed and seed yields. However, at new introduced soybean area with no indigenous rhizobium addition of N resulted in increased N, fixed and seed yields. (7) Phosphorus fertilization at rate 8 kg P2O5/rai increased in double times both yield and N2 fixation of soybean at pod-filling stage. (8) The inoculum of the selected trains under this research program were proved high effectiveness in farmer's field condition. With Rhizobium inoculation can increase yield 40-120 kg/rai, particularly with the most appropriate field management. Inoculants can be produced commercially using these selected strains of rhizobia. The inoculation of rhizobia to soybean are strongly recommended for high N2 fixation and yield production.