The growth of soybean (Glycine max (L.) Merr.) under saturated soil culture (SSC) has potential for use in the rice-based agriculture of the humid tropics. Higher yields achieved with this system could be due to increased nodule activity (N₂ fixation) and to delayed nodule senescence. We report a study to assess SSC under wet-season conditions in northern Thailand, and to quantify N₂ fixation so that N budgets could be constructed for the eight treatments (two irrigation systems, SSC and CI (conventional irrigation); two cultivars of soybean, SJ5 and NW1; two levels of fertilizer N, 0 and 50 kg/ha).

Largest effects of SSC were on nodulation; they were evident at the first sampling (V4) and remained throughout growth. Nitrogen fixation activity reflected the patterns of nodulation. The SSC plants fixed more N₂ during early growth because of their reduced uptake of soil N even though accumulation of total N was less. Thereafter (R3 to R7), N₂ fixation by SSC plants was greater than for CI plants both because they derived a higher proportion of crop N from N₂ fixation (Pfix) and had increased accumulation of total N. By physiological maturity, the SSC plants had fixed, on average, 44% more N than plants under CI. Overall Pfix values ranged from 38 to 74% and N balances (fixed N-seed N) based on harvested shoots ranged from −55 to + 25 kg N/ha. Balances for the SSC and CI treatments, averaged over cultivar and fertilizer N, were zero and −34 kg N/ha. Effects of SSC versus CI on the accumulation of above-ground dry matter and on production of seed were insignificant (P>0.05). Fertilizer N did not affect seed yield, but reduced significantly N balances through suppressive effects on nodulation and N₂ fixation.

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