A new fertilization method with deep placement of slowrelease N fertilizers, such as coated urea and lime nitrogen (LN) (calcium cyanamide) at 20 cm depth was found to promote soy bean seed yield. In the present study, the effect of deep placement of LN was investigated on different parameters such as growth, N accumulation, N 2 fixation activity and yield of soy bean by applying LN at different rates in the rotated paddy field of Niigata, Japan. In addition to the basal fertilizer, ammonium sulphate (16 kg N ha À1 ), deep placement of LN was conducted by applying various amounts such as 50 kg N ha À1 (A50), 100 kg N ha À1 (A100) and 200 kg N ha À1 (A200) at 20 cm depth in separate plots. A 15 N-labelled LN fertilizer was also employed for each of the above treatments to calculate N utilization from LN in separate plots. Soya bean plant growth and N 2 fixation activity were periodically analysed. Both plant growth and N accumulation were found to increase with LN treatment compared with control plants. An increase in N 2 fixation activity was found in the A100 plots. The total seed yield was the highest in the deep placement of LN with A100 (73 g per plant) compared with other treatments. The visual quality of harvested seeds also showed that A100 enhanced the quality of seeds compared with other treatments. Thus, it is suggested that N fertilization management with particular reference to optimum amount of fertilizers is important for maximum growth, N 2 fixation and enhancement of seed yield of soy bean.

It is well known that nodulation and nitrogen fixation of soybean are generally depressed by the application of N fertilizers. We previously reported that a new fertilization method consisting of deep placement of slow‐release N fertilizers, such as coated urea and lime nitrogen (calcium cyanamide) at a 20 cm depth enhanced soybean growth and seed yield. In the present study, the effect of deep placement of various 15N‐labeled N fertilizers (100 kg N ha−1) was compared in converted paddy fields located at Niigata Agricultural Research Institute, in Nagaoka. Fertilizers consisting of 15N‐labeled ammonium sulfate (AS), urea (U), coated urea (CU) or lime nitrogen (LN) were applied at a 20 cm depth for nodulated soybean (cv. Enrei) and for a non‐nodulated isogenic line (En1282). The plants without deep placement of N fertilizer were used as control (Cont) for both Enrei and En1282. Whole plants were sampled at maturing stage, and 15N abundance and N concentration in each part were determined. The evaluation of the amounts of Ndfa, Ndfs and Ndff was conducted by the 15N dilution method using En1282 as a reference plant. Also the amounts of Ndfa and Ndfs+Ndff were estimated by the N balance method for comparing Cont and the other treatments. In all the deep placement treatments, the non‐nodulated line exhibited only 36–40% of total dry weight and 16‐30% of total N accumulation compared with the nodulated line, due to N deficiency associated with the lack of nitrogen fixation. In both nodulated and non‐nodulated soybeans, plant growth and the total amount of accumulated N showed the highest values by the application of LN, followed by CU, U, AS and Cont, respectively. The value of the seed dry weight of Enrei was highest in the LN (73 g plant−1), followed by CU (63 g plant−1), U (47 g plant−1), AS (37 g plant−1) and Cont (26 g plant−1) treatments. The value of Ndfa estimated by the 15N dilution method was higher in the LN (3.6 g N plant−1) and CU (2.8 g N plant−1) treatments than in the U (2.3 g N plant−1) and AS (2.2 g N plant−1) treatments. In addition, the value of Ndfa estimated by the N‐balance method was also higher in the LN (4.1 g N plant−1), CU (3.9 g N plant−1), U (2.8 g N plant−1) and AS (2.6 g N plant−1) treatments than in the Cont treatment (1.5 g N plant−1). The recovery rate of N fertilizers in Enrei was higher in the LN (43%) and CU (36%) treatments than in the U (21%) treatment and lowest in the AS (8.5%) treatment. These results indicate that deep placement of LN and CU is effective to improve soybean growth and seed yield. The combination of “deep placement” and “slow‐release N fertilizers” is important to supply N until the maturation stage, resulting in the increase of seed yield.

We have shown in previous reports that a new fertilization method with deep placement of slow‐release N fertilizers, such as coated urea and lime nitrogen (calcium cyanamide), at a depth of 20 cm promoted soybean growth and seed yield. In the present study, the effect of deep placement of lime nitrogen (which gave the highest yield in our previous experiment) was compared at various depths in a rotated paddy field at the Niigata Agricultural Research Institute. The objective of the present study was to analyze growth, N2 fixation activity and seed yield, and to examine visually the quality of soybean seeds cultivated at different depths with the slow‐release fertilizer, lime nitrogen. In addition to the conventional basal application of mixed chemical fertilizer (including ammonium sulfate 16 kg N ha−1) in 0–10 cm soil layers, deep placement of lime nitrogen was conducted by applying at various depths (10 cm [D10], 15 cm [D15] and 20 cm [D20]) the same amount (100 kg N ha−1) in separate plots. 15N‐labeled lime nitrogen fertilizers were also used for each of the above treatments to calculate N absorption in relation to fertilization depth. Soybean plant growth was periodically analyzed and the quality of harvested seeds was also visually examined (hereafter referred to as “visual quality”). The total seed yield was highest in the deep placement of lime nitrogen (100 kg N ha−1) at D20 (69 g plant−1) compared with D15 (59 g plant−1), D10 (57 g plant−1) and the control (48 g plant−1). The visual quality of harvested seeds showed that lime nitrogen (20 cm depth and 100 kg N ha−1) enhanced the quality of seeds compared with the other treatments, in which the good quality seeds (hereafter referred to as “good seeds”) based on dry weight were 24 g plant−1 (control), 37 g plant−1 (D10), 33 g plant−1 (D15) and 45 g plant−1 (D20). Thus, it appears that N fertilization management with particular reference to the depth of fertilizer placement is important for maximum yield and enhancement of seed quality of soybean.

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