Comparison of the depth of placement of lime nitrogen on growth, N2 fixation activity, seed yield and quality of soybean (Glycine max (L.) Merr.) plants

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) in 0-10 cm soil layers, deep placement of lime nitrogen was conducted by applying at various depths (10cm [D10], 15 cm [D15] and 20 cm [D20]) the same amount (100 kg N /ha) in separate plots. sup(15)N-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) at D20 (69 g /plant) compared with D15 (59 g /plant), D10 (57 g /plant) and the control (48 g /plant). The visual quality of harvested seeds showed that lime nitrogen (20 cm depth and 100 kg N /ha) 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 (control), 37 g /plant (D10), 33 g /plant (D15) and 45 g /plant (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.