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Fluid Journal : Fluid Journal 1993-1995
2 Fluid Journal Late Spring 1993 Table 1. Fertilizer treatments applied (1988 to 1992). 1 None 2 N 40 lbs/A Urea or UAN1 3 B 0.4 lb/A Soluble Sodium Borate 4 Mg 0.25 lbs/A Epsom salts 5 N+B 6 N+Mg 7 B+Mg 8 N+Mg+B 9 Mn 0.15 lb/A Mn EDTA Several different soybean cultivars were examined over four years. Plots were sprayed in the early evening to maximize the length of absorption time and minimize foliar burn. Spray volume was 25 gal/A 1 Low biuret urea used for sprays and UAN in some fertigations. Table 2. Greatest and mean ( ) annual yield increases from late-season N and B applications. Application 1988 1989 1990 1991 Fertigate N 9 (5) 6 (3) 5 (2) 2 (0) Fertigate N+B ------ ------ 6 (2) 3 (0) Dribble N ------ ------ ------ 5 (0) Dribble N+B ------ ------ 6 (1) 3 (0) Dribble B ------ ------ ------ 5 (0) Spray N ------ ------ 4 (2) 6 (0) Spray N+B ------ ------ 2 (1) 7 (3) Spray B ------ ------ 3 (1) 3 (1) emphasis was placed on sprays in the 1992 studies. Economic response Determining the probability of receiving an economic response to N and/or B application can only be accomplished by analysis of results over several years. In general terms, the costs for ferrigated and spray applications should be less than $10/A. Therefore, yield increases of two bushels/A or greater should be economical. Such response was obtained in each of the four years we have fertigated N on soybeans at R3 to R5. Economic responses were also obtained over the last three years from applications of B and combinations of N and B---both fertigated and sprayed. The problem with recommendation of these applications is that mean increases often have not been economical (Table 2). In particular, cultivars have been an important response variable. However, improved application technology, developed during this research, has contributed to economically significant results obtained in 1992 (Figure 1). Using this technology, we are optimistic that economically important results can be attained. High N demand Soybean plants have high nitrogen requirements for seed production. The major part of nitrogen is accumulated in the seed during the pod-filling stages. As much as 95 percent of the total N is found in the seeds at harvesttime. To satisfy this high nitrogen demand, N2 fixation increases during reproductive growth and a portion of the N is remobilized from vegetative parts. The proportion of the accumulated or remobilized N depends on the cultivar, the rhizobium strain and growing conditions. Nutrient shortage in the pod-filling stages may contribute to nutrient deficiency and early senescence. Duration of the grain- filling period is one of the main factors limiting soybean productivity. A well-established soybean rhizobium symbiosis may be effective in providing enough N for high grain yield, but a number of cultural factorsinfluence N2 fixation and can influence yield. So, the quantity of nitrogen in the soybean derived from atmospheric nitrogen near the end of the plant's growth cycle may vary substantially. The amount depends on soil N availability, cultivar, rhizobium strain, fertilizer applied and environmental conditions. A small quantity of combined nitrogen may promote overall accumulation as well as plant growth and yield. Soybean responses to soil fertilizer applications have not been consistent. The effect of N fertilizer application during vegetative stages on nodulation and N2 fixation is complex and variable. Large N applications have a tendency to decrease nodulation as well as N-fixation and have not generally resulted in significant increases. In recent years, many studies have shown foliar-N applied with other nutrients, such as boron, holds some promise. Foliar applications have resulted in leaf bums and inconsistent results. Studies on sandy soils in Georgia, however, have demonstrated that UAN or urea via sprinkler irrigation or dribbling at R3 to R5 showed no foliar bum and often resulted in yield increase. Studies in Missouri also suggest that yield may be enhanced by B applications either by infusion or foliar sprays. The yield increase was due to increased branching. Since fertigation is not a possibility in most soybean systems, it is important to develop foliar application methods as well as fertigation methods. Practice promising The following conclusions on the value of adding nitrogen to soybeans could be reached only after several years of compiling data on trials. 1. Spray application of N, B, Mg and combinations thereof during the reproductive stages of soybean growth, as well as fertigations and dribble applications, have resulted in significant and economic yield responses. 2. Responses greater than 5 bu/A have been limited to irrigated, sandy soils and responses on loamy soils have generally been less than 5 bu/A, in studies over a five-year period.
Fluid Journal 1996-1998