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Fluid Journal : Early Spring 2011
9 The Fluid Journal Early Spring 2011 mineral N uptake during the legume growth cycle. Although controversial, N applications to soybeans could be a possibility to improve grain yields. Although many studies have not shown responses, several studies have indicated positive Summary: Given the development of fluid fertilization and availability of fluid application equipment in particular, there is the possibility of delivering specialty fluid nitrogen (N) fertilizers by dribbling or knifing at a phenological moment when soybean canopy is small enough to allow the traffic of terrestrial applicators without jeopardizing the proper biological fixation mechanisms. A study of using slow/controlled release fertilizers is therefore in order to determine a timing of N availability that offers better synchronization with N demand. Better synchronization with N demand sought via better timing of N availability to crop. A Look At Increasing Late N Availability In Soybeans The Fluid Journal • Ofﬁcial Journal of the Fluid Fertilizer Foundation • Early Spring 2011 • Vol. 19, No. 2, Issue #72 Dr. Ricardo Melgar Nitrogen requirement of legumes can be met by both mineral assimilation and symbiotic N fixation. The plant N requirement may not be met during early vegetative and later productive phases by N2 fixation. Symbiotic fixation begins only after nodule formation, which is preceded by the colonization of the rhizosphere and the infection of legume roots by Rhizobium. Thus, mineral N may be a critical source of N for grain legumes during both the early vegetative and late reproductive periods. The period of high N requirement for soybeans is from the R3 to R6 growth stages. A 1974 study reported that 25 to 60 percent of the N in a mature soybean comes from N fixation and the other 40 to 75 percent comes from the soil. The contributions of symbiotic and mineral N sources to total plant N are determined by legume N requirement and mineral N supply, provided an effective Rhizobium symbiosis is ensured. When mineral N uptake is less than the N requirement, N2 fixation potential can be considered to be equal to the aggregate of per day deficits in crop. Under Argentinean conditions, broadcasting urea is difficult because of lack of appropriate machinery. Spreaders of large working capability are not common. Other sources like ammonium nitrate are prohibited by law. However, some increasing use of UAN and NS solutions is occurring among farmers. Fluid potential Given the development of fluid fertilization and availability of fluid application equipment in particular, there is the possibility of delivering fluid N fertilizers by dribbling or knifing at a phenological moment when the soybean canopy is small enough to allow the traffic of terrestrial applicators. However, that moment may be too early if ready- available applied N stops or slows down severely the symbiotic fixation process. Further product development by industry may help to improve allocation of fluid N products at a time of planting or shortly afterwards when farm equipment can move over fields. Excepting fertigation, it would be more cost efficient than foliar or aircraft "N on Soybeans Could Improve Yields" responses. Nitrogen applied at the R1 to R5 growth stages has been shown to increase soybean yields. A study in Kansas reported significant yield increases to N fertilizer supplied as urea or UAN in soybeans under irrigation. In the Pampean region of Argentina, we had some small but consistent responses to ready-available N applied by hand at R1 (data not presented). While evidence exists for late application responses, operationally it is difficult to manage when fertilizer has to be applied on a dense soybean