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Fluid Journal : Winter 2016
agronomic inputs (Ruffo et aI., 2015) Biomass and grain production are significant factors influencing total nutrient accumulation (Ning et aI., 2012), and would presumably influence fertilizer recovery. Nutrients with high harvest index values (i.e., high relative partitioning of total nutrient accumulation to corn grain) typically require soil availability for a longer duration in combination with unique remobilization tendencies to supply grain with needed nutrients (Bender et aI., 2013). When combined with the increased yield responses observed at high populations (Table 3), this may at least partially explain the greater recovery efficiency of Nand S with increasing densities (Table 4). At the highest planting population, the increase of Nand S recovery efficiencies represents an additional 13.2 Ibs Nand 1.0 Ibs S on a per acre basis. Potassium accumulation occurs during a relatively abbreviated period before flowering and has a low nutrient harvest index , which may explain the limited effect of population level on K recovery efficiency Soybean yield. Significant sources of variation for grain yield in soybeans included fertigation (P = 0.001), variety selection (P < 0.001), and the interaction between variety and fertigation (P < 0.063). Fertigated soybean yields ranged from 60.2 to 68.1 bu/A (Table 5) with an average yield response of +2.3 bu/A (P = 0.001) over the irrigated treatment. The effect of relative maturity as a function of variety selection did not influence the response to fertigation, though we believe the highest yield potential requires a well- managed responsive soybean variety that is planted as early as permissible for the region. Findings from the current study suggest that six of the 15 varieties had significant yield increases from fertigation averaging 4.8 bu/A, and subsequently were classified as 'Responsive' (Table 5). The remaining varieties were classified as 'Non-Responsive' to supplemental fertigation. Individual seed mass was most consistently increased with fertigation (+3.7 mg/seed; P = 0.002), unlike seed number, which varied among varieties (data not shown). Understanding an individual variety's genetic predisposition for tolerance to intensive management may have far-reaching implications. Hybrid management evaluations in corn using varying N fertilization rates and planting densities have been Winter 2016 used by the University of Illinois Crop Physiology Laboratory (Haegele and Below, 2012) to classify a hybrid's responsiveness to crop management. Further insight allows producers to position specific hybrids in certain field locations and/or environments for optimal crop performance. Because of the indeterminate nature of soybeans and its compensatory yield component responses, classification of soybean varieties for their responsiveness to agronomic management has been especially difficult. Findings from this research suggest that responsiveness to supplemental fertilization may be used as a proxy for tolerance of additional agronomic practices. Interestingly, a variety's magnitude of yield response to fertigation in the current study was at least partially predictive of its responsiveness to other agronomic management factors (e.g., fertilization, row spacing, crop protection products, etc.) in separate university trials (Bender, 2015), highlighting the importance of proper variety selection. Biomass & nutrient recovery. At physiological maturity (i.e., R8) a random subset of soybean varieties (n = 6) were sampled for measurement of biomass and nutrient accumulation. Varietal differences influenced total biomass accumulation , which ranged from 7,436 to 8,114 Ibs/A in fertigated conditions (Table 6). Of sampled varieties, four were considered 'Non-Responsive' and two were described as 'Responsive' to the fertigation treatment. Only yield-responsive varieties measured significant increase in biomass accumulation (+411 Ibs biomass/A) with fertigation. The increased total dry weight in yield-responsive varieties also resulted in greater N (+23 0 /0), K (+3 0 /0), and S (+9 0 /0) recovery efficiencies compared to the non-responsive varieties (Table 6). The fertilizer industry and sustainability efforts promote increased nutrient use efficiency of applied fertilizers through best management practices such as the '4R Nutrient Stewardship' program (Bruulsema et aI., 2012). Nutrient recovery efficiencies vary widely across crops, soil conditions, fertilizer application practices, and yield levels, though typically range from 5 0 /0 to 30 0 /0 across nutrients in soybeans (Bhupinder et aI., 2012; Bender et aI., 2015). In the current study, recovery efficiencies ranged from 3 to 44 % for N, -26 to 21 0/0 for K and 1 to 16 0 /0 for S (Table 6). The The Fluid Journal delayed planting in 2014, due to the SOl system installation, shortened the period of vegetative growth and likely reduced total biomass production, yield potential, and ultimately nutrient uptake. The recovery efficiencies of yield- responsive varieties, however, were two to three-fold greater than non-responsive varieties and further highlight how variety selection interacts with agronomic management. Furthermore, we believe that cultural practices that permit a more timely planting date and the use of a full complement of nutrients (i.e., phosphorus and other micronutrients) will further improve crop yield potential and recovery efficiency of applied nutrients. Summing up The unique attributes of an SOl system include in-season application of crop nutrients positioned in or near the root zone microenvironment. In the current study, this tool was used to assess how these attributes not only improve crop productivity, but also achieve increased nutrient recovery. Corn. Fertigated N, K, and S, in addition to a base N application, increased corn grain yield by 20 to 22 bu/A. Although significant yield increases to fertigation were realized at each population level, the greatest responses occurred at higher populations and suggest that improved nutrient management can be used as a strategy to ameliorate the negative effects associated with the increased planting densities that are needed for higher corn grain yields. Soybean germplasm exhibited variation in response to fertigation, with an average yield improvement of 4.8 bu/A across 'responsive' varieties. 'Responsive' varieties also measured significantly greater plant biomass, improving the recovery efficiency of N, K, and S by as much as threefold over 'non-responsive' varieties. Collectively, we believe these results highlight the utility of SOl fertigation to not only increase grain yield and nutrient recovery across genotypes, but also as a tool to characterize genotypes for response to other agronomic considerations. Dr. Ross R. Bender is a Sr. Agronomist at The Mosaic Company and Dr. Fred E. Below is Professor at the University of Illinois. 14