Even though soybean [Glycine max (L.) Merr.] fixes N from the air, N demand at the R4 to R6 (pod‐fill) growth stages of high‐yielding, high‐protein soybean may be greater than the amountsss of available N with normal farming practices. In Kansas, supplemental N has not been recommended on soybean regardless of yield level. Research was conducted at eight sites over a 2‐yr period on irrigated soybean to evaluate effects of N rates (0, 20, 40 lb/acre) and sources (urea‐ammonium nitrate solution [UAN], ammonium nitrate, urea, urea + N‐[n‐butyl] thiophosphoric triamide NBPT]) on leaf N concentrations, grain yield, and grain protein, and oil concentrations. The NBPT used is a commercially available urease inhibitor. Most N was broadcast but UAN was applied over the top of the canopy through flat‐fan nozzles. All applications were made at the R3 growth stage. Nitrogen concentrations in leaf samples taken 2 to 3 wk after N application were unaffected by N fertilization. Soybean yields were increased significantly by late‐season N application at six of eight sites; the average increase was 6.9 bu/acre or 11.8 %. Both of the nonresponsive sites had yields averaging under 50 bu/acre, whereas responsive sites yielded 56 bu/acre or more. Soybean plants at all locations were well nodulated. In most cases, 20 lb N/acre provided positive responses. Late‐season N fertilization increased grain protein and oil concentrations at some sites, but overall combined analysis indicated nonsignificant effects. Nitrogen sources performed similarly. Application of UAN resulted in leaf burn, which probably reduced yields at the 40 lb N/acre rate. However, this would not be a problem for producers applying UAN through irrigation systems where the UAN would be much more diluted. Even when well nodulated, soybean with high yield potential may not be able to take in enough N to achieve maximum yield of high quality grain. Public and private soil test labs and crop consultants may have to reevaluate N recommendations for soybean with high yield potential. Supplemental N application at the R3 growth stage can provide positive economic returns for producers growing irrigated soybean with high yield potential. Research Question Can soybean symbiotically fix enough N to produce high yields under irrigation? Producers of irrigated soybeans are now routinely achieving yields in excess of 60 bu/acre. A soybean crop yielding 70 bu/acre requires nearly 250 lb N/acre. Late‐season supplemental N may increase yields. In addition, future soybean marketing strategies may include protein and oil concentrations. Late‐season supplemental N has potential to affect these seed quality considerations. The objective of this research was to evaluate the effects of late‐season supplemental N on soybean grain yield and composition. Literature Summary Previous research on N fertilization of soybean has produced inconsistent results. Much of the earlier work evaluated N applied either preplant, at planting time, or early in the growing season. Results from previous work in...

Response of soybean [Glycine max (L.) Merr.] to changes in row spacing and seeding rate have been variable. Some researchers have reported grain yields to be higher with the use of narrow row spacings. Other investigators have found that wide row spacings provided grain yields equal to or greater than those obtained with narrow row spacings. This study was designed to determine the influence of environment on the optimum row spacing and seeding rate for soybean. Eleven field experiments were established in Kansas from 1991 to 1993. Four seeding rates in 1991 and five seeding rates in 1992 and 1993, ranging from 52 272 to 261 360 seeds/acre, were used in 8‐ and 30‐in. rows. At high yielding sites, maximum grain yields were higher with 8‐in. rows than with 30‐in. rows. If moisture stress reduced grain yields, maximum yields were greater with 30‐in. rows than with 8‐in. rows. Response to changes in seeding rate varied between row spacings depending upon environmental conditions. Under high‐yielding conditions, grain yields were maximized with 30‐in. rows at approximately 115 000 seeds/acre, whereas seeding rates of 203 000 to 232 000 seeds/acre were required to maximize grain yields with 8‐in. rows. Under conditions of limited soil moisture, grain yields were not affected by changing seeding rates. At sites with adequate soil moisture, mature plant heights increased as seeding rates increased. At moisture‐deficient sites, plant height was not significantly affected by increased seeding rates. Research Question Recommendations for soybean row spacing and seeding rate are generally constant within a geographical area regardless of yield goal or yield potential. This research was designed to determine the influence of environment on the optimum row spacing and seeding rate for soybean. Literature Summary Many researchers have reported that soybeans planted in narrow row spacings produced higher yields than did soybeans planted in wider row spacings. Other investigators found little or no yield increase with the use of narrow row spacings. Some researchers have reported that soybeans planted in narrow rows had greater water‐use efficiency than did soybeans planted in wider rows. Other investigators, however, found that, under conditions of severe water stress, water‐use efficiency was greater with wide rows than with narrow rows. Yield response to changes in plant population usually have been small and often inconsistent. Generally, increasing plant populations has increased plant height at maturity. It has been reported that higher plant mortality occurred with wide rows than with narrow rows. Study Description This research was conducted from 1991 to 1993 at 11 dryland locations in Kansas. In 1991, four seeding rates ranging from 52 272 to 209 088 seeds/acre were used in 8‐ and 3041‐1. rows. In 1992 and 1993, an additional seeding rate of 261 360 seeds/acre was added. Plant populations were determined 5 wk after planting. Plant lodging, mature plant heights, and grain yields were measured at maturity. Corsica was the ...

Crop residues produce alleochemicals that may inhibit corn [Zea mays (L.)] seed germination and early growth. Studies were conducted in which residues of corn, soybean [Glycine max (L.) Merr.], oat [Avena sativa (L.)], and mixed grass hay were extracted under N2 gas or air. Organic debris was removed and half of each extract was filter sterilized. Corn seeds were incubated in the extracts for 96 h at 25 °C. Percent germination, and lengths of coleoptile, radicle, and secondary roots were measured. Residues extracted under N2 gas or air did not differ significantly in their toxicity. Nonsterile residue extracts decreased germination to 74% for soybean and oat straw and 27% for corn and hay residues. Sterile extracts did not affect germination. Nonsterile soybean and oat extracts did not reduce coleoptile lengths but did reduce radicle and secondary root lengths by 34% compared to the water treatment. Sterilized extracts reduced radicle and secondary root lengths by 63%. Nonsterile corn and hay extracts reduced coleoptile lengths by 42% and radicle and secondary root lengths by 81%. A second extraction was performed by incubating the residues without aeration at 25 and 0.5 °C. Seed germination for treatments with nonsterile extracts obtained at 25 °C were similar to those for nonsterile extracts of Exp. 1. Extraction at 0.5 °C and filter sterilization also improved germination. Soybean and oat extracts did not strongly inhibit coleoptile lengths; however, a 61% reduction occurred in radicle and secondary root lengths for the sterilized, 0.5 °C extract. Corn and hay residues were generally more inhibitory to coleoptile, radicle and secondary root lengths; however, no consistent effects were observed from temperature and sterilization treatments.

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Nitrogen fertilizer use efficiency in corn (Zea mays L.) production could be improved if farmers adjusted N fertilizer rates for different yield potentials caused by variations in cultural practices. Five studies were conducted in Ohio, three on Hoytville silty clay (fine, illitic, mesic Mollic Ochraqualf) and two on Riddles silt loam (fine‐loamy, mixed, mesic Typic Hapludalf), to assess the effects of planting date, plant population, N rate, and timing of N application on yield of two corn hybrids grown using no‐tillage methods. Increasing plant population increased grain yield in only one of five site‐year comparisons, but did not increase the amount of N required to produce maximum yield. Crop yield potential varied depending on site‐year, with maximum grain yield indicated by response functions ranging from 5.5 to 11.3 Mg ha−1 for early‐planted corn, and 3.0 to 10.1 Mg ha−1 for late‐planted corn. Nitrogen rates needed to achieve maximum yield varied from 0 to 205 kg N ha−1 for early‐planted corn and from 0 to 175 kg N ha−1 for late‐planted corn, depending on site‐year. In most site‐years, late‐planted corn required less N to achieve maximum yield than early‐planted corn. The reduction in N requirement averaged 60 kg N ha−1, and was directly related to the magnitude of the yield reduction associated with delayed planting. Split applications of N generally produced yields equivalent to application of all N at planting. Split application can offer many farmers a way to use lower N rates to compensate for delayed planting.