Potassium availability in agronomic crops has recently decreased due to periodic drought conditions, soil compaction, reduced K applications, lower frequency of soil testing, and higher K fertilizer requirements because of increasing corn (Zea mays L.) yields and larger soybean [Glycine max (L.) Merr] acreage. Little research has evaluated the effects of foliar K applications on low to medium soil test K claypan soils, which are susceptible to drought and surface compaction. Field research was conducted in 2001 and 2002 to determine soybean response to foliar‐applied K fertilizer compared with a preplant application and evaluate the cost‐effectiveness of these treatments. Potassium fertilizer (K2SO4) was either broadcast‐applied at 140, 280, and 560 kg K ha−1 as a preplant application or foliar‐applied at 9, 18, and 36 kg K ha−1 at the V4, R1–R2, and R3–R4 stages of soybean development. Soybean grain yield increased 727 to 834 kg ha−1 when K was foliar‐applied at 36 kg ha−1 at the V4 and R1–R2 stage of development in 2001 and 2002. Foliar‐applied K at the R3–R4 stage of development increased grain yield but not as much as V4 or R1–R2 application timings. Treatment cost‐effectiveness ranked preplant K at 280 kg ha−1 = preplant K at 140 kg ha−1 > preplant K at 560 kg ha−1 = V4 or R1–R2 foliar‐applied at 36 kg ha−1. Foliar K did not substitute for preplant K in this research; however, foliar K may be a supplemental option when climatic and soil conditions reduce nutrient uptake from the soil.

The soybean cyst nematode (SCN), Heterodera glycines, is one of the most important pathogens of soybean. Periodic monitoring of SCN population densities and virulence phenotypes is necessary for developing management strategies utilizing resistant cultivars, the primary strategy used to combat this pest. Therefore, we conducted a statewide survey of Missouri to determine SCN population densities and virulence phenotypes during 2015–2016 and compared these results with a similar survey conducted in 2005. SCN population densities were determined for 393 soil samples representing 74 soybean-producing counties across eight geographical regions of Missouri. Eighty-eight percent of samples tested positive for SCN, up from 50% in 2005, and population densities ranged from 125 to 99,000 eggs per 250 cm3 of soil. The virulence phenotypes of 48 SCN populations also were determined. For this, female indices (FI) were calculated by dividing the mean number of females that develop on the roots of a set of resistant soybean indicator lines by the mean number of females that develop on the roots of susceptible cultivar Lee74 after 30 days in the greenhouse then multiplying by 100 to obtain a percentage. Notably, all SCN populations evaluated during 2015–2016 had a FI > 10 on PI 88788, the most widely used source of resistance in Missouri, in contrast to 78% in 2005. Moreover, 50% of these populations had a FI > 50 on PI 88788, up from 16% in 2005. Forty-three percent of populations tested also had a FI > 10 on Peking, the second most common source of resistance by farmers. Our results show that over the last decade, SCN has become more prevalent in Missouri fields. Additionally, the percentage of individuals within SCN field populations that are virulent on PI 88788 and Peking has markedly increased. The results stress the importance of rotating cultivars with different types of resistance when using resistant cultivars to manage SCN.

Field measurements of NH3 volatilization from both surface‐applied and incorporated turkey (Meleagris gallopavo) manure (20 g of N m−2, which compares with 16 Mg of manure ha−1) and urea (20 g of N m−2) were made using the forced‐draft technique for a 6‐d period in early June 1991. The soil (temperature and water potential) and atmospheric parameters (relative humidity, wind speed, and air temperature) affecting NH3 volatilization rate were also measured. Ammonia losses for manure and urea followed a diurnal pattern; maxima occurred shortly after midday (between 1200 and 1800 h), when soil temperature was nearing maximum and wind speed was highest, and minima occurred during midnight to early morning hours (between 2400 and 0600 h), when the soil temperature was lowest and relative humidity was highest. Significant positive correlations between rates of NH3 volatilization and soil temperature, atmospheric temperature, and wind speed were observed. Relative humidity was generally negatively correlated with NH3 volatilization. Ammonia volatilization from the surface‐applied manure was highest immediately after application and gradually decreased with time, whereas ammonia volatilization from surface‐applied urea did not start until Day 2 and reached a maximum on Day 6. The total NH3 loss from incorporated manure was 23 times lower than that from surface‐applied manure (0.5 vs. 11.5 kg of N ha−1), whereas NH3 loss from urea was eight times lower when incorporated (0.2 vs. 1.5 kg N ha−1) during a 6‐d period. Total N loss (6 d) from surface‐applied urea was eight times lower than an equivalent amount of inorganic N in turkey manure (1.5 vs. 11.5 kg of N ha−1). Ammonia volatilization from turkey manure and urea was found to be influenced by a combination of environmental factors and the method of application.

Controlled-release N fertilizer, such as polymer-coated urea (PCU), may be a fall N management option for wheat (Triticum aestivum L.) grown in poorly-drained claypan soils. Field research evaluated (1) urea release from fall-applied PCU in 2006 and 2007; (2) broadcast fall-spring split (25%:75%) of N sources; and (3) a single fall (100%) application of PCU, urea, urea plus NBPT (N-(n-butyl) thiophosphoric triamide] (U + NBPT), ammonium nitrate (AN), or urea ammonium nitrate (UAN) at 0, 56, 84, and 112 kg·N·ha −1 on wheat yield, wheat biomass, N uptake by wheat, and frost-seeded red clover (FSC) (Trifolium pratense L.) forage yield (2004)(2005)(2006)(2007). PCU applied in fall released less than 30% urea by February. Urea released from PCU by harvest was 60% and 85% in 2006 and 2007, respectively. In poorly-drained soils, wheat yields ranked PCU > AN > U + NBPT > urea ≥ UAN over the rates evaluated for fall-only application. PCU was a viable fall-applied N source, with yields similar to or greater than urea or U + NBPT split-applied. Split-N applications of AN, urea, UAN, and U + NBPT generally resulted in greater wheat yields than a fall application. Enhanced efficiency fertilizers provide farmers with flexible options for maintaining high yielding production systems. OPEN ACCESSAgronomy 2014, 4 498

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