Residual herbicides applied to summer cash crops have the potential to injure subsequent winter annual cover crops, yet little information is available to guide growers’ choices. Field studies were conducted in 2016 and 2017 in Blacksburg and Suffolk, Virginia, to determine carryover of 30 herbicides commonly used in corn, soybean, or cotton on wheat, barley, cereal rye, oats, annual ryegrass, forage radish, Austrian winter pea, crimson clover, hairy vetch, and rapeseed cover crops. Herbicides were applied to bare ground either 14 wk before cover crop planting for a PRE timing or 10 wk for a POST timing. Visible injury was recorded 3 and 6 wk after planting (WAP), and cover crop biomass was collected 6 WAP. There were no differences observed in cover crop biomass among herbicide treatments, despite visible injury that suggested some residual herbicides have the potential to effect cover crop establishment. Visible injury on grass cover crop species did not exceed 20% from any herbicide. Fomesafen resulted in the greatest injury recorded on forage radish, with greater than 50% injury in 1 site-year. Trifloxysulfuron and atrazine resulted in greater than 20% visible injury on forage radish. Trifloxysulfuron resulted in the greatest injury (30%) observed on crimson clover in 1 site-year. Prosulfuron and isoxaflutole significantly injured rapeseed (17% to 21%). Results indicate that commonly used residual herbicides applied in the previous cash crop growing season result in little injury on grass cover crop species, and only a few residual herbicides could potentially affect the establishment of a forage radish, crimson clover, or rapeseed cover crop.

Core Ideas A comparison of ammonia volatilization from granular urea treated with two commercially available nitrification inhibitors and the interaction with the urease inhibitor, NBPT. Nitrification inhibitors increased ammonia volatilization from surface‐applied granular urea compared to untreated urea. When treating urea with nitrification inhibitors, a urease inhibitor should be applied to reduce ammonia volatilization losses. Soil type specific recommendations may be needed when applying nitrification/urease inhibitors to urea in order to maximize N use efficiency. Nitrification inhibitors reduce the rate of transformation of ammonium to nitrate, which may increase ammonia volatilization from urea‐based N fertilizers. The objective of this study was to quantify ammonia volatilization from surface‐applied granular urea treated with combinations of NBPT and the nitrification inhibitors, DCD and nitrapyrin, under controlled laboratory conditions for three soils. Nine laboratory trials evaluated select combinations of application rates for NBPT, DCD, and nitrapyrin‐treated urea. Trials were conducted on three soils selected for differences in pH, organic matter, and cation exchange capacity (CEC). Cumulative N loss as ammonia from untreated granular urea was 40.1, 44.5, and 38.2% of applied N for the Raub, Wheeling, and Pella soils, respectively. When the urease inhibitor, NBPT was applied alone at 900 mg NBPT kg−1 urea, N loss through volatilization was reduced 48.5, 43.3, and 77.9% for the Raub, Wheeling and Pella soil types, respectively. Of the soil tested, cation exchange, texture, and organic matter had the greatest impact on the rate and cumulative ammonia loss from surface‐applied untreated urea and urea treated with the urease/nitrification inhibitors. Nitrification inhibitors prevented the conversion of ammonium to nitrate by DCD and nitrapyrin, increasing ammonia losses of N in five out of six trials compared to untreated urea. When using nitrification inhibitors with surface‐applied urea, ammonia losses were decreased with the application of NBPT as well.

Core Ideas Soil K at 0‐ to 15‐ and 0‐ to 30‐cm depths were excellent predictors of full‐season soybean relative yield.Tissue‐K concentration can be used for calibrating fertilizer‐K rate and in‐season K management.Soil sampling to 30‐cm depth would reduce fertilizer amount and cost for soybean on low cation exchange capacity soils. Quantifying soil‐K availabilities at deeper depths may be necessary to determine optimum fertilizer‐K rate for soybean [Glycine max (L.) Merr.] grown on low cation exchange capacity (CEC) soils that are prone to K leaching. We characterized full‐season soybean response to fertilizer‐K across 19 coarse‐textured low‐CEC sites during 2013 and 2014. Mehlich‐1 soil‐K concentrations at 0‐ to 15‐ and 0‐ to 30‐cm depths better correlated with relative yield and explained 90% of relative yield variation compared to 77% for 0‐ to 60‐cm depth. Critical soil‐K concentrations were similar for relative yield, V5 plant‐K concentration, and R2 leaf‐K concentration, ranging from 48 to 73 mg K kg−1 for 0‐ to 15‐cm and 41 to 63 mg K kg−1 for 0‐ to 30‐cm depths. Soil‐K concentrations less than this critical range accurately predicted positive yield responses to fertilizer‐K 89% of the time for 0‐ to 15‐cm and 80% for 0‐ to 30‐cm depths. Plant‐ and leaf‐K concentrations were equally good in predicting relative yield with critical concentrations of 19 to 22 g plant K kg−1 and 18 to 21 g leaf K kg−1. Plant‐K concentration was better than leaf‐K concentration in diagnosing K‐deficient sites. Calibration model confirmed that soybean requires no fertilizer‐K to maximize yield for soil‐K concentrations above the critical ranges at both depths. However, for K‐deficient soils, soil‐K concentrations at 0‐ to 30‐cm depth resulted in 7 to 32% less fertilizer‐K requirements than 0‐ to 15‐cm depth, indicating the value of deeper sample in recommending fertilizer‐K for soybean grown on coarse‐textured low‐CEC soils.

 Mehlich III K ranged from 30-400 mg kg -1 across cotton regions reporting K deficiency. Over half of site-years reported soil K levels less than the Mehlich III critical level. A lint yield response to soil applied K fertilizer was determined at five of the 23 site-years. Inconsistent results indicate K dynamics in the soil -plant system need further investigation.

Ammonia (NH) emissions from animal manures can cause air and water quality problems. Poultry litter treatment (PLT, sodium bisulfate; Jones-Hamilton Co.) is an acidic amendment that is applied to litter in poultry houses to decrease NH emissions, but currently it can only be applied once before birds are placed in the houses. This project analyzed the effect of multiple PLT applications on litter properties and NH release. Volatility chambers were used to compare multiple, single, and no application of PLT to poultry litter, all with and without fresh manure applications. A field component consisted of two commercial broiler houses: one had a single, preflock PLT application, while the other received PLT reapplications during the flock using an overhead application system. In the volatility chambers, single and reapplied PLT caused greater litter moisture and lower litter pH and , relative to no PLT. After 14 d, NH released from litter treated with reapplied PLT was significantly less than litter with both single and no applications. Furthermore, total N in litter was greatest in litter treated with reapplied PLT, increasing its fertilizer value. In the commercial poultry houses, PLT reapplication led to a temporary decrease in litter pH and , but these effects did not last because of continued bird excretion. Although one preflock PLT application is currently used as a successful strategy to control NH during early flock growth, repeat PLT application using the overhead reapplication system was not successful because of problems with the reapplication system and litter moisture concerns.