Studies were conducted at two sites in South Dakota in 1992 and at one site in 1993 to measure the effect of velvetleaf on corn growth and yield. Velvetleaf was overseeded in corn rows and thinned to densities of 0, 1.3, 4, 12, and 24 plants/m2. Velvetleaf leaf area index and total biomass were positively correlated with velvetleaf density. Biomass per velvetleaf plant and corn biomass were correlated negatively with velvetleaf density. The percent corn yield reduction was similar for locations and years in spite of large yield differences. Maximum yield loss estimated by a hyperbolic yield reduction model was 37.2% with a loss of 4.4% per unit velvetleaf density.

Ridge application of N fertilizer has been promoted as a management tool to reduce nitrate movement. However, anhydrous ammonia applicator knives produce slots, which may impact water and nitrate movement. The objective of this study was to determine the effects of N fertilizer placement by an anhydrous ammonia applicator on nitrate movement within a ridge tillage system. Anhydrous ammonia was applied in a subsurface band to the ridge or valley areas of a ridge tillage system at the rates of 0 or 200 kg N ha−1. Rainfall (17 cm) was applied with a drop‐type artificial rainfall simulator 3, 10, and 24 d after fertilizer application in a fallowed field. Percolating water was collected in grid lysimeters (15 by 15 cm) located 75 cm below the soil surface of a Brandt silty clay loam (fine‐silty over sandy or sandy skeletal mixed Pachic Udic Haploboroll). Rainfall timing and N fertilizer placement influenced N fertilizer loss. The percentage of applied fertilizer collected in lysimeters when rainfall occurred 3 and 10 d after fertilizer application was 0 and 15%, respectively. When rainfall occurred 24 d after application, 49 and 73% of the applied N was leached through the profile from valley and ridge treatments, respectively. Increased N loss in the ridge treatment may have resulted from the fertilizer slot remaining open during the rainfall, while in the valley treatment the slot closed.

Accurate fertilizer recommendations require representative soil samples. However, it is often prohibitively expensive to collect the number of samples required to insure accurate recommendations. The objectives of this project were to determine inorganic N variability and sampling requirements in six fields with different cultural practices. Inorganic N concentrations and variability were evaluated in six corn (Zea mays L.) fields (20 to 80 acres) with management histories. Samples from the 0 to 2 ft soil depth were collected from a 200 ft grid. At each grid point, five samples were collected along transects between corn rows. Soil samples were analyzed for ammonium and nitrate N. Sampling location relative to the fertilizer band influenced inorganic N concentrations. By considering water and N inputs into the row and interrow areas separately, the relative importance of residual N remaining in row and interrow areas of the soil could be estimated. This estimate can be used as a guide for management decisions. Results shows that the best strategy for sampling banded fields may include collecting 15 to 30 composite cores from a zone located halfway between the row and the band. Research Question Fertilizer recommendations are only as good as the soil sample collected. Typically, a composited sample contains between five and 20 separate cores from a field. However, when N fertilizer is band applied, the fertilizer recommendation may be highly inaccurate if the composite sample contains this number of cores. Techniques are needed to accurately assess residual soil N and reduce sampling errors without increasing sampling requirements. Literature Summary Four different sources of error (lab, temporal, spatial, and management induced) have to be considered in developing soil sampling protocols. Reducing errors associated with any one of these parameters should reduce fertilizer recommendation errors. Study Description Soil samples from the surface 2 ft were collected from a minimum grid spacing of 200 by 200 ft in six fields. At each grid point, five individual samples were collected at different points relative to the fertilizer band. The total number of soil samples collected from each field ranged from 190 to 455. Samples were analyzed for ammonium N and nitrate N. Applied Questions How do we sample fields where N was band applied? If samples were randomly collected from a no‐tillage field where 120 lb of N/acre was band applied, then for the estimated residual N to be within 20% of the mean 80% of the time, approximately 57 individual cores should be composited. Many farmers are unwilling to collect this number of samples. Systematic soil sampling may reduce sampling requirements. However, samples must be collected from soil zones that contain both fertilizer and soil N. For example, if samples are collected from crop rows, then residual N in the field may be underestimated, while if collected from bands then residual N may be overestimated. An efficient sampling protocol for the Elkton no‐tillage field where 120 l...

The loading of organic substrates into shallow aquifers may follow seasonal cycles, which will impact the transport and fate of agrichemicals. The objective of this research was to measure temporal changes in the groundwater dissolved organic C (DOC) and nitrate concentrations. Groundwater monitoring wells were installed and sediment samples from the aquifer were collected in 1991. Sediment samples were used to evaluate denitrification potentials, while water samples were collected at periodic intervals in 1992 and 1993 from the surface of the aquifer. Water samples were analyzed for nitrate-N and DOC-C. Denitrification was observed in sediment amended with nitrate and incubated under anaerobic conditions at 10°C. Addition of algae lazed biomass increased denitrification, establishing that denitrification was substrate limited. In the aquifer, DOC concentrations followed seasonal patterns. DOC concentrations were highest following spring recharge and then decreased. Peak timing indicates that freezing and thawing were responsible for seasonal DOC patterns. These findings show that seasonally driven physical processes, such as freezing and thawing, influence organic substrate transport from surface to subsurface environments, and that this process should be taken into account when assessing agrichemical detoxification rates in shallow aquifers.

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