Predicting leaching of residual soil nitrate-nitrogen (NO3-N) in wet climates is important for reducing risks of groundwater contamination and conserving soil N. The goal of this research was to determine the potential to use easily measurable or readily available soilclimatic-plant data that could be put into simple computer models and used to predict NO3 leaching under various management systems. Two computer programs were compared for their potential to predict monthly NO3-N leaching losses in western Oregon vegetable systems with or without cover crops. The models were a statistical multiple linear regression (MLR) model and the commercially available Nitrate Leaching and Economical Analysis Package model (NLEAP 1.13). The best MLR model found using stepwise regression to predict annual leachate NO3-N had four independent variables (log transformed fall soil NO3-N, leachate volume, summer crop N uptake, and N fertilizer rate) (P < 0.001, R 2 = 0.57). Comparisons were made between NLEAP and field data for mass of NO3-N leached between the months of September and May from 1992 to 1997. Predictions with NLEAP showed greater correlation to observed data during high-rainfall years compared to dry or averagerainfall years. The model was found to be sensitive to yield estimates, but vegetation management choices were limiting for vegetable crops and for systems that included a cover crop.
Hudson Minshew, John Selker, Delbert Hemphill and Richard P. Dick
John Z. Burket, Delbert D. Hemphill and Richard P. Dick
Cover crops hold potential to improve soil quality, to recover residual fertilizer N in the soil after a summer crop that otherwise might leach to the groundwater, and to be a source of N for subsequently planted vegetable crops. The objective of this 5-year study was to determine the N uptake by winter cover crops and its effect on summer vegetable productivity. Winter cover crops [red clover (Trifolium pratense L.), cereal rye (Secale cereale L. var. Wheeler), a cereal rye/Austrian winter pea (Pisum sativum L.) mix, or a winter fallow control] were in a rotation with alternate years of sweet corn (Zea mays L. cv. Jubilee) and broccoli (Brassica oleracea L. Botrytis Group cv. Gem). The subplots were N rate (zero, intermediate, and as recommended for vegetable crop). Summer relay plantings of red clover or cereal rye were also used to gain early establishment of the cover crop. Cereal rye cover crops recovered residual fertilizer N at an average of 40 kg·ha-1 following the recommended N rates, but after 5 years of cropping, there was no evidence that the N conserved by the cereal rye cover crop would permit a reduction in inorganic N inputs to maintain yields. Intermediate rates of N applied to summer crops in combination with winter cover crops containing legumes produced vegetable yields similar to those with recommended rates of N in combination with winter fallow or cereal rye cover crops. There was a consistent trend (P < 0.12) for cereal rye cover crops to cause a small decrease in broccoli yields as compared to winter fallow.
Helene Murray, Richard P. Dick, Daniel Green-McGrath and Larry S. Lev
Calls for increased farmer involvement in research and extension programs have been numerous and well supported. One approach to integrate the collective knowledge and experience of scientists and farmers is through whole farm case studies (WFCS). An interdisciplinary team of 34 research and extension personnel at Oregon and Washington State Universities conducted WFCS of 16 vegetable and small fruit farms.
The WFCS process proved useful in developing an interdisciplinary team, and the vast majority of team members stated they would consider using the WFCS approach again; but the primary constraint cited was the amount of time required. Team members gained a better understanding of the complexity of farms and identified areas requiring further research. Farmers stated they enjoyed participating and discovered new information that will assist them in managing their fanning systems. Farmer-developed innovations were identified that are useful to other farmers and to the research process.