1992 Annual Report for AW92-006
Cover Crops Incorporated with Reduced Tillage on Semi-Permanent Beds: Impacts on Nitrate Leaching, Soil Fertility, Pests, and Farm Profitability
Summary
Objectives
1. Quantify the potential for new cover crop/reduced tillage systems on semi-permanent beds to improve nitrogen cycling and decrease nitrate leaching as compared to conventionally farmed systems.
2. Compare vegetable crop yields on cover-cropped/reduced tillage and conventionally-farmed management systems.
3. Develop innovative tillage practices that minimize and energy requirements for cover crop incorporation.
4. Determine positive and negative effects of cover crops and reduced tillage on disease insect pests.
5. Calculate costs and benefits of each management system to determine relative profitability, energy savings, and labor requirements associated with using cover crops and reduced tillage on semi-permanent beds, using a computerized budget generator (Budget Planner).
6. Use a soil/plant process model (EPIC) to assess short-term and long-term effects of using cover crops and reduced tillage on crop productivity, leached nitrate, and profitability.
7. Conduct field demonstrations and local meetings, and prepare publications to inform growers of the environmental advantages, management feasibility, risks, and profitability of adopting cover crops and new tillage practices.
Abstract of Results
Development of new management practices is necessary for annual row crops, which use large amounts of nitrogen and water, and create a high potential for ground water contaminants by nitrates. Our previous work has shown that in intensive vegetable production systems, winter cover crops can deplete soil nitrate during the winter rainy season, and improve soil nitrogen recycling after the incorporation, even when there is only a short fallow (unseeded) period between vegetable crops. One purpose of this project is to develop cover crop management techniques, including reduced tillage on semi-permanent beds, which are cost-effective and practically suited to intensive vegetable production schedules.
Research plots were established in 1992 at three commercial farm sites (two-year studies), and at two experimental sites (one-year studies). The on-farm sites were cover cropped during the winters of 1992-93 and 1993-94, and then planted with two successive vegetable crops in both years. The studies at the experimental sites were conducted to determine the effect of cover crop incorporation on microbial carbon and nitrogen dynamics, and subsequent crop yield where management conditions could be more closely controlled. The soil types at these sites ranged from loam to clay.
Cover crops reduced nitrate leaching during winter by as much as 70 percent compared to winter fallow soil, due to plant nitrogen uptake and to reduced drainage from plant transpiration. High nitrate levels in the lower soil depths of the fallowed plots persisted at all sites into the early stages of vegetable production, when frequent irrigation may cause further leaching. The short-term effects of cover crop incorporation, including plant nitrogen availability, insect and disease incidence, and soil nitrogen and microbial biomass, were the focus of intensive post-incorporation soil sampling at all sites. This data suggests that the effects of one year of low carbon/nitrogen cover cropping are short-term and temporary, and are usually not detectable by the time the first cash crop is planted. Only one site produced a significantly higher lettuce yield and nitrogen content after one year of cover cropping. Sustained differences in soil nitrogen content and microbial biomass were found to extend into the vegetable crop period after two successive years of cover cropping on the clay loam soil.
Studies at the experimental sites showed if cover crops have a low carbon/nitrogen ratio at the time of incorporation, cover crop nitrogen may be rapidly mineralized, and contribute significantly to crop available nitrogen. However, if cover crops with a high carbon/nitrogen ratio are incorporated under soil moisture conditions, delayed nitrogen cycling dynamics may actually reduce crop available nitrogen compared to fallowed soils in the early stages of the subsequent crop.
At the on-farm sites, other impacts of cover cropping were also measured. Increases in certain soil arthropods (insects) and funguses, which were found in some cover cropped plots after incorporation, did not last into the vegetable cropping period, and had no effect on crop yield.
Economic Analysis
Detailed management input records were compiled for each commercial site to allow a comparison of energy use, labor requirements and costs of cover cropping, using a computerized budget generator. The cost of cover cropping was found to be minimal relative to the cost of producing a vegetable crop (3-5%), and approximately 14 percent of the costs were for management operations required on winter fallowed ground, such as listing beds and cultivating furrows. It appears that the more significant risk in winter cover cropping lies in the potential for lost revenue if the cover crop disrupts the vegetable planting schedule.
The window of opportunity to grow and incorporate a cover crop, and then prepare the field for vegetable planting is very short in this production system. The use of semi-permanent beds and minimum tillage techniques may be the critical management component to make cover cropping a viable option in intensive vegetable production.
Potential Contributions
Management techniques to grow and incorporate cover crops on semi-permanent raised beds have been developed and demonstrated for intensive vegetable production systems on commercial sites. This study has shown that cover crops significantly reduced nitrate leaching during winter. The use of cover crops to deplete soil nitrate, and thereby reduce the potential for nitrate contamination of ground water during winter rains has been demonstrated for a range of soil types. No yield reduction or increase in disease incidence of vegetable crops has been found as a result of cover cropping.
Cover cropping on semi-permanent beds may reduce cultivation requirements compared to planting and disking on flat ground, and therefore save energy and labor costs.
Producer Adoption
Winter cover cropping is becoming more widespread in the Salinas Valley, as the threat of nitrate leaching to the ground water becomes more widely known, and the value of building soil fertility through the addition of organic residues becomes more accepted. This project has generated interest, evidenced by high participation at field days, and an increasing number of growers now planting winter cover crops on raised beds and using minimum tillage techniques at incorporation.
All of our cooperators are growing winter cover crops as a result of their participation, and two of them are committing substantially more acreage in addition to our research sites to cover crops. We have received at least ten requests for cover crop seed; several offers of commercial sites from different growers for future cover crop research. Including drip irrigation sites; interest farm the largest local fertilizer/farm service company in management techniques to reduce nitrate leaching; and, interest from a seed company in becoming local source for cover crop seed.
New Hypotheses
Further research is needed on modifying tillage practices to improve nitrogen cycling. Data from this study has revealed that a significant, short-term surge and decline in nitrogen cycling result from the first spring tillage in these intensively-managed, highly –tilled cover cropped and fallowed soils. It is hypothesized that this is caused by microbial stimulation in response to the mixing and aeration of tillage, and then exhaustion of the supply of water or carbon, causing the microbial population to die back. Further research is needed to determine the extent of nitrogen loss, and how tillage practices can be modified to increase soil carbon and nitrogen retention.
Also, minimum –tillage techniques have been shown in this study to provide several advantages, such as allowing earlier entry into the field under wet conditions, and reducing operating costs compared to conventional disking. Further research is needed to determine the optimal tillage practices to reduce soil compaction in vegetable production, which in turn will effect nutrient cycling, drainage, disease incidence and more.
Reported in 1996.