Tillage Practices for Improving Nitrogen Cycling and Soil Quality
1. Describe soil carbon (C) and nitrogen (N) dynamics immediately after tillage.
2. Examine the effect of organic matter (OM) additions on C and N dynamics after tillage.
3. Identify management options that minimize short-term C and N loss by altering the type, timing or frequency of tillage, both during crop production and during the winter fallow.
4. Analyze the costs and benefits of the new management practices in terms of economic feasibility and agroecosystem health.
5. Demonstrate these tillage options in the context of commercial agricultural practices to show their practical applicability to growers.
This project conducted on-farm trials to examine management options for cover crops, compost, and alternative tillage practices for lettuce production in coastal California. In the intensive, large-scale vegetable production systems of the Salinas Valley, growers typically rely on frequent tillage and add little OM to the soil, so that levels of soil OM have decreased markedly in the last hundred years. We established two monitoring studies on growers’ fields. One study examined the changes in yield, plant disease, soil microbial biomass with three different minimum tillage methods for retaining semi-permanent beds. The other study investigated the combined effects of reducing tillage and increasing OM addition with cover crops and compost. In addition, daily responses of soil microbial activity were monitored after tillage in order to study the disruptive nature of tillage events.
In the three-year on-farm study on minimum tillage practices, shallow implements resulted in higher severity of lettuce drop disease and lettuce yield than did deeper implements. Soil microbial biomass C did not show consistent responses to tillage. The most advantageous minimum tillage method was a grower-designed, five-step set of operations to chisel, disk, and rip the soil, but keep beds intact for several years. There was less soil compaction with this method than with conventional tillage that re-makes beds between crops with disks and bed shapers.
In the two-year on-farm study on the effects of compost, cover crops and tillage practices, clear advantages to increased OM additions were observed after one year. Simultaneous use of cover crops and compost significantly increased soil microbial biomass for almost a year thereafter. Nitrate decreased after cover cropping, reducing the potential for leaching loss. Lettuce yield generally increased in the next spring and summer after these OM additions. Weed density also decreased. Shallow minimum tillage produced lower lettuce yield than conventional tillage. Weed density was not affected by tillage treatment. Insect pests and diseases were not affected by either OM or tillage treatment. Economic analysis is now underway, as the experiment will end in April 2000.
Overall, excellent cooperation with growers of large vegetable operations resulted in clear recommendations for increasing OM inputs with simultaneous addition of compost and cover crops, and for avoiding prolonged use of shallow minimum tillage equipment without intermittent deeper tillage. Our outreach programs have delivered the findings of this project to approximately 400 people and we have developed several written publications, and more are in progress.
These on-farm projects showed a clear yield advantage for lettuce produced with compost, cover crops, and tillage methods that deeply till the soil. Organic matter additions also increased soil microbial biomass, a measure that is often considered indicative of higher soil quality. Weeds were also less numerous with organic amendments. Cover crops decreased the potential for nitrate leaching. Conventional tillage, however, proved to be more valuable than shallow minimum tillage. Shallow minimum tillage was shown to increase the propensity for lettuce drop disease after a two years, so should only be used periodically.
Farmer Adoption and Direct Impact
Our project has received wide interest and visibility, not only from farmers, but also from compost producers, farm service company personnel, and members of government agencies (see list of outreach activities above). Our cooperation with major growers in the region has impact in the region, since findings are being used in developing management regimes for several hundred acres farmed by these growers.
Growers have expressed particular interest in the finding that compost alone had no effect on soil microbial biomass or lettuce yield, but when applied in conjunction with cover cropping, had a long-lasting effect. Also, the potential for lettuce drop disease with shallow minimum tillage has garnered much interest, since this disease is very difficult to eradicate. At a recent meeting, the decrease in weed density with the use of cover crops and compost was presented for the first time, and this finding was particularly interesting to the growers in the audience.
Future Recommendations or New Hypotheses
Our results suggest several hypotheses that will be important to pursue in new experiments. 1) Adding a cover crop residue may provide a very readily available source of C that stimulates microbial growth temporarily. Simultaneous addition of a more recalcitrant C source as compost appears to have supplied the microbes with available C for an extended period of time. Higher microbial biomass appears to be important for nutrient mineralization that enhances plant growth and yield. 2) Sclerotinia minor disease may become more severe in shallow tilled systems due to accumulation of sclerotia in zones with high root length density. 3) Organic amendments may favor increased biological activity that in turn results in weed seed degradation.
This summary was prepared by the project coordinator for the 2000 reporting cycle.