Fall Planted Cover Crops May Improve Tomato Yields
Gene Miyao1 and Paul Robins2
1UC Cooperative Extension, Yolo-Solano Counties
2Yolo County Resource Conservation District
Planting fall cover crops in fields that will later be planted to processing tomatoes is a departure from the goal among many tomato growers of minimizing weed vegetation prior to seedbed preparation. Vetch cover cropping attempts to maximize the vetch's vegetative growth to exclude weed competition, increase plant biomass for soil incorporation, fix atmospheric nitrogen and reduce winter runoff. Incorporating the green manure crop can be a challenge for tomato growers, with uncertain rainy weather conditions and planting schedules that for tomatoes are defined by calendar days.
A field test in the southern Sacramento Valley near Woodland established with a 1997 fall planting of a common vetch-pea mix found there may be some benefits to this practice. The trial was a 3-acre planting in a commercial field, with cooperator Blake Harlan of Harlan and Dumars. The cover crop was drilled into dry beds, and germinated after late fall rains. As expected, growth was slow during the winter and early spring. The peas were able to grow and develop during the cooler temperatures, compared to vetch, which grew more rapidly during late February and March. Ideally, vetch should be allowed to reach its maximum growth, normally by early April in the Sacramento Valley, before chopping and incorporating the plant biomass to maximize the benefit.
Our cover crop was terminated earlier to meet a planting schedule adjustment. After mid-March, a herbicide desiccant was sprayed and the vetch-pea cover was mowed prior to incorporation. The vetch-pea mix was at that time over 2 feet tall. It should be noted that bed preparation was more difficult because the plant debris caught on and wrapped around cultivator tools.
Once the vetch was mowed and incorporated, and the beds prepared, greenhouse grown plants were transplanted into the beds, one line per bed, in late March between rain episodes.
Our plot design was a randomized complete block with 6 replications with each plot 3 beds wide by 100 feet long. We evaluated two factors: 1) fallow vs. cover cropping with a vetch-pea mix and 2) spring-applied sidedress nitrogen rates of either 0, 50, 100, or 150 pounds of N per acre. Sidedressed N, as urea, was applied soon after transplants were well established. All other cultural practices were those commonly used by growers in our local area. Furrow irrigation combined with spring rain accounted for all the irrigation the crop received.
We monitored growth of the tomato plants throughout the season. Plant tissue samples, petiole as well as whole leaf, were collected at 3 separate growth stages and sent to a UC lab at Davis. Overall, we found that tomato vine size and canopy cover were each reduced over 14% when no sidedressed N was applied, regardless of the presence of the cover crop. We calculated the cover crop fixed over 100 lbs. of N per acre from the top growth. However, tomato yields suffered when grown solely on the nitrogen fixed by the vetch-peas and without benefit of supplemental applied N. We did not see a substantial fertilizer N benefit from the cover crop nor detect large N differences from lab analysis of tissue samples.
We were encouraged by a 5% yield increase and a soluble solids improvement with the cover crop over the fallow-bed treatment, however. Yields were increased from 36.5 tons/acre in the fallow beds compared to 38.4 tons in the cover cropped beds. Applied N alone did not explain the yield enhancement. We speculate that incorporation of a leguminous biomass may have been important in changing underground factors such as soil microbial activity. Soluble solids were also increased from 4.7 to 4.9%. Fruit color was reduced from 23.7 to 24.3 using the Processing Tomato Advisory Board's measurements.
We are indeed hopeful the cover crop practice will aid tomato growers. We remain hesitant to base recommendations on a single year's worth of positive results. We will continue to evaluate the benefit of fall cover cropping and hope to develop some recommendations in the future.
We expect the cover cropping will be more attractive to growers transplanting after late April. These planting periods will allow more vegetative growth of the cover crop and leave more time for tillage to bury the green manure.
Support and assistance was provided by the Harlan and Dumars, California Department of Food and Agriculture's Fertilizer Research and Educational Program, Timothy, Stewart and Lekos Seeds, and the Processing Tomato Advisory Board. Edited by Christine Jutzi, California Vegetable Journal.
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