Fall, 1996 (v8n4)

Project Update

Cover Crops in Central Coast Farming Systems
by Davis Chaney, SAREP

Farmers and ranchers often question whether or not cover crops can supply enough nitrogen for the cash crops they grow. This question is particularly important in vegetable cropping systems where the amount and timing of nitrogen uptake is crucial to both the yield and quality of the harvested crop. Two SAREP-funded projects coordinated by Richard Smith, a UC Cooperative Extension farm advisor in San Benito County, are providing answers to these questions for farmers in California's Central Coast Region.

Organically Grown Bell Peppers

In 1992 Smith received funding to explore the feasibility of using cover crops prior to planting bell peppers, an unusually long-season crop with a high demand for nitrogen (about 250 pound per acre over a five-month growing season). Supplying sufficient nitrogen to bell peppers in organic production systems is particularly challenging. Can the soil be conditioned to meet this demand over 150 days? A well grown leguminous cover crop can supply 200 to 250 pounds of nitrogen that the subsequent crop can use, but according to Smith, the release of this nitrogen reaches a peak about 75 to 80 days following incorporation into the soil. Under these conditions, will there be enough nitrogen for the remainder of the growing season?
In Smith's study, nitrogen from a lana vetch cover crop (about 200 pounds per acre) was supplemented with 0, 25, 50, and 100 pounds of nitrogen in the form of feather meal. Feather meal is an organic form of nitrogen that releases nitrogen over 120 days. Smith felt that the slow release form of nitrogen would complement the relatively fast release by the cover crop. The results of his study indicate that the cover crop nitrogen was running out toward the end of the season, as evidenced by the petiole nitrate levels which were below the conventional critical level of 700 ppm in the 0 and 25 pounds per acre treatments, and the fact that the unfertilized plants had turned yellow by the end of the season. Nitrogen applications of 50 and 100 pounds per acre as feather meal provided adequate nitrogen to the bell peppers over the season to maintain the recommended petiole nitrate levels, but, Smith said, this added nitrogen did not lead to any significant increase in yield. When Smith repeated his study in 1993, the additional nitrogen in the 50 and 100 pounds per acres treatments did result in significant yield increases.

Organic, Conventional Systems

In 1995 Smith began a new study aimed at providing organic farmers with better information about how to monitor the nitrogen status of their crops. As the bell pepper study revealed, growers that utilize organic sources of nitrogen such as compost and cover crops are able to provide enough nitrogen to meet the crop demands, but they have to pay close attention to how that nitrogen is released over the course of the season. University of California researchers have been conducting a number of studies over the past several years to determine critical levels of nitrate in the fresh sap of vegetables (as measured by hand-held selective electrode nitrate meters) to aid growers in making quick fertility decisions. These tests have proven to be useful in conventional production systems using chemical fertilizers. There is increasing evidence, however, that organic farms have lower soil and plant nitrate levels and yet are achieving yields comparable to conventional systems. Thus the applicability of these tests and established critical values may be of limited value in organic farming systems.
In order to determine the value of the fertility tests to organic growers, Smith set up a project to monitor nitrogen in four farms producing long day onions: two organic farms using compost, cover crops and feather meal; and two conventional farms. Soil nitrate and ammonium were monitored, as well as nitrate levels in the fresh sap of root tissue and the total nitrogen in the onion tops.
As found in other studies, there were dramatic differences in the levels of soil nitrate and ammonium between conventional and organic systems. Nitrate levels were similar at the beginning of the season (about 20 ppm), but by mid-June nitrate in the conventionally farmed soils increased to 76 ppm and remained significantly greater than in the organic system until the end of the season. Nitrate levels on the organic farms remained constant at about 15 ppm through the entire season. A similar pattern was found with soil ammonium. Nitrate in the onion plants (both roots and tops) was also lower in the organic system than in the conventional system, according to Smith.
One of the organic farms had a severe pink root disease problem, so yields could not be considered in the final analysis. The yields from the other organic farm, however, were comparable to the yield from the two conventional fields, even though the soil nitrate and ammonium levels were low and the levels of nitrate-nitrogen in the roots and total nitrogen in the tops were much lower.
Smith said these results indicate that quick test technology measuring nitrate-nitrogen in the fresh sap may not be a useful diagnostic tool for organic farmers. The organic onions in this study obtained reasonably adequate supplies of nitrogen even though plant nitrate levels were low, so the question arises, how did they do this without the large pools of nitrogen in the soil? Smith is planning further research to help find an answer to this question and to determine whether revised critical levels are needed for organic systems.

For more information, contact: Richard Smith UCCE San Benito County 649-A San Benito Street Hollister, CA 95023-3952. (408) 637-5346.

 

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