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Home HortScience A Winning Combination for Organic Broccoli Crops
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SANTA CRUZ, CA—Organic farmers, who don't use chemical fertilizers, are constantly searching for new ways to grow crops and profits. A study reports on an effective combination of organic winter cover crops and fertilizers that increased returns and yields in organic broccoli crops.

Previous studies have found that winter cover crops can improve soil and water quality, limit erosion, and suppress weeds. Some species can be planted after a crop is harvested, allowed to grow, then tilled back into the soil before planting the next year's crop. The nitrogen (N) found in the off-season plant is incorporated naturally into the soil and becomes available to fertilize the subsequent crop.

Legumes provide atmospheric nitrogen while cereals recycle residual soil nitrate. Combining the two increases the amount of plant cover and the benefits of both. "Use of a legume/cereal mixed winter cover cropping is a common practice among organic growers on the central coast of California," reports a study published in HortScience, led by University of California Santa Cruz researcher Joji Muramoto. The study noted that the central coast of California is a prime location for broccoli—the region's growers supplied almost 30% of the U.S. supply of conventional and organic broccoli in 2007.

Muramoto's team set their experiment in two locations. The first site (Site A) had been an organic farm for 33 years. The researchers planted mixed legume/cereal crops consisting of oats, beans, and wooly pod vetch on the entire plot. These crops were mowed in May; any shoots that remained above ground were raked away on the "without cover crop" plots. All plots were tilled and an organic fertilizer (feather meal) was applied at four concentrations. Broccoli was planted in late June and harvested in mid-September. An additional application of blood meal, another organic fertilizer, was applied at four concentrations in late July.

Another trial was conducted at the second site (Site B), which had been used for cover crop trials for the previous years. A mixed legume cover crop consisting of oats, beans, wooly pod vetch, and peas was planted only on some of the plots in mid-October. Feather meal was applied in four concentrations to all of the plots in late April, and the broccoli was harvested in late June and early July. The trial was repeated using the same plots and methods during the following year.

Broccoli crops from both sites were evaluated for biomass and nitrogen content at three stages: early, midgrowth, and harvest. The amount of biomass stayed about the same for all cover crop plants, but the first year Site B cover crop plants had higher nitrogen concentrations. Site A produced much more and heavier broccoli than Site B due to its higher soil N fertility brought about by the long organic management. At Site A and the first year Site B, the increase in yield with cover crops was about equal to the yields without cover crops that received the low concentrations of fertilizer. In contrast, in the second year Site B, broccoli yield decreased by use of cover crops due to soil inorganic N immobilization caused by the low-N cover crops incorporation.

The study results showed that adding cover crops helped to make broccoli crops more profitable by reducing the amount of additional fertilizer needed for optimum plant growth in a field with a long history of organic management, or when cover crop N content was sufficiently high to prevent immobilization.

"This study underscores the critical need for appropriate supplemental N fertilization to achieve economically viable yields in the successive organic broccoli," Muramoto said, adding that cover crops can reduce the need of nitrogen fertilization.

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The complete study and abstract are available on the ASHS HortScience electronic journal web site: http://hortsci.ashspublications.org/content/46/8/1154.abstract

Founded in 1903, the American Society for Horticultural Science (ASHS) is the largest organization dedicated to advancing all facets of horticultural research, education, and application. More information at ashs.org


 

Original Article:

Nitrogen Contribution of Legume/Cereal Mixed Cover Crops and Organic Fertilizers to an Organic Broccoli Crop
Joji Muramoto, Richard F. Smith, Carol Shennan, Karen M. Klonsky, James Leap, Miriam Silva Ruiz, and Stephen R. Gliessman
HortScience 46:1154–1162. [Abstract][Full Text][PDF]

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