Sustainable Agriculture Literature Review - Boulder County

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! !! "#$%&!'())!! !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!*+!,-./!01#/2&! 5.3 Organic Matter, Cover Crops, and Green Manures Soil organic matter and humus have been found to create a healthy and fertile soil structure. 387 Most soils in Colorado have low amounts of organic matter, containing less than 1.5 percent. 388 Soils containing higher percentages of organic matter or humus are better able to retain water and nutrients and resist erosion. Cover crops and green manures can serve as organic material sources for agricultural systems, helping to decrease the potential for soil erosion while improving soil tilth, water-holding capacity, 389, 390 stability, and structure. A cover crop is any crop that is grown to provide soil cover mainly for the purpose of reducing wind and water erosion. A green manure is any field or forage crop, including a cover crop, that is incorporated into the soil while the crop is still green or flowering. Cover crops and green manures can be annual, biennial, or perennial plants that grow in a pure or mixed stand during all or part of the year. In addition to providing ground cover they may also help suppress weeds and reduce diseases and insect pests. 391 Factors affecting the economics of cover crop use include: 392 ! The cash crop grown ! The cover crop selected ! Time and method of establishment ! Method of termination ! The non-market value applied to the environment, soil productivity, and soil protection benefits derived from the cover crop ! The cost of nitrogen fertilizer and the fertilizer value of the cover crop ! The cost of fuel In the literature reviewed, there are three types of costs associated with using cover crops: direct, indirect, and opportunity costs. The direct costs are affected by the cost of seeds, water and energy, as well as nitrogen fertility dynamics in cover crop systems. Direct costs are typically associated with cover crop establishment and are particularly high for legumes. Studies state that the costs of establishment can be ten times higher for leguminous cover crops than for grasses because of large seed size, seed dispersal mechanisms, and the generally weak emergence of leguminous cover crops. 393 However, the increased cost of the legume cover crop seed can be offset by the value of nitrogen that legumes can replace from other fertilizers. 394 The cost of legume seeds is minor compared to the cost associated with delaying a main crop planting in order to 395, 396 plant cover crops, and is only practical in some cases. Another direct cost associated with cover crops and green manures is water use. Regardless of which tillage system is used, the risk of early-season soil water depletion by cover crops remains the same. !,342#.5#6/1!78$.%3/23$1! 9.21$#23$1!:1;.1
! !! "#$%&!'())!! !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!*+!,-./!01#/2&! season. This is dependent on rainfall distribution in relation to crop development.” 399 Water consumption by a cover crop is a concern in areas with less than 30 inches of precipitation per year, like <strong>Boulder</strong> <strong>County</strong>. 400 Additionally, turning green manure into the soil requires additional time (labor) and energy expense, compared to not planting a cover crop. 401 The indirect on-farm costs include hindering the establishment of the succeeding cash crop and cover crop management problems that impede realization of the expected benefits (i.e. over-vigorous cover crops that are hard to kill or incorporate). 402 Some of these potential disadvantages are highlighted in Figure 19. Also, cover crops have been shown to harbor insects, diseases, and nematodes that could be harmful to a cash crop, but conversely, have been used in conservation tillage systems to attract beneficial insects. Understanding these specific crop and pest interactions can be complex and is best evaluated at individual farms. 403, 404 For additional information see Pest Management. ! "%#" !,342#.5#6/1!78$.%3/23$1! 9.21$#23$1!:1;.1
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SUSTAINABLE AGRICULTURE LITERATURE
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Sustainable Agriculture Literature Review - Boulder County

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