Sustainable Agriculture Literature Review - Boulder County

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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.
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