Sustainable Agriculture Literature Review - Boulder County

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The most effective cover and green manure crops are legumes, which are nitrogen-fixing
plants. 412 Common legumes include: peas, beans, alfalfa, clover, and vetch. 413 Nitrogen
production from legumes is a key benefit of growing cover crops and green manures.
Estimates of nitrogen accumulations by leguminous cover crops range from 40 to 200
pounds of nitrogen per acre, with 50 percent of the nitrogen accumulations available in
the first year. 414 Cultural and environmental conditions, such as a delayed planting date,
poor stand establishment, limits of irrigation water available, and drought, limit legume
growth and reduce the amount of nitrogen produced. Conditions that encourage good
nitrogen production include optimum soil nutrient levels, soil pH, and adequate soil
moisture. Typical nitrogen yields from a variety of common legumes are presented in
Figure 21.
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Figure 21: Average Biomass and Nitrogen Yields from Sample Legumes 415
Alfalfa is a cool season leguminous plant noted for improving soil tilth and having a
higher potential yield than any other forage crop in Colorado. It is also known for being
drought tolerant and adapted to growing at higher elevations. 416 Alfalfa can be rotated
with grain and seed crops to increase yield. Much of this increase can be attributed to
the nitrogen-fixing ability of alfalfa, similar to other legumes. 417 The average lbs/acre of
nitrogen yields for alfalfa ranges from a low of 44 lbs to a high of 308 lbs. 418 The impact
on the nitrogen in the soil may be even greater when alfalfa is grazed instead of hayed
because of nutrient recycling. 419 An additional benefit of rotating alfalfa is its ability to act
as a living mulch, such as in an alfalfa-corn rotation. However, this can present
challenges when the alfalfa is rotated with certain crops, such as soybeans, because
they are slower growing and more susceptible to competition from living mulch. 420
Bio-char
Additional soil nutrient inputs can come from livestock manure and bio-char. Bio-char is
a charcoal rich in nutrients and organic carbon that is produced from partially burning
biomass. The application of bio-char improves soil fertility by adding and retaining soil
nutrients. Bio-char is more effective at retaining most nutrients and keeping them
available to plants than other organic matter such as common leaf litter, compost, or
manures. 421
Using bio-char can be complicated because its chemical composition needs to be
closely matched to the soil to which it is applied. Therefore, using bio-char from local and
regional settings will likely prove to be more effective than using bio-char that has
traveled long distances. The majority of bio-char projects are small scale, but are
increasingly considered as a local means of replenishing soil nutrients.
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