Sustainable Agriculture Literature Review - Boulder County

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Farmers can also prevent excess nitrogen application by utilizing precision agricultural
practices and applying nitrogen directly to the plant root at the right time of year.
Precision agriculture can increase the efficiency of applying fertilizer, which reduces
fertilizer costs, but the economic viability of precision farming needs to be evaluated on a
farm-by-farm basis. The economic benefits of precision agriculture depend on whether
the realized increase in revenue is sufficient to cover the costs of the technology. 437
When excess nitrogen is applied, non-legume cover crops can be planted following
traditional growing cycles to take up any nitrogen that was leftover from the cash crops.
Nitrogen availability and use can be improved with erosion control and reduced- or no-till
practices. Planting cover crops on fallow fields prevents nitrogen and carbon loss from
erosion and no-till practices reduce disturbance and destabilization of soils. 438
Nitrogen fertilizers have varying effects on different crops because of differences in root
systems. In a study done in the Arkansas Valley in Southwest Colorado, onions and corn
were rotated to test these varying effects. Onions have shallow root systems; therefore,
nitrogen fertilizers are more likely to leach in the groundwater, because less of the
fertilizer is recovered by the crop. By rotating crops such as wheat or corn with onions,
the alternate crop can help extract residual nitrogen. The study found that rotating crops
can help recover some residual nitrogen; together, both crops recovered around 39
percent of the total nitrogen fertilizer. However, this does not solve the concern of
groundwater contamination. 439
Manure
Animal manure, especially solid and slurry manure, contains large amounts of organic
matter. The continued application of manure can supply plant nutrients and slow down
the depletion of soil organic matter. 440 Manure organic matter can contribute to improved
soil structure, resulting in improved water infiltration and greater water-holding capacity
leading to decreased crop water stress, soil erosion, and increased nutrient retention. An
extensive literature review of historical soil conservation experiment station data from 70
plot years at seven locations around the United States suggested that manure produced
substantial reductions in soil erosion (13-77 percent) and runoff (1-68 percent). 441 While
manure offers many benefits for soil health, it can also pose risks. Continuous overapplication
of manure can result in increased salinity, degradation of surface water from
phosphorus runoff, or groundwater contamination by nitrates. Application of manure is
best applied based upon crop nutrient needs determined by soil testing to ensure
appropriate application rates. 442
Typically, there are about 1,000,000 cattle at any one time in Colorado, with each 1,000pound
animal producing between 50 and 60 pounds of manure and urine per day. The
nutrients in the manure excreted from these cattle have a fertilizer value of $34.2 million
annually in 2007 dollars. 443 In Boulder County there are over 15,000 cattle, which
produce roughly 381 tons of manure. 444 Figure 22 shows the approximate nutrient
445, 446
content of manure and the projected value per ton in 2001 dollars.
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