Sustainable Agriculture Literature Review - Boulder County

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5.4 Soil Amendments
Soil amendments are instrumental to soil health management. Soil amendments are
simply any material that is mixed into the soil, however, they do not imply that the
material is helpful or harmful to soil health or plant growth. Organic soil amendments can
either be organic material, such as compost and manure, or an organic fertilizer that
does contain a certain guaranteed level of nutrients. 422 In Colorado, compost is
considered a soil amendment even if it does not contain microorganisms. Fertilizer
differs from compost because legally it must guarantee a certain level of nutrients.
Technically, mulch is not considered a soil amendment because it is applied to the
surface of the soil rather than mixed in. Selecting the best amendments to achieve the
desired results, using amendments appropriately, not over-amending, and evaluating
their productivity are all functions of managing soil health. 423
Nitrogen Applications
One of the main problems associated with nitrogen fertilizer is that it is typically over
applied. In many regions, the nitrogen levels in groundwater exceed EPA standards. 424
The USDA conducted tests in Colorado’s South Platte River Basin aquifer and found
that in 70 percent of their test sites within ten miles of feedlots, nitrate levels were above
EPA standards. 425 This illustrates that manure and other nitrogen fertilizers can easily
cause water pollution if they are not managed properly. For this reason, it is necessary
that nitrogen fertilizers be efficiently used. 426 Application rates for nitrogen fertilizers can
be calculated using several different methods. In the Western Great Plains region,
nitrogen algorithms using field productivity are generally used to decide the appropriate
use of nitrogen. 427 The effects of nitrogen over application are explained in more depth in
section 4. Water Pollution.
Nitrous oxide emissions are primarily generated from synthetic fertilizer applications,
excessive nitrogen applications, and nitrogen leaching. 428 Nitrous oxide has a global
warming potential of 310 – the impact of nitrous oxide is 310 times more severe than
carbon dioxide – and is the largest contributor to agricultural GHG emissions. 429
Excessive nitrogen applications can be mitigated with slow- or controlled-release
synthetic fertilizers, manures, and/or legumes. 430, 431 Nitrogen leaching and runoff can be
prevented through erosion control using cover crops, reduced tillage, increased soil tilth,
precision applications to coincide with crop usage, and improved irrigation management.
In Boulder County, leaching losses are mostly due to furrow and flood irrigation.
Nitrous oxide is released from excess nitrogen applications that are not used by crops.
All forms of nitrogen applications can contribute to nitrogen losses, but because legumes
fix nitrogen from the atmosphere and fulfill remaining nitrogen needs using soil nitrogen,
the primary contributor of excess nitrogen is synthetic nitrogen and, to a lesser degree,
manure. About 50 percent of the nitrogen released from manure or legume crops
becomes available to the plants in year one, and most of the remaining nitrogen is used
to build up soil humus and improve overall soil fertility. 432 Similarly, the crop plants
absorb about 50 percent of nitrogen contained within synthetic nitrogen fertilizers; but
instead of the remaining nitrogen being used to build soil fertility, the nitrogen is released
from the soil as nitrous oxide and runoff or leaching. 433, 434 Using a slow- or controlledrelease
synthetic fertilizer, manure, and/or legumes can help to prevent excess nitrogen
435, 436
losses.
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