The availability of nutrients in manure to a crop is dependent on the chemical makeup
of the nutrients. With nitrogen, three different measurements are given in a manure
analysis:
- Total Kjeldahl Nitrogen (TKN) is the total amount of organic and ammonia nitrogen in the
sample.
- Ammonia nitrogen (NH3) is the amount of inorganic nitrogen that is readily converted to
plant available forms.
- Organic nitrogen is determined by the difference between total nitrogen and ammonia
nitrogen.
Once the manure in storage has been well-agitated, the manure should be sampled for
nutrient analysis. Once handling of manure begins, volatile gases such as ammonia are
given off and the nutrient value of the nitrogen in the manure decreases. Therefore, it is
important to anticipate how much of a certain nutrient will be lost before it can be
utilized by the crop.
Volatilization of ammonia represents a significant cause of nitrogen loss. Loss of
ammonia depends upon method of application and time before incorporation, as shown on the
application chart for liquid manure:
Method of Liquid Manure Application |
% N Loss |
*broadcast, no incorp. (2-3 days) |
25-35 |
broadcast, immediate incorp. (< 2 days) |
15-25 |
broadcast, no incorp. on cover crops |
35 |
injection |
0-2 |
*irrigation (3 days or less) |
25-35 |
*If time to incorporation is greater
than three days, nitrogen losses can be 40-60 per cent for broadcasting and 60-80 per cent
for irrigation. For solid manure, volatilization losses from broadcasting may be less than
those reported for liquid manure. |
Factors conducive to volatilization:
- high soil pH (>7.0);
- soils high in calcium carbonate (lime);
- soils with low retention ability for ammonium: e.g., low clay content, low organic
matter, low cation exchange capacity;
- high soil or atmospheric temperature;
- liquid fertilizer applied onto dry soil;
- high wind velocity and/or highly aerated soils;
- high rate of fertilizer application;
- depth of incorporation/penetration < 2 cm.
Nitrate leaching is the form of nutrient loss that receives the most environmental
concern. Leaching of nitrate-nitrogen occurs if excess nitrates are not utilized by the
crop and are moved down the soil profile by infiltrating water.. If nitrates move out of
the root zone, the nitrates are no longer available to the plant and eventually end up in
the groundwater, where they may cause health problems in drinking water. Nitrogen
fertilizers should be added to the soil only at rates that can be utilized by crops during
a growing season. Nitrates should be present in the soil for the shortest possible time to
minimize the likelihood of leaching. The Farm Practices
Guidelines recommends no more than 150 lbs/ac of nitrate-nitrogen present in the top
four feet of the soil profile and no more than 20 lbs/ac of nitrate in each subsequent
foot of the soil profile.
Factors contributing to nitrate leaching to groundwater:
- coarse-textured or extensively cracked soils
- high concentration of nitrates in the soil profile due to excessive applications of
nitrogen fertilizers, including manure
- significant precipitation to move nitrates downward
- limited plant root zone (due to plant species, time of year) to intercept nitrates
- groundwater table present (higher risk if closer to surface)
- use of irrigation
Not all of the above conditions must be met for nitrate leaching to occur.
To prevent nitrate leaching:
- Identify areas in the landscape where nitrate leaching into groundwater is most likely
to occur (these are usually areas with coarse-textured soils and shallow water tables)
- Test soil every year. Manitoba's Farm Practices
Guidelines recommends no more than 150 lb/ac of nitrate-nitrogen in the
top four feet of the soil profile and no more than 20 lb/ac of nitrate-nitrogen in
each subsequent foot of the profile. Refer to Appendix F, Monitoring Soil Nitrate, in
Manitoba's Farm Practices
Guidelines.
- Monitor groundwater levels and sample groundwater for nitrate content annually or when
possible. Environmental guidelines have set 10 parts per million (ppm) as the
maximum nitrate content for drinking water consumed by humans.
- Apply only as much nitrogen fertilizer to the crop that can be used in one growing
season. Adjust commercial nitrogen fertilizer additions when manures have been applied.
- Minimize the window of opportunity for nitrate leaching. The time when nitrate is
present in the soil to the time when it is utilized by the crop should be as short as
possible. Spring applications are less risky than fall applications, and split
applications of nitrogen during the growing season are less risky than applying all the
nitrogen at the time of planting.
- If high levels of nitrate-nitrogen are found below four feet, use crops suited to your
operation for retrieval of deep-leached nitrates.
Denitrification, like volatilization, can result in loss of fertilizer nitrogen. The
difference between the processes is that volatilization occurs due to exposure to the
atmosphere, whereas denitrification is loss of fertilizer nitrogen due to the absence of
oxygen. The rate of denitrification decreases with depth and increases with temperature
because it depends on biological activity.
Factors conducive to denitrification:
- soils with high organic matter (5 per cent or greater);
- limited oxygen, due to high water content, rapid respiration or compaction;
- neutral or alkaline pH (7.0 or greater);
- temperatures > 2˚C;
- chemodenitrification (denitrification without microbial activity) requires low pH, but
may be significant in freezing soils with high salt concentrations and high nitrite
content.
To prevent denitrification:
- Avoid high applications of nitrogen to areas in the landscape with high water tables and
intermittent ponding.
- Follow the same practices to prevent nitrate leaching.
The overall management practices to consider are:
- Incorporate manure as soon as possible.
- Test soil regularly to monitor nitrogen utilization and possible losses.
Tisdale, S. L., Nelson, W. L. and Beaton, J. D. 1985. Soil Fertility and Fertilizers.
4th ed. New York: Macmillan Publishing Company.
Farm Practices Guidelines for Producers in Manitoba
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