Previously we looked at the Nitrogen Cycle and determined that natural methods of increasing the nitrogen contents of most soils is possible, but a legume must be used in the crop rotation. For thousands of years, humans had little impact on the nitrogen cycle. The strong bond found in nitrogen gas prevented its simple conversion to other, much more useful, nitrogen compounds. People were therefore entirely dependent upon bacteria for the initial nitrogen fixation. Once fixed in the nitrogen cycle, nitrogen compounds could be collected and used as fertilizer.
In more recent times as agricultural systems changed the use of a legume in the cropping rotation was no longer economically feasible because of the availability and low costs of nitrogen fertilizers. Nitrogen is the plant nutrient most universally needed to meet the requirements for high crop yields. It is also most responsible for green lawns, since it is an important constituent of chlorophyll.
Prior to 1916, sources of fixed nitrogen were found in Chile. This outcrop was due to a vast number of sea birds which nested, and deposited their fecal matter along its coasts. Over thousands of years these "natural" deposits called "Guano" accumulated and became several feet thick. A huge industry developed to supply this Chilean saltpeter to the rest of the world. With synthetic production almost non-existent, the world was entirely dependent on the Chilean resource for fertilizers and high explosives. See History of N Production
Supplying N to the Soil
The ability of Minnesota's soils to supply nitrogen is estimated by considering: 1) soil nitrate level, 2) percent organic matter, and 3) soil texture. A soil test for nitrogen can be used for recommendations as to how much N fertilizer is needed, but it is only valid if rainfall does not leach nitrates from the root zone before plant uptake.
For Nitrate leaching to occur, two conditions are needed: 1) the soil must
contain soluble nitrates, and 2) water must move through the soil. Reductions
in nitrate losses can be achieved by: 1) improving nitrogen fertilizer
placement, 2) applying part of the fertilizer N later in the growing season,
and 3) using slow-release fertilizers or nitrification inhibitors and currently
recommended soil test procedures for fertilizer management.
If an N soil test is not done another way to determine the need for nitrogen is by basing the decision on: 1) the nitrogen-supplying capabilities of the soil ( % OM < 3=low, > 3%=med or high), 2 ) the cropping history, 3) the crop to be grown, and 4) the expected yield of the crop.
Where corn yield goals are high or more than 130 bushels per acre, more nitrogen will be recommended. In Minnesota the "yield goal potential" is determined by the climate and soils of each region of the state. The natural continued decomposition of organic matter will release nitrogen for plant growth, thus soils with less OM will require more nitrogen fertilizer.
Forms of Nitrogen
Generally, we can say the N that costs the least is the best. However, there are often additional considerations. Some nitrogen materials like anhydrous ammonia, while being the least expensive require considerable safety precautions and may be more costly to apply. The list below provides the forms of the different nitrogen carriers. Soil specialists will generally agree that one form does the job as well as another when both are properly applied at equal rates of actual nitrogen. However, we do need consider some other factors. Tables of Fertilizer Materials
Nitrogen Fertilizers
Shortly before the outbreak of World War I, two
Germans developed a method for producing synthetic ammonia. The first plants
using this "Haber-Bosch Process" were constructed shortly after the
outbreak of the war. They had discovered that ammonia could be made by placing
nitrogen gas and hydrogen gas in a high pressure chamber. With the addition of
a suitable catalyst, and a little heat to speed things up, vast quantities of
fixed nitrogen could be produced. Other countries soon copied the process and
quickly scaled up their own synthetic ammonia production capabilities, like the
plant in Crystal City, Missouri. When the war was over, fixed nitrogen
continued to be produced in large amounts because of its use as a fertilizer
and explosive.
Different Forms of N Fertilizer Materials- All ammonium based fertilizers are derived from Anhydrous Ammonia.
1) Water Soluble: Ammonium nitrate, Ammonium Sulfate, Anhydrous Ammonia; Most effective for rapid green-up when soil temperature is below 55-60 °F (before May 1) strongly acidifying to soil. May cause burning of turf if not watered-in immediately.
2) Water Soluble Organic: Urea ; Rapid - Slightly less available than soluble, inorganic forms when soil temperature is below 55-60° F, but other characteristics are similar. May cause burning of turf if not watered-in immediately.
3) Water Insoluble Natural Organic: Processed sewage sludge (Milorganite), Tankage Seed meals; Moderate to slow release; also contains some phosphorus. Release to nitrogen forms most rapid when soil temperature is above 55-60°F. Minimum danger of burning turf.
4) Water Insoluble Synthetics: Ureaformaldehydes, Osmocote, Slow release; Slow nitrogen release until soil temperatures are above 55-60°F. Normally mixed with soluble, readily available forms. Minimum danger of burning turf when used alone.
One
important factor in choosing a form of nitrogen is the rate of
nitrogen release. This is very rapid for the water soluble materials like
ammonium nitrate and urea. However, these materials may readily burn vegetation
like turf if they are not washed off the leaf blades. They don't really burn
the turf, but, because they are a salt, they will adsorb water from the leaf
blade, thus killing that portion of the leaf. When a large amounts of
fertilizer is spilled on turf, the fertilizer needs to be picked up, rather
than washed into the soil. If it is washed in, the high salt level may kill the
turf.
Nitrogen Deficiency Symptoms
This
corn field is showing nitrogen deficiency symptoms. The older lower leaves are
the first to show the deficiency of N, since N is mobile in the plant. Notice
that N deficiency yellowing occurs along the midrib of the corn leaf. For
almost all plants, the yellowing of the plant will take place when N is
deficient.
Nitrogen Losses
The main goal in a Nitrogen fertilization program is to keep the nitrogen available for plant use, while at the same time keeping it from becoming a pollutant or being lost to the atmosphere via denitrification. When the nitrogen applied needs to remain in the root zone and available to plants over a longer period of time, natural organic or synthetic organic nitrogen fertilizers are recommended.
Leaching of nitrate nitrogen( NO3- form) will take place when gravitational water moves through the soil profile. Nitrates can pollute groundwater and create health hazards for humans and animals. Go to N Water Pollution to view an example of this kind of pollution. Also the web page at Nitrates provides for more information about nitrates and human health and nitrogen contamination of ground water or go to Nitrates and Nitrites .
Nitrogen in Rocks
There is a growing interest in the role of bedrock in global nitrogen cycling and potential for increased ecosystem sensitivity to human impacts in terrains with elevated background nitrogen concentrations. Nitrogen-bearing rocks are globally distributed and comprise a potentially large pool of nitrogen in nutrient cycling that is frequently neglected. For more information on nitrogen in rocks go to the abstract by Holloway and Dahlgren
Nitrogen losses can also occur when denitrification takes place. This often occurs when soil become saturated or compacted and oxygen becomes deficient for soil organisms. For a look at a flooded corn field go to Flooded Corn
For more information on nitrates in drinking water go to Minn Dept. Health
Go to Chapter 4
Fertilizer Chapters
© Terence H. Cooper & Regents of the University of Minnesota, 2007. The University of Minnesota is an equal opportunity educator and employer.
