Soil Nitrogen
Background:
Nitrogen is one of the major elements required for life. It is an essential for plant life because it
stimulates above-ground growth, and produces the rich green color that is characteristic of a healthy
plant. Although molecular nitrogen (N2) makes up 78% of the atmosphere, this form of nitrogen
cannot be used by animals and most plants to make essential amino acids and proteins. This
molecular nitrogen must first be “fixed” (combined with oxygen or hydrogen) to compounds such as
ammonia (NH3) or nitrate (NO3-), or some other organic form of nitrogen.
Some nitrogen fixation occurs by lightning and some by bluegreen algae, however, the bulk of
nitrogen fixation is performed by bacteria living in the soil. One variety of nitrogen fixing bacteria
lives free in the soil, while the other variety lives within the root nodules of legume plants (soybean,
peanut, beans, clover, alfalfa). The decomposition of materials on the forest floor is also a source of
nitrogen because ammonia or ammonium (NH4+) is produced in the process of decomposition. The
movement of nitrogen from the atmosphere to inorganic forms such as ammonia and nitrate,
followed by the incorporation of nitrogen into plant matter is represented by the nitrogen cycle,
which is shown in the figure below.
If soil is deficient in nitrogen, plants become spindly, stunted, and pale. The rate of plant growth is
usually proportional to the rate at which nitrogen is supplied. However, an excess of nitrogen can
damage plants just as overfertilizing the lawn can burn and damage the grass. Because nitrogen is an
element essential to plant growth, but is most often found in relatively low quantities in forest soils,
nitrogen, along with phosphorus, often becomes the nutrient that limits plant growth and therefore
limits forest productivity. The ammonium ion can bind to clay particles to remain in the soil,
however, the nitrate ion does not and is often washed or “leached” from the soil by water. Most of
the nitrogen in soils will be found in the upper soil horizons and as a consequence, nitrogen can
easily be depleted in soils when a disturbance occurs.
If the soil lacks adequate amounts of organic matter, resulting in poor aggregate structure, porosity,
and high levels of compaction, nitrogen will be leached (or washed) from the soil. Additionally,
when the erosion process removes the upper soil horizons, including deposited organic matter,
nitrogen sources are removed from the ecosystem.
Procedure:
Step 1:
Soil Extraction: Extract the nitrogen (in the form of nitrate) from the soil
Method: KCl extraction
1.
2.
3.
4.
5.
Measure a 35g sample of soil into a specimen cup.
Add 75mL of 2M KCl solution. Securely cap each cup.
Swirl each hour for 24 hours (it can sit unswirled overnight) or use a shaker-table for 1 hour.
Filter the liquid through filter paper.
Use extract promptly or freeze until needed. The solution may be kept for 24 hours in the
refrigerator.
Step 2:
Testing for soil nitrate
Method: Hach Nitrate Test Kit #1468-03
1.
2.
3.
4.
5.
Place 5mL of extract into the control tube and place into the color wheel container.
Place 5mL of extract into the testing tube and add the contents of one nitrate reagent packet.
Cap the tube and shake vigorously for one minute.
Place the testing tube into the color wheel container.
Holding the container up to the light, slowly turn the wheel until the colors in the two
windows match.
6. Record the nitrate level in ppm.
Mechanism:
Mixing the soil with KCl floods the soil particles with cations, causing all other cations in the
soil to be “knocked off” and come into solution. This is important if you are measuring
ammonium (NH4+) in the sample Nitrate is soluble in water, but needs to also be extracted
with KCl.
Source: Chris Buyarski, Sarah Hobby’s lab tech, July 16, 2010
2M KCl.
This is the extractant recommended by Keeney and Nelson (1982) to extract
exchangeable NH4 +-N. When NO3- -N and NH4 + -N are both to be measured in soil
samples, 2 M KCl is the extractant of choice. Because of its high Cl- content, 2 M KCl
also effectively extracts exchangeable NO3 - -N from soils.
To prepare, dissolve 150 g of KCl in 1L deionized water.
Source:
http://ag.udel.edu/extension/agnr/pdf/soiltesting%202009/CHAP4-2009-kg.pdf
Recommended Soil Testing Procedures for the Northeastern United States
Last Revised 10/2009
Pg. 29