Download Soil Nitrogen

Soil Nitrogen
Ammonia
Ammonium
Amino Acids
Protein
Nitrate
Nitrogen Fixation
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 A – CHEMetrics Nitrate Test Kit
Use the soil filtrate from the Soil pH procedure as the solution for analysis with the CHEMetrics Nitrate
Test Kit.
This kit gives results for nitrate levels under 5 ppm and can read as low as 0.1 ppm. The nitrate kit with 30
ampoules is $ 53.70, however, there is a educational kit with 10 ampoules for $13.20 (Although this seems
like a better deal, two kits are required to achieve the same range as the larger kit. One kit measures 1-5
ppm, the other 0-1 ppm). There are also kits available at the same cost for ammonia and nitrite nitrogen.
Procedure B – Test Kit Procedures for Nitrates and Ammonia Inorganic Nitrogen Test Kit Procedures
*Reprinted from LaMotte Company Test Instructions
Equipment and Materials (with LaMotte part numbers)
Extraction Tubes 0704
Plastic Soil Measure
Transfer Pipet
1 mL Pipet
0.5 g Spoon
Spot Plate
Ammonia Color Chart
Nitrate Color Chart
Nitrite Color Chart
0819
0364
0354
0698
0159
1302
1315
1310
Universal Extract Solution 5173
Ammonia Test Solution
Nitrate Test Reagent #1
Nitrate Test Reagent #2
Nitrite Test Reagent #1
Nitrite Test Reagent #2
Nitrite Test Reagent #3
5103
5146
5147
5151
5152
5153
Extraction Procedure
The following extraction procedure uses *Universal Extracting Solution to produce a single soil extract
which is used in all of the three inorganic nitrogen tests, ammonia, nitrate, nitrite.
1.
2.
3.
Fill the extraction tube to 14 mL line with extracting solution.
Use the plastic soil measure to add two level measures of soil to the tube. Cap and shake for one
minute.
Using a piece of filter paper and plastic funnel, filter the soil solution. Catch the filtrate in the
second extraction tube.
Ammonia Nitrogen
A fertile soil may be expected to give a low ammonia nitrogen test reading, unless there has been a recent
application of nitrogenous fertilizer in forms other than nitrate. The rapid disappearance of ammonia after
fertilizer application indicates the desired transformation of the ammonia to the more available nitrate
compounds. In forest soils ammonia is the most abundant available form of nitrogen. If there is a
satisfactory rate of nitrogen transformation, the humus layers of a forest soil will produce very high
concentrations of ammonia nitrogen.
1.
2.
Use a transfer pipet to transfer 4 drops of the general soil extract (filtrate) to one of the large
depressions in the spot plate.
Add one drop of *Ammonia Nitrogen Test Solution. Stir with a clean stirring rod. Allow to stand
for one minute.
3.
Compare the resulting color against the Ammonia Color Chart. The color chart expresses the
results in relative values from very low to very high. Values can be converted to ppm according to
the table below.
Very low
Low
Medium
High
Very high
0–5
5 – 10
40
100
150
ppm
ppm
ppm
ppm
ppm
Nitrate Nitrogen
1.
2.
3.
4.
5.
Use a 1 mL pipet to transfer 1 mL of the general soil extract (filtrate) to one of the large
depressions in the spot plate.
Add 10 drops of *Nitrate Test Reagent #1
Use a .5 g spoon to add one level measure of *Nitrate Test Reagent #2
Stir thoroughly with a clean stirring rod. Allow to stand for five minutes.
Match the sample color with the Nitrate Nitrogen Color Chart. The color chart gives nitrate values
in lb/acre. Convert this value to ppm.
1 ppm = 2 lb/acre