Exercise 11: Determination of Soil Nitrate

Nitrogen is applied to the soil as a fertilizer element more often, and in larger quantities, than any other element. Plant response to nitrogen fertilizer is often dramatic, and the economic returns are usually quite favorable. Even so, it is important to avoid using too much nitrogen fertilizer both because of reduced profit and potential pollution hazards.

Decisions regarding the proper amount of nitrogen fertilizer to apply should be based on an understanding of several factors. One important factor is how much nitrogen will be required by the crop. The available nitrogen supplied by the soil also is important because it can be deducted from the crop requirement. The amount the soil will supply, however, is often difficult to determine because nitrogen relations in soils are quite complex.

The amount of soluble nitrate (NO₃^–) and/or exchangeable ammonium (NH₄⁺) present in a soil sample at a particular time can be measured with reasonable accuracy, but this may be a poor indicator of how much nitrogen will be available for the crop. Several factors contribute to this difficulty in evaluating available nitrogen. One factor is that nitrate moves up and down with soil water so much that it is difficult to obtain a representative sample. A second factor is that most of the nitrogen supplied by the soil must be released from organic matter by decomposition processes during the growing season and, therefore, would not be revealed as available nitrogen when the test is run. Also, some of the nitrate and ammonium nitrogen currently present in a soil might become unavailable through microbial activity or might be lost through volatilization or leaching.

The total amount of nitrogen present in a soil sample can be measured, but most of it is present in complex organic forms that are unavailable to growing plants. Some of this nitrogen will be made available by organic matter decomposition while the crop grows. This amount is much larger and more important than the amount of available nitrogen present at any one time. But the rate of release by decomposition is variable and difficult to predict accurately.

This exercise will determine two aspects of soil nitrate. The first determines the amount of soluble nitrate present in the soil and is known as the pre-sidedress soil nitrate test (PSNT). The second is the determination of the amounts of nitrate the soil will mineralize during a two-week incubation, which one assumes is related to the amount of nitrogen mineralized during the growing season.

Pre-sidedress Soil Nitrate Test (PSNT)

A tool useful to determine the amount of nitrogen fertilizer to apply specifically for corn is the Pre-sidedress Soil Nitrate Test (also called the Late Spring Nitrate Test). This test is done when corn plants in the field are 6 to 12 inches tall and involves sampling the surface 12 inches of soil. The samples must be dried within 24 hr or frozen until they can be dried for analyses. The laboratory test will determine the concentration of nitrate available in the soil. In the Northeast to Midwestern regions of the US, the critical PSNT ranges from 21 to 25 mg NO₃-N per kg. This critical concentration is considered to be the number above which a response to N does not occur. Nitrate concentrations also can be measured on the farm by using commercially available kits.

Interpretation of the PSNT results for making fertilizer N recommendations vary by state. Below is the interpretative information for corn after soybean or corn after corn from ISU pamphlet 1714. Other data exist in the pamphlet for manured soil and first or second year corn after alfalfa.

The first step in making a fertilizer recommendation for corn after corn or corn after soybean is to select a critical concentration for nitrate (i.e., the concentration that distinguishes between adequate and inadequate supplies of available N). A critical concentration of 25 mg/L N is appropriate in the absence of additional information.

The second step is to adjust the critical concentration if excess rainfall occurred at the site shortly before the soils were sampled. Reducing the critical concentration by 3 to 5 mg/L is advised if rainfall is more than 20% above normal amounts between April 1 and time of soil sampling.

The third step is to estimate fertilizer needs by subtracting the concentration of soil-test nitrate (mg/kg or ppm N) from the chosen critical concentration (mg/kg N). This value is then multiplied by 8. A factor of 8 is used because studies have shown that it usually takes about 8 lb of N/acre before planting to increase soil-test nitrate-N by 1 mg/L.

Examples:

1. A soil test of 15 mg/kg and a critical concentration of 25 mg/kg results in a recommendation of 80 lb of N per acre to be applied.

(25 mg/kg – 15 mg/kg) × 8 = 80 lb N/acre needed

2. b)A soil test of 35 mg/kg and a critical concentration of 25 mg/kg indicates that the soil already has approximately 80 lb more N than needed.

(25 mg/kg – 35 mg/kg) × 8 = -80 lb N/acre

Nitrogen Mineralized During Two-week Incubation

(You will not measure in class but understand this for quizzes and exams)

This portion of the exercise is designed to measure the soil’s potential to release (or mineralize) nitrogen through organic matter decomposition. Mineralization is the result of microbial action that converts organic nitrogen into ammonium nitrogen. The ammonium ions (NH₄⁺) are oxidized by nitrifying bacteria to nitrite (NO₂^–) and then on to nitrate (NO₃^–) when oxygen is available. Thus, the end product of mineralization is either ammonium ions under reducing conditions or nitrate under oxidizing conditions. Some nitrogen-release procedures have used anaerobic incubation of soil samples (by putting the samples in bottles containing enough water to cover the soil) followed by a test for ammonium nitrogen. The other alternative consists of aerobic incubation and a test for nitrate nitrogen, as will be used in this exercise.

The general outline of the procedure is first to remove the nitrate initially present in the sample, then incubate the soil to allow the indigenous population of microorganisms to decompose organic matter and to oxidize the released ammonium to nitrate, and finally to again remove the nitrate and measure how much was produced during the incubation period. Nitrate is readily soluble and can be removed from the soil by leaching with water. To be reproducible, the temperature conditions must be controlled during the incubation period required for the decomposition process.

Measurements

The Iowa State Soil and Plant Analysis Laboratory uses a Lachat Flow Analyzer to measure the concentration of nitrate (NO₃^–) in the solutions. This procedure involves the cadmium (Cd) reduction method where the nitrate is first reduced to nitrite by copperized Cd. The resulting nitrite is converted to a diazonium salt that when combined with a coupling reagent forms a reddish purple azo compound that can be measured colorimetrically. The intensity of this reddish purple color is proportional to the concentration of nitrate plus nitrite in solution.

Another way of measuring NO₃^– ions in solution uses a nitrate ion-selective electrode. The procedure looks somewhat similar to that used for determining pH electronically. Properly equipped meters can measure either pH or any of several specific ions for which special electrodes are available. The nitrate ion electrode contains a porous membrane that is saturated with a water-repellent, ion-exchanger liquid. When the nitrate-containing solution contacts this membrane, the nitrate binds to the ion exchanger and is transported across the membrane, which results to an electrical potential difference between the two sides of the membrane. This difference is measured relative to a constant potential generated with an external reference electrode. There are several ions that can interfere with this technique.

A third way of measuring NO₃^– ions in solution is a rudimentary method using test strips that gives a quick answer. We will use this third method in this laboratory although this method would not be used in routine laboratory analyses but it will give us general ideas of determined values. The test strips (Hach Company, Loveland, CO) are impregnated with appropriate chemicals that give a color responses in the presence of NO₃^–-N (range 0 – 50 mg/L) and NO₂^–-N (range 0 – 3 mg/L).

Note: Solution ppm = mg/L; there are 1000 mg in one gram and there are 1000 mL in one liter (thus parts per million or ppm, wt:vol).

 

Procedure

I. Pre-sidedress Soil Nitrate Test (PSNT)

1. Weigh 5.0 ± 0.3 g of air-dried soil (record exact weight) into each of two 50-mL centrifuge tubes.
2. Add 25.0 ± 0.1 mL of distilled water to each tube.
3. Shake the soil solution for 15 min using the flatbed horizontal shaker, and centrifuge at 3000 rpm for 10 min.
4. Pass the supernatant through a 12.5 cm Whatman No. 5 filter and collect the filtrate.
5. Dip nitrate strip or place droplets of filtrate onto nitrate test strip and begin timing.

Your instructor has also prepared standards containing 3 ppm and 30 ppm NO₃^–-N for you to validate the readings.

6. At 30 seconds, compare nitrite pad to color scale.

We expect nitrite levels in soils to be low.

7. At 60 seconds, compare nitrate pad to color scale and record estimate. If the value obtained is 50 ppm or greater, dilute your sample and retest.

The nitrate pad measures both nitrate and nitrite, so subtract the nitrite values, if any, from the 60-second reading to obtain nitrate levels.

8. Convert ppm in solution to ppm in soil.

Multiply the ppm in solution by the solution/soil factor (mL extractant/g soil used) to obtain ppm (mg/kg) nitrate-N in the soil. Because the nitrate that was in 5 grams of soil was extracted and diluted into 25 mL of solution, the concentration in solution must be multiplied by 25/5 (or 5) to obtain concentration in the soil.

 

II. Solvita Test for SLAN

The procedure and detailed information are taken from Solvita booklet. The Solvita soil test is a tool which easily measures soil organic nitrogen that has a potential to be mineralized into inorganic nitrogen forms, nitrate -nitrogen and ammonium nitrogen. In topsoils, nearly half of organic nitrogen is in various forms of amino-acids and amino sugars linked to humus and microbial tissue. The SLAN test provides a measure to quantify this pool of nitrogen. The ammonia gas (NH3) is released from amino-acids when soil is reacted with an alkali (2N NaOH). The results are read with the Digital Color Reader (DCR).

1. Weigh 4 g ± 0.02 g of the pre-dried and 2mm sieved soil into each plastic beaker.
2. Add 0.1 g ± 0.005 g plant material to one of the plastic beakers and mix well. This rate of residue equals 5 tons / acre.
3. Label the jars with and without the residue. Label – Don’t guess.
4. Please the plastic beakers into the glass jars.
5. Add 10 mL of 2N NaOH on the top of the soil in the beaker slowly. Be careful NOT to pour NaOH solution quickly.
6. Holding the edge of the plastic, place the Solvita paddle (probe) next to plastic beaker in glass jar.
7. Put the paddle with gel facing out next to the clear side of the jar.
8. Immediately screw the lid on jar tightly and record the time.
9. Store jars at room temperature (25 °C) for 24 hours.
10. After 24 hours (± 30 minutes), remove paddle from glass jar.
11. Insert paddle (gel up) into the DCR and press the READ button.
12. The DCR reports the SLAN color number on the first line and the ppm NH₃-N on the second line. Record both.

Calculations

To get mg/kg (ppm), multiply mg N result indicated by DCR by appropriate dilution value:

13. Calculate the amount of nitrate-N released by each soil in lb/acre-furrow slice (1 acre-furrow-slice = 2,000,000 lb of soil).

Multiply the ppm (1) in solution by the solution/soil factor (mL extractant / g soil used) to obtain ppm (2) in soil nitrate-N released by the soil. Then multiply this value by 2 to determine the lb/acre-furrow slice of nitrate-N released by the soil (ppm multiplied by 2 gives pp2m or lb per acre-furrow-slice soil or lb per 2,000,000 lb soil).

 

 

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