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10.10: Activity 3 - Soil Nitrate-Nitrogen Analysis

  • Page ID
    14702
  • Name: ______________________________

    Section: _____________________________

    Student ID#:__________________________

    Adapted from: T.K. Hartz. 2010. Using the Pre-Sidedressing Soil Nitrate ‘Quick Test’ to Guide N Fertilizer Management. UC Vegetable Research and Information Center.

    Safety Considerations: Read hazard information below on today’s experiment. Wear appropriate eye protection. Avoid contact with skin for all corrosive and irritating reagents. If contacted, flush with copious amounts of water and inform your instructor immediately.

    Hazard information: Calcium chloride (0.01 M CaCl2) may cause eye, skin, and respiratory tract irritation.

    Procedure

    1. After viewing each of the three treatment containers with the binocular microscope, carefully add 50 ml of the 0.01 Molar calcium chloride extracting solution to each of the containers.
    2. Using a glass stirring rod, stir mixture for 30 seconds. Rinse stirring rod and return to proper location.
    3. Allow containers to sit undisturbed for 15 minutes while soil particles settle.
    4. After the settling period, obtain one nitrate strip for each container and carefully tilt the container to about a 45-degree angle to allow enough depth of solution for dipping of the test strip. Dip the test strip with tab facing down to avoid floating organic matter. Lightly shake off any excess solution and read the nitrate color from the test strip against the color chart at one minute after dipping. The color pad at the end of the test strip is the one to read for nitratenitrogen.
    5. Cleanup: Place the nitrate test strip into a trash can and thoroughly rinse the plastic container with tap water to remove all soil and extraction solution, dumping rinse material into bucket in sink. Place plastic container in proper storage container.

    Nitrate Observations

    Color of the test strip

    Amount of nitrate produced (ppm)

    Control
    Alfalfa
    Sawdust

    Nitrate Questions

    1. Rank the treatments (control, alfalfa, sawdust) in terms of the amount of nitrate produced in each treatment, from greatest to least amount of nitrate produced.

    Immobilization of nitrogen occurs when the carbon to nitrogen ratio (C:N) is high (≥30:1) and an insufficient level of ammonium and nitrate exist in the soil to meet the needs of the microorganisms that are decomposing the organic matter. Usually, immobilization occurs when the nitrate level is nearly zero. Immobilization occurs during the initial part of the degradation of most organic materials in the soil. After a sufficient period of time, the microbes will run out of energy after they have decomposed most of the organic material and release large amounts of carbon dioxide. Once the microbes have run low on energy, they begin to release extra ammonium into the soil by the process of mineralization. The ammonium produced in the soil is rapidly converted to nitrate by the nitrifying bacteria. Consequently, after mineralization, the nitrate level in the soil amended with an organic material will be higher than was the nitrate level of the original soil.

    2. Based on the discussion above, briefly explain the ranking (#1) (the amount of nitrate produced in each treatment).

    3. Do your results for the amount of nitrate in each treatment make sense in terms of the C/N ratios of the various treatments?

    4. What would be the effect on plants if a finely ground organic matter having high C/N (85/1 straw) were added to the soil?

    5. What 3 practical steps can be taken to reduce nitrogen immobilization by microorganisms and the resulting nitrogen deficiency in the plants when applying high C/N organic materials to soils for the next crop?

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