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8.4: Activity 3 - Soil Nitrate

  • Page ID
    14647
  • Name: ______________________________

    Section: _____________________________

    Student ID#:__________________________

    Source: NRCS Soil Quality Test Kit Guide, Ch 7

    At home before lab:

    1. Collect approximately ½ cup of soil from a garden or farm. Make sure to sample only to a 0-3inch depth. To collect a representative sample, collect three samples across the field to capture variability. Determine the texture of your soil sample and record it.

    Materials needed to measure soil nitrate:

    • 1/8 cup soil collected by student prior to lab

    • Filter paper

    • 120-mL plastic container with lid
    • Eye dropper
    • Nitrate/nitrite test strips
    • Stopwatch or timer
    • Distilled water

    Procedure (in lab)

    1. Familiarize yourself with the directions on the side of the bottle of nitrate strips.
    2. Prepare soil-water sample
      1. Thoroughly mix soil sample before taking 2 tablespoon sub-sample and placing in plastic container. Record weight of subsample.
      2. Add 30 mL of distilled water to the soil subsample, creating a 1:1 soil to water ration on a volume basis
      3. Put the lid on the container and shake vigorously 25 times.
    3. Fold filter paper in half (semi-circle), the fold again into a quarter-circle.
    4. Insert filter paper into subsample
      1. Open filter paper into a cone shape and push it quickly into the container with the soilwater mixture until it touches the bottom of the container. Wait for an eye dropper-full of the solution has seeped through to the inside of the filter paper.
    5. Place drops on Nitrate Strips
      1. Using the eye dropper and one nitrate/nitrite test strip, place 1 or 2 drops of the filtered solution on each of the strip’s two pads. Note the time.
      2. Note: one pad measures the amount of nitrite and the other measures the amount of nitrite and nitrate combined. Nitrite rarely occurs in measurable amounts in the soils, so nitrite readings from the test strips are not recorded.
    6. Measure and record nitrate
      1. Align the nitrate/nitrite test strip with the bottom of the bottle with your thumb corresponding to the diagram on the bottle.
      2. After 60 seconds, compare the first pad furthest from your thumb) along the nitrate scale and estimate the amount according to the degree of color change. Record that value on the worksheet in ppm. This is an estimate of the nitrate-N concentration in the solution.

    Calculations:

    Estimated (lb NO3-N/acre)=

    \( \mathrm{\dfrac{(ppm\; extract\; NO_3-N) \times (depth\;of\;soil\;sampled\;in\;cm) \times bulk\;density \times 0.89}{10}} \)

    Exact (lb NO3-N/acre)=

    \( \mathrm{\dfrac{(ppm\; NO_3-N) \times (volume\;water\;used) \times (depth\;of\;soil\; sampled,\;cm) \times bulk\;density \times 0.89}{(dry\;weight\;of\;soil) \times 10}} \)

    Volume of water used = 30mL + [dry weight of soil x soil water content (g/g)]

    **ppm in extract = lb NO3-N/acre / 43.6 = lbs/1000 sq ft

    Note: for garden topsoil, bulk density can be assumed to be 1.1 g/cm3 and 1.5 g/cm3 for subsoil. For all other soil textures, see table below.

    Table 1. Soil bulk density according to texture

    Soil Textures

    Avg bulk density (g/cm3)

    Organic matter

    0.22

    Sand

    1.56

    Loamy sand

    1.54

    Sandy loam

    1.5

    Loamy sand

    1.45

    Silt loam

    1.2

    Sandy clay loam

    1.63

    Silty clay

    1.55

    Clay loam

    1.45

    Silty clay loam

    1.4

    Note: The maximum nitrate-N reading on the nitrate/nitrite test strip container is 50 ppm. If the sample reading falls into the 50 ppm category, the sample can be diluted to get a better estimate of the actual amount over 50 ppm. To dilute the sample, fill the eye dropper with filtered solution and place five drops in a plastic container. Add five drops of distilled water; mix gently by swirling the container. Take a reading with a new test strip as stated in Step 4. Multiply the estimated nitrate-N in ppm by 2 before using the calculations. If the nitrate reading falls into the category of 50 ppm again, repeat the dilution steps, and multiply the estimated nitrate-N in ppm by 4.

    Soil sample site description

    Soil sample texture
    Bulk Density

    Soil NO3-N ppm (est.)

    Estimated Soil NO3-N (lb NO3-N/acre)

    Exact Soil NO3-N (lb NO3-N/acre)

    N-Availability Questions

    1. Using Table 2, determine three vegetables that would have soil-N requirements met based on your soil sample.

    2. Compost might typically contain about 1% total N, but only 10% of that becomes available in the first year. So typical compost would contain 0.1% NO3-N, meaning about 1000 lb of compost is needed for each lb of N to be added, and substantially increase SOM content. Explain how compost can provide a supply a slow release of nutrients and long-term benefits to soil health.

    Table 2. Nutrient requirements of common vegetables

    Screen Shot 2021-06-24 at 08.36.34.png

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