Soil Nutrient Analysis: Nitrogen, Phosphorus, and Potassium

1. Extraction of Nitrogen (Nitrate NO₃^-)

1. Turn on the balance, set a weigh boat on top, and zero the balance.
2. Use a spatula to weigh out 10 g of soil (dried and sieved) and transfer to a labeled 100-mL beaker.
3. Weigh out 0.1 g of calcium sulfate and transfer it to the beaker.
4. Using a 25-mL graduated cylinder measure 20 mL of deionized water and transfer to the beaker.
5. Repeat steps 1.1 - 1.4 for each nitrogen soil sample.
6. Thoroughly mix the contents of each beaker with a stir rod.
7. Secure samples on a table-top shaker and shake for 1 min.

2. Extraction of Phosphorus and Potassium

1. Turn on the balance, set a weigh boat on the top, and zero the balance.
2. Use a spatula to weigh out 2 g of soil (dried and sieved) and transfer into a labeled 100-mL beaker.
3. Use a 25-mL graduated cylinder to measure 20 mL of Mehlich 2 soil extractant into the cylinder. Transfer to beaker.
4. Repeat steps 2.1 - 2.3 for each phosphorus and potassium sample.
5. Thoroughly mix the contents of each beaker with a stir rod.
6. Secure samples on a table-top shaker table and shake for 5 min.

3. Nutrient Extraction Filtration - This step will be performed for all three analytes (nitrate, phosphate, and potassium)

1. Secure one end of the funnel hose onto a vacuum jet.
2. Secure the other end of the hose to the side arm of the flask.
3. Assemble the funnel by snapping together the cylinder and perforated top disk. Place the assembled funnel on top of the side-arm flask by inserting the rubber stopper into the top of the flask to secure the funnel on top.
4. Place 1 clean filter paper on top of the funnel.
5. Turn on the vacuum jet.
6. Slowly pour soil extract solution into the funnel, allowing the extract to drain away from the soil and into the bottom of the funnel flask.
7. Pour filtered extract into a new, labeled 50-mL beaker. This filtrate will be analyzed as is.
8. Remove funnel, discard filter paper, and rinse funnel and flask with deionized water. Use air jet to dry funnel and flask.
9. Repeat steps 3.3 - 3.7 for each soil sample.

4. Sample Analysis with Color Comparator for Nitrate

1. Label one color viewing tube “S” for sample and another color viewing tube “B” for blank.
2. Thoroughly rinse both color viewing tubes with deionized water. Shake the tubes to remove the remaining rinse water.
3. Add a small amount of the sample extract (prepared in steps 1.1 - 1.7) about ¼" deep to the color viewing Tube marked “S”. Cap the tube with a rubber stopper and shake it for 3 s. Discard this solution.
4. Add the sample extract to both tubes until the meniscus is even with the 5-mL mark on the tubes (bottom of the frosted area).
5. Add the contents of one NitraVer 5 Powder Pillow to the tube marked “S”. Cap and shake the tube vigorously for exactly one minute.
6. Immediately place tubes “S” and “B” into the comparator with tube “B” in the outside hole and tube “S” in the inside hole.
7. Wait 5 min, then hold the color comparator up to a light source. Rotate the disc until the color in the window for tube “B” matches the color in the window for tube “S”. Record the concentration value (mg/L) that displays in the lower window of the color comparator box.
8. Repeat steps 4.1 - 4.7 for all replicates and record the average.
9. Repeat step 4.8 for all nitrate samples.

5. Sample Analysis with Color Comparator for Phosphate

1. Using the 2.5-mL dropper, add 2.5 mL of the filtered sample extract (prepared in steps 2.1 - 2.6) to a 25-mL graduated cylinder.
2. Dilute to the 25-mL mark with deionized water, cap with stopper, and invert to mix.
3. Label one color viewing tube “S” for sample and another color viewing Tube “B” for blank.
4. Thoroughly rinse both color viewing tubes with deionized water. Shake the tubes to remove the remaining rinse water.
5. Add a small amount of the diluted extract about ¼" deep to the color viewing tube marked “S”. Cap the tube with a rubber stopper and shake it for a few seconds then discard this solution.
6. Add the sample extract to both tubes until the meniscus is even with the 5-mL mark on the tubes (bottom of the frosted area).
7. Add the contents of one PhosVer 3 Powder Pillow to the “S” tube. Cap and shake the tube vigorously for one minute.
8. Immediately place tubes “S” and “B” into the comparator with tube “B” in the outside hole and tube “S” in the inside hole.
9. 3 min after completing Step 5.8, hold the color comparator up to a light source. Rotate the disc until the color in the window for tube “B” matches the color in the window for tube “S”. In a lower display area on the box, the color disk will simultaneously display the concentration value corresponding with the color intensity chosen. Record the concentration value that displays in the window.
10. Repeat steps 5.1 - 5.10 for all replicates and record the average.
11. Repeat step 5.10 for all phosphorus samples.

6. Reagent Addition and Analysis for Potassium

1. Using a 1-mL dropper, add 3 mL of potassium sample extract (prepared in steps 2.1 - 2.6) to a 25-mL cylinder.
2. Add DI water to the 21-mL mark on the cylinder. Firmly cap the cylinder with a rubber stopper and invert to mix.
3. Add one potassium 2 reagent powder pillow to the cylinder.
4. Add 3 mL of Alkaline EDTA solution to the cylinder.
5. Cap the cylinder and invert several times to mix. Allow solution to stand for 3 min.
6. Add the contents of one potassium 3 reagent powder pillow.
7. Firmly cap the cylinder and shake vigorously for 10 s.
8. Allow the solution to stand for 3 min as a white turbidity develops.
9. While looking straight down into the cylinder, slowly insert the potassium dipstick vertically into the solution until black dot is no longer visible from above the cylinder.
10. Hold dipstick in that position and rotate the cylinder so it can be seen the scale on the dipstick. Look across the surface of the scale on the dipstick. Record the number on the dipstick scale where the surface of the sample meets the dipstick scale.
11. Repeat 6.1- 6.10 for all replicates and average. Repeat 6.11 for all potassium samples.
12. Refer to the potassium conversion table to determine the concentration of potassium in soil samples. Locate the dipstick reading on the left column and record the corresponding mg/L concentration on the right column.