Last updated 11-13-07
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Testing Water & Soil
Experiment #13: Soil Testing
(old name was lab5, exp3)

Often when you see fertilizer products, you see a set of three numbers on the packages. In the left bag, we see "32-3-4" and the right package "9-15-20." These numbers are referring to three fertilizers: Nitrate, Phosphorus, and Potassium. They are always listed in this order. The numbers are percentages by weight.



NITROGEN: Nitrogen is necessary for new growth and green plants.

If you've read the Building Block tutorials in the lecture course, you may remember that nitrogen is needed to make amino acids and DNA.

An amino acid has an "amino" group, which always has a nitrogen. Amino acids are chained together to make proteins. Proteins create structure and act as enzymes.

Nitrogen is absorbed as nitrate (NO3)- or ammonium (NH4)+.

Nitrogen is also used in chlorophyll.  Notice the four nitrogen atoms that hold the magnesium (Mg) atom.
Looking at DNA to the left, you will see several nitrogens are needed in the rings. So nitrogen is needed for fast plant growth because as cells divide, DNA needs to be duplicated.
PHOSPHORUS:  The molecule that provides energy for all living things is called Adenosine Triphosphate (ATP). To the left is the structure of this molecule. Notice the three phosphates using three phosphorus atoms. It is absorbed as H2PO4- and HPO4-2. Phosphorus is also part of DNA (see previous picture).

POTASSIUM: Potassium ion (K+) is needed to control the water uptake in the plant. The plant to the left shows a deficiency of potassium.  Potassium is also needed for the assembly of proteins, ATP, and used in photosynthesis. It is absorbed simply as the ion (K+)

In this fertilizer, the zero means there is no phosphorus. In some areas by lakes or rivers, it is recommended that fertilizers not have phosphorus because the phosphorus can over stimulate growth of algae in the lakes or rivers. The numbers "15" are interpreted as 15% of the total weight of the fertilizer is nitrogen and 15% is potassium.

This is my front yard. The area near the sidewalk has trouble growing anything. I decided to test the soil in this area to see if there were any deficiencies of nitrogen, phosphorus, or potassium.

In your kit is a glass jar with a stack of plastic cups inside. You will use one of the smaller plastic cups. You will also need a filter paper from this jar. We want to place mark on the small cup so we can measure out 100 mL of soil.
One way to know where that mark should go is to place 100 mL of water into the cup. Use your 100 mL graduated cylinder. Fill it with tap water.
Pour the 100 mL of water in to the cup.

Your kit has a few sheets of small paper labels. You can take one and stick it to the side of the cup or use a pen to mark the level of 100 mL.

Pour the water out. The label will now serve as the 100mL mark.


Take a picture of where you are sampling the soil.

Use a spoon, spade, or knive to dig into the soil. You probably want to take dirt that is a few inches down. That's where the roots are. So you may want to go 3 inches or much deeper depending how deep the roots of a new plant might start at.
Place soil into the cup.

Fill cup up to the 100 mL mark. That way we use a consistent amount of soil to compare results.


Use distilled water to measure out 100 mL of water using the graduated cylinder.

(Note: the picture is only showing about 40 mL of water. You want a whole 100 mL)

Pour the 100 mL of distilled water into the cup with the 100 mL of soil.
The soil needs to be stirred. In your kit is a stirring rod with a rubber end (called a rubber policeman). Don't use the rubber end; it's too soft. Just use the other end to stirrer.
You will need a filter paper. So open the glass jar in your kit. Remove the plastic cups and retrieve a filter paper.
First fold the filter paper in half.
Now fold the filter paper but not evenly. Fold it as shown in picture.
Open one side of the folded filter paper.
Place the filter paper in the funnel and place the funnel on top of the 100 mL graduated cylinder.
A trick in minimizing solids from getting poured out with the liquid is to pour against a rod. The rod directs the liquid into the funnel (reduces splashing) and it blocks the solids from coming out with the liquid. This process is called "decanting," which to pour off a liquid without disturbing the "sediment."
Let the water pass through the filter paper leaving the soil behind.
Keep pouring water from the cup with the soil until you collect at least 10 mL of liquid. More is OK. Yours might be clear or have a brownish tint like mine did here.

Open up your Soil Test tablet cannister and find the foil covered tablets labeled "FLOC EX" on one side and "TesTabs" on the other side. The other tablets you will see are "POTAS" (potassium), "PHOS" (phoshorus) and "NITRATE". For now, you just need the "FLOC EX" tablet.

Tear open the "FLOC EX" foil and drop one tablet into the graduated cylinder. This tablet is a flocculent. A flocculent causes some impurities in the water to become insoluble and settle to the bottom. This will make the liquid more clear and easier to check color changes.

If you want the tablet to dissolve easier, you might want to hook up the portable air pump. Connect about one foot of the tubing to it and let it bubble air through the water in the graduated cylinder. This will speed up the dissolving of the tablet.

We need to dilute the liquid we got through the funnel four fold. To do this we can measure out 10mL of the soil-water extract into a 10mL graduated cylinder.
Now pour the 10mL into the small 40mL beaker.

Pour some distilled water into the 250mL beaker and then transfer enough distilled water to bring up the liquid level in the 50mL beaker up to the 40mL mark. This dilutes the soil-water extract four-fold.


We are doing this because we are using four times the soil that is normally used for these soil tests. Using more soil will better represent the soil we are testing. However, using four times more soil will also increase the concentration of nitrogen, phosphorus, and potassium four-fold. The final color chart for the tests expects lesser amounts, so we are diluting the soil extract by four to make it match the color charts.

Pour the diluted soil extract into a test tube. Fill the test tube about 3/4 full.

This is the test tube that you will test nitrogen levels.

Find the foil wrapped tablet with "NITRATE" printed on it. Tear open one side and place one tablet into the test tube with the diluted soil extract.

Set your stop watch to zero and then start it when the tablet is placed in the test tube.

Put the cap on the test tube and start shaking it until the table dissolves. Also watch the stop watch. At five minutes you compare the color of the liquid to that of the chart.
At five minutes compare the color of the liquid with the chart for Nitrogen. I'd say this soil sample contains a high amount of nitrogen. I kind of expected this since I had placed a large amount of cow manure on the soil awhile back.
For the phosphorus test, we use 25 drops from our diluted soil extract. Use the plastic pipette. 25 drops is a way to measure out 1 milliliter fairly accurately. Or you can use the 10mL graduated cylinder.
Distilled water is now added to the text tube to bring up the level to about 3/4 full. This will dilute the soil extract even further. For the color chart to be accurate, this amount of dilution was necessary.

Locate the foil wrapped tablet labeled "PHOS." (short for phosphorus)

Also, set your stopwatch to zero.

After dropping in the "PHOS" (phosphorus) test tablet, start the stopwatch.

Place cap on test tube and start shaking until dissolved.

After five minutes you will check the color of the text tube water with the color chart.

At five minutes compare the color of the text tube with that of the color chart for Phosphorus. I'd say that this sample of soil is low in phosphorus. This is good to know. Now I know what to buy for my soil.


Now on to the potassium test.

For the potassium test, fill another test tube about 3/4 full with the diluted soil extract in the 40mL beaker.

Add one tablet from the foil packet labeled "POTAS"


Again start the timer after you add the tablet.

I found the "POTAS" (potassium) tablet hard to dissolve. So I took the microspatula and used it to break up the tablet before capping the test tube and shaking it.

Now hold the test tube over the black squares next to the potassium "L M H" squares. The more potassium the more opaque the liquid will be. I estimate my sample to be between the M (medium) and H (high) levels of potassium.


So it looks like the only thing my soil needs is phosphorus. Nitrogen and potassium levels seem good.

Take a picture of the final three test tubes after the tests have been performed. I realize the colors will be different than they were at the 5 minute mark. It may help to place some white paper behind them to help the colors stand out.

(As always, I need a picture of you doing some step in this lab. Perhaps take a picture of you shaking a test tube or holding up a test tube and looking at it.)

Email the results you got to me along with the three pictures from this lab.

Different computers since Jan. 5, 2005
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