Testing Your Soil

This photo shows a test capsule for a potassium in the soil test. Testing your soil with a simple kit can give you results that let you know what types of fertilizers to add. This one shows the results of a K test, or a potassium test. By matching the color of the left column of test liquid to the colors of the scale, t shows between an adequate and sufficient amount of potassium. — Testing your soil with a simple kit can give you results that let you know what types of fertilizers to add. This one shows the results of a K test, or a potassium test. By matching the color of the left column of test liquid to the colors of the scale, t shows between an adequate and sufficient amount of potassium.

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In our last post, The Major Elements Plants Need: N-P-K, we talked about the three important elements that contribute to your plant’s growth. This post will talk about testing your soil so that you know what is in your soil already that you may or may not need to add. Looking at your plants’ leaves (and possibly roots if the plant has died) may tell you what elements you may need to add, but if that doesn’t work, you need a soil test.

Some soil tests can test the big three: N-P-K. Some can go much deeper and cover calcium, magnesium, and other minor nutrients. For our purposes, we are going to discuss the big three with the addition of alkalinity/acidity.

pH (Acidity and Alkalinity)

First, let’s discuss alkalinity and acidity. Alkalinity and acidity are measured on a scale called pH. The chemical description of pH is that it is number of hydrogen ions in a water solution. Whether we can remember that tidbit isn’t the important part. The important part is that pH ranges in numbers of 0 to 14, where below 7 is acidic and above 7 is alkaline. The lower the number, the more acidic the soil. The higher the number the more alkaline the soil.

This pH scale not only shows how the numbers represent acidity and alkalinity, it show the effects on nutrients as you go from very acid to very alkaline. (Courtesy of Mainland Aggregates)

Most experts agree that it is a difficult task, if not an impossible task, to change the pH of your soil by more than a number or two. But you still need to know, in general, what your soil is so that you can choose your plants with an eye to what their needs are in terms of acidity or alkalinity.

Effect of pH (Acidity and Alkalinity) on Plants

There are just some plants that are going to be hard to grow in your neck of the woods. Although the ease of growing a plant can be the result of many things, alkalinity and acidity rule out many plants for your area, unless you are lucky enough to live in a neutral area.

Blueberries are not going to be happy in my backyard because they thrive where the soil is acidic. Yes, you can amend the soil and raise the acidity, but it is going to take a lot of work. In my case, even the water here is also alkaline, so I would need to amend the soil to raise the acidity, and then water the blueberries with distilled water or rainwater.

Photo of Blueberry bush. Blueberries! Oh so luscious! However, you better make sure you have acidic soil, or are willing to put in a lot of work amending your soil. Photo courtesy of Gurneys.com — Blueberries! Oh so luscious! However, you better make sure you have acidic soil, or are willing to put in a lot of work amending your soil. Photo courtesy of Gurneys.com

Grammy’s Alkaline Soil

I tend to just deal with the alkalinity of my soil rather than trying to change it. For instance, where I live the soil is alkaline, and strongly alkaline. What that means in practice is I don’t grow perennials that need an acidic soil.

One problem with very alkaline soil is that it is hard for iron to dissolve in the water for uptake into a plant. That can lead to a plant that looks like it’s low on nitrogen, but it is actually fine with nitrogen. Such a plant needs a little something acidic to dissolve the iron.

Plants that Grow In Alkaline Soil

Blooming Back Yard has a nice list of vegetables that do best in alkaline soil: Asparagus (6.0-8.0), Beans, pole (6.0-7.5), Beet (6.0-7.5), Brussels Sprouts (6.0-7.5), Cauliflower (5.5-7.5), Garlic (5.5-8.0), Kale (6.0-7.5), and sweet pea (6.0-7.5).

This photo shows a head of cauliflower. This cauliflower from my garden is one vegetable that likes to grow in alkaline soil. — This cauliflower from my garden is one vegetable that likes to grow in alkaline soil.

And they also have a list of flowers that like alkalkine soil: Anchusa, Borage, California Poppies, Lavender, Lily of the Valley, Phacelia, Polemoniums, Trifolium (Clovers), Viper’s Bugloss, Wild Marjoram. Of these, I’ve only grown borage, California poppies, lavender, trifolium (clovers), and Wild Marjoram.

I’ve grown many other flowers with much success. For instance, my favorite flowers to grow are zinnias, verbenas, wild bergamots, obedient plants, calendulas, as well as others. The point is that you may be able to grow many things, but some things are going to require a lot of care because of their acid/alkaline preference.

Using a Soil Test and My Results

It is easy to test for alkalinity/acidity. It is also possible to test for N-P-K, although perhaps not so easily. With the results of these tests, you can adjust what kinds of fertilizers you might add to help your plants along.

I got a soil test kit from Amazon. Then the fun began. Okay, not so much fun but interesting results.

Procedure for Testing your Soil

I dug a 4-inch-deep hole to collect my sample of soil.

This photo shows a hole in the garden and a yardstick showing it is 4 inches deep. The instructions for the soil test say to dig a hole 4 inches deep and then gather your soil from there. — The instructions for the soil test say to dig a hole 4 inches deep and then gather your soil from there.

The test instructions say to avoid touching the sample with your hands. I was also careful to use a clean bucket and other implements. Lay the soil out on a tray to dry, and remove any weeds, stones, or other big particles. Using a clean implement, crush any lumps of soil. Mix thoroughly.

Testing your Soil for pH

For the pH test, you fill the little container with a tiny bit of soil up to the line indicated. Then with the provided dropper you add water. The first time I used regular tap water. The second time I used distilled water. It made a huge difference. Finally, you add the contents of a capsule provided by the test and shake it up. After the soil settles and the color develops in about a minute, you have the results.

This is the result using tap water. You can see that the color is very blue and doesn;t match any of the scale on the right.

This is the result using distilled water. You can see that my soil is very alkaline.

Here are the results of the pH test, the first with tap water and the second with distilled water. You can see that I am dealing with extremely alkaline soil. Acidic amendments may help, and some frequently recommended amendments are definitely not going to be good for my soil and my plants.

Testing Your Soil for N-P-K

For the N-P-K tests, you make a solution of soil and water first.

Testing Your Soil Procedure

You use 1 cup of soil and 5 cups of distilled water (or a 1 to 5 soil/water ratio). After shaking the soil/water solution, you let it settle for 24 hours.

This photo shows a mason jar containing soil and water. For the test you add 1 part soil to 5 parts water, shake thorooughly, and let sit for 24 hours. — Add 1 part soil to 5 parts water, shake thorooughly, and let sit for 24 hours.

Using the dropper, you add the liquid part of the solution to fill the little containers for the N, P, and K tests. You add the matching color of capsule to the container, close the lid, and shake thoroughly. Allow the color to develop for 10 minutes.

Results of N-P-K Tests

Here are the results of the nitrogen, phosphorus, and potassium tests.

Testing your Soil for Nitrogen

This photo show the nitrogen test which by the color of the solution shows shows a deficient amount of nitrogen in the soil. Time to add compost and other sources rich in nitrogen. — The nitrogen test shows a deficient amount of nitrogen in the soil. Time to add compost and other sources rich in nitrogen.

It looks like my soil needs nitrogen, which is not surprising since I took my soil sample at the end of the hot summer after growing tomatoes there. Nitrogen is used heavily by tomatoes, and the heat also decomposes nitrogen. (This is another good reason to move your plants around to different areas of the garden, so that plants with different needs will perhaps be moved to a better spot for them.)

Testing your Soil for Phosphorus

The soil test shows an adequate amount of phosphorus. We are looking for approximately the same shade of the color, not an exact match. No need to add more.

Testing your Soil for Potassium

This photo shows the potassium test (K) shows an adequate suppy of potassium in the soil. — The potassium test (K) shows an adequate suppy of potassium in the soil.

The P and the K tests look like those two elements are adequate.

Conclusion

I’m not too happy about the ease of interpreting the soil test. The colors do not match the scale exactly. Even so, we are able to get an approximate scale of how much of each nutrient is in the soil. For an even more precise measure, you should check with your extension agent for a nearby lab that does soil testing.

It is important do to the soil tests so that when you fertilize, you are not wasting money by adding a nutrient that is already there.