How to Do a Soil Test and How to Adjust pH


If some of the plants in your garden are not growing well, then the soil composition may be to blame.  The level of pH or nutrients might be too high or too low, but the only way to tell is with a soil test.

So, how do you perform a soil test?  One way is to send a soil sample to your local agricultural extension and pay a small fee.  You can also perform a soil test yourself, using a digital soil analyzer or a test kit. However, these tests will not provide as much information as a test from an extension.

Let’s go into a little more detail about how you perform a soil test using each of these methods.  Then, we’ll talk about ideal pH levels and why it matters.  Finally, we’ll look at ways to raise, lower, and maintain soil pH.

How to Do a Soil Test

As mentioned above, there are a few different methods you can use to test your soil for pH or nutrients.  They will vary in terms of time, convenience, and cost, as well as the level of detail and accuracy you will get.

Send a Sample to an Agricultural Extension

This method will probably take the most time.  However, the level of detail and accuracy is unmatched, since your soil will be tested in a lab by trained scientists.

soil
Getting a soil test from an agricultural extension service can help you to address problems with pH or nutrient levels in your soil.

The first step is to find your local agricultural extension office.  Check out the USDA website to search for agricultural offices by state.

Once you get to the website of an office near you, find the soil testing area of the site.  For example, the University of Massachusetts Amherst site has a “Services” tab, and under this tab there is a “Soil and Plant Nutrient Testing Laboratory” link.

The cost at UMass Amherst is $20 for a garden soil test, plus extra for additional tests.  (The cost will vary depending on your state).

Your office’s website should also provide detailed instructions on soil sampling.  For reference, you can check out the soil sampling instructions on the UMass Amherst site here.

They suggest that you collect soil from a depth of 6 to 8 inches, removing stones and other debris, and breaking up any soil clumps.  Then, put about one cup of dry soil in a plastic Ziploc bag, and send it in.

Do not send wet soil samples to the lab!  If your soil is wet, put it on a plate to dry it out overnight before you bag it.

Also, include a crop code so that they know what you are growing.  That way, they can send recommendations for pH and nutrients.

It can take a few weeks to get your results back, and possibly longer at busy times of year (spring), so try to send a sample in the fall.

A routine soil test from a lab should tell you pH, acidity, and levels of nutrients including phosphorus, potassium, calcium, magnesium, iron, manganese, zinc, copper, and boron.

For more information, check out my article on what a soil test tells you.

Use a Digital Soil Analyzer

This method can be costly, depending on which soil analyzer you choose to buy.  Also, the results will not be as detailed as those from a lab.

However, a digital soil analyzer is fairly easy to use, and you can get the results quickly.  In addition, you can reuse the analyzer to perform multiple soil tests.

This is useful after you make adjustments to the soil, such as adding compost, fertilizer, or lime.  It is also helpful if you have a large garden, since the pH and nutrient levels can vary from place to place.

You can find digital soil analyzers online or in stores such as Home Depot, Lowes, or other garden centers.  The cost can vary from $25 to over $100.

Most digital soil analyzers will test soil pH, soil temperature, and soil fertility.  The soil fertility reading is based on the combined levels of nitrogen, phosphorus, and potassium (NPK) in the soil.

A digital soil analyzer has a digital display where results are shown, buttons to select your test, and a cord that connects to a metallic soil probe.  After turning on the analyzer and selecting your test, you put the probe into the soil, and wait until a reading is displayed.

Unfortunately, a digital soil analyzer will not be able to give you recommendations on how to adjust your soil pH, but we will cover that later on.

Use a Soil Test Kit

A soil test kit relies on reactions between your soil and test chemicals to display colors, which will indicate the soil pH and nutrient levels (NPK). To learn more about NPK in soil and fertilizers, check out my article on NPK ratios.

Often, soil test kits will give a range instead of an exact number, so they are not the most accurate option.  However, they are probably the cheapest option, since you can find soil test kits starting at $5 or so.

Again, you will probably want to test soil at a depth of 6 inches or so.  The reason is that your plant’s roots will be drawing nutrients from below the surface of the soil.

soil sample
This is probably too much soil for a sample – a test kit does not need much soil to work with.

Make sure to wear gloves so that you don’t contaminate the soil sample that you take.  Take a few tablespoons’ worth of soil, and remove any stones or other debris.

Spread the soil on a tray and leave it out to dry overnight.  When it is dry, put the soil in the test chamber, up to the indicated soil line.

Next, break open a test capsule for what you want to test (pH or nutrients), and pour the powder into the test chamber, over the soil.

Then, add distilled water (which is pH neutral – don’t use tap water!) to the test chamber, up to the indicated water line.

Place the cap on top of the test chamber to seal it, and then shake the chamber to mix the soil, powder, and water together.

Wait for a little while (a few minutes should do), and the water should change color.  You can then match the color to one of those listed on the test chamber (or in the instructions) to get an idea of the pH or nutrient level.

What is the Ideal Soil pH Level for Your Garden?

Now that you know how to test your soil pH, what does it mean, and what levels should you look for?  Remember that pH ranges from 0 to 14.  Anything below 7 is acidic, and anything above 7 is basic (or alkaline).  A pH of 7 is neutral.

The ideal pH level for your garden will depend on the plant, but most plants do well with a pH between 6.0 and 7.0.  Some exceptions are blueberries and azaleas, which prefer slightly more acidic soil.

Getting the right soil pH is essential for the plants in your garden to grow properly.  A soil pH that is too high or too low can lead to a nutrient deficiency, even if there is plenty of a nutrient in your soil.

The reason is that each nutrient has a specific pH range where the plant can easily absorb the nutrient.  Outside of this range, nutrient availability drops off quickly.

To learn more, check out this chart from Research Gate about nutrient availability by pH level.

How to Adjust Soil pH (Raise, Lower, or Maintain)

The best way to keep the plants in your garden healthy is to maintain adequate nutrient levels and keep the pH within an acceptable range.  One great way to do this without artificial fertilizers is to use compost.

Compost
Compost is one of the best ways to maintain nutrients and pH levels in your soil.

By adding grass, leaves, and food scraps (not meat or dairy) to a compost pile and letting it sit, you will end up with nutrient-rich soil that will help to maintain the pH in your soil.  If you have worms in your compost bin, they will create compost even faster, and it will be even better for your garden.

For more information, check out my article on how to keep soil pH stable.

Sometimes, when you are starting your garden, the soil may have been depleted by a previous gardener, due to a lack of composting, fertilization, and crop rotation.  If your soil pH is abnormally high or low, there are ways to change the pH.

If your soil pH is too low (acidic), you can raise it by adding lime (calcium carbonate) to the soil.  For more information, check out my article on how to raise your soil pH.

calcium carbonate
Adding lime to your soil increases pH, and also increases calcium levels.

If your soil pH is too high (basic or alkaline), you can lower it by adding sulfur to the soil. For more information, check out my article on how to lower your soil pH.

Cautions About Nutrient and Soil pH Levels

Remember that when you add certain materials to your soil, you can create unintended consequences.

For instance, let’s say that your soil is too acidic.  To solve this problem, you add lime (calcium carbonate) to your soil.

Now the pH is at an appropriate level.  However, since you added lime, the calcium levels in the soil may be too high.

If calcium levels are too high, it can block the uptake of magnesium or potassium by your plants.  This is true even if there is plenty of magnesium or potassium in your soil!

In effect, the calcium, magnesium, and potassium are “competing” for uptake by a plant’s roots.  Too much calcium means that the plant does not absorb enough magnesium or potassium.

Magnesium is the central atom in a chlorophyll molecule (the molecule that makes plants green), so you can imagine the problem there.  Potassium is used by the plant to develop roots and flowers, so a shortage will result in weak plants or lack of flowers.

chlorophyll molecule
Chlorophyll has a central Magnesium atom.

To get detailed analysis and recommendations on nutrients you need, sending a soil sample for testing at your local agricultural extension may be the right move.

Conclusion

By now, you have an idea of how to test your soil for pH and nutrient levels, and why these levels matter for your garden.  You also know what to be careful about when making changes to your soil.

I hope that this article was helpful.  If you have any questions or ideas on how to adjust soil pH, please leave a comment below.

jonathon.david.madore

Hi, I'm Jon. Let's solve your gardening problems, spend more time growing, and get the best harvest every year!

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