Soil Analysis for Gardeners

article image
by Lorain Ebbett-Rideout
Gardening/Soil Analysis

Whether you’re an experienced gardener who wants to learn more about your existing garden plots or a beginner interested in whether your newly dug garden plot will bear fruit, there are several tests you can employ to test your garden soil; some for free or at minimal expense. Even the most costly tests — a full laboratory soil analysis — are surprisingly affordable.

As with most things, you can start with a simple reality check. If you are examining an existing garden plot, what have you grown there successfully? Have you had unexplained difficulties growing certain crops but not others? If your gardening experiences have been mostly positive — with disease, lack of sunlight, or lack of watering attributed to most failures — your garden soil is likely fairly healthy. However, if some (or all) crops will simply not grow, or will not thrive, you may have a soil-related problem.

If you’ve dug a new garden, what was growing there previously will give you some hints of the soil’s fertility. If the site was covered with grasses or otherwise lush, low-lying vegetation, the soil is probably suitable for gardening. If the site was largely barren, perhaps dominated by a few large plants, you can expect some problems. You should observe any prospective garden patch to ensure it receives full sun.

Drainage test

One of the most important elements in suitable garden soil is its drainage. If the soil retains too much water, the roots of plants can rot, and produce will grow poorly there. A simple drainage test, however, will tell you everything you need to know. To perform the test, pick one or more low-lying spots in the garden to test. If the garden is completely level, test the middle. To perform the test, dig a hole 1 foot deep and 6 inches across. Fill the hole with water and let it drain. Then fill the hole with water again. Now that the surrounding soil is saturated with water, measure how long it takes to drain. If the drainage time is less than 4 hours, you can plant most vegetables without problem. In the case of extremely well-drained soil, the water may be gone within minutes. Poor drainage is most often seen in soils with too much clay and not enough sand.

Soil structure

The components of soil can be broken down into three components: clay, sand, and silt. In the right proportions, these combine to make loam — a rich, well-draining type of soil that is optimal for gardens. A very quick method for assessment of your soil structure is to take a handful of moist, although not soaking wet, soil and squeeze it into a ball. If you open your hands and the ball immediately crumbles, your soil is sandy. If it stays in a ball, even when prodded with your finger, the soil has a lot of clay. If the soil remains as a ball, but crumbles relatively easily when prodded with a finger, it is loamy — a nice blend of sand and clay that has the potential to make great garden soil.

The structure of your soil — with regards to the amount of clay, sand, and silt — can be examined in more detail with a Mason jar test. All you need for this is a Mason jar and its lid. To perform the test, take a soil sample and fill a quart-sized Mason jar 1/3 to 1/2 full. To take the soil sample, scrape away the top inch of soil and dig the soil beneath. If there are any large rocks or large hunks of organic matter (roots from previous crops, for example), remove these. Next, fill the Mason jar with water, leaving about 1/2 to 1 inch of air space on top. (Some gardeners add a couple drops of liquid dishwashing detergent to this water — this may help the soil particles separate a bit, but isn’t strictly needed.) Now shake. And when I say shake, I mean seriously shake the contents of the jar for at least 2 minutes. If everything inside the jar does not look uniformly suspended by this point, keep shaking. In order for the test to work, everything in the jar needs to be evenly suspended throughout. Then let the contents settle. With some sandy soils, this may happen within a few hours. For soils with extensive clay and silt, it may take 48 hours. The longer you leave the jar, the more distinct the three layers will be.

To read the test, simply measure the depth of each layer and express each as a percentage of the whole. For example, if the layer of sand (the bottom layer) is 1 inch thick, and the entire soil layer is 2 inches thick, the sand layer is 50 percent of the total thickness. Of the soil components, sand is the heaviest and forms the bottom layer. Silt is the next heaviest and forms the middle layer. Clay is the lightest and forms the top layer. Depending on soil composition, the layers will vary not only in texture, but in color. Ideally, garden soil should contain 40 percent sand, 40 percent silt, and 20 percent clay.

In addition to the layers of soil described, you may find organic matter floating on top of the water. This may be peat and a possible indicator of acidic soil. Or, you may find a layer of white crystals at the bottom of the jar. This is chalk, an indicator of alkaline soil.

With a shovel, a trowel, a Mason jar, and an afternoon, you can perform all the previous tests and gather some useful information about your garden soil. There are a couple other pieces of information that will help, but they take more time and effort to obtain.

Acidity or alkalinity

The pH of your garden soil — a measure of how acidic or alkaline it is — is good to know. Most garden vegetables grow in soil with pH values from 5.5 to 7.5. A pH of 6.5 is good all-around pH. Outside of these pH ranges, plants have difficulty absorbing some nutrients and may be more susceptible to disease. Measuring the pH of soil is not difficult, but you need to have a decent pH meter.

Most garden centers sell cheap test kits, often for $5 to $10, that purport to measure pH. You scoop some soil into a test chamber, add some water and an indicator solution, and compare the color of the water to a color chart. Likewise, for $15 or more, many garden centers sell “pH probes” that you stick into your soil and read the pH from a needle. Paper pH test strips are also inexpensive and easy to find. Unfortunately, these can be hard to interpret or not measure pH on a fine enough scale to really be useful. Most home gardeners can distinguish between soil that’s in the right ballpark and soil that’s completely out of whack. However, to get a good, narrow-range pH reading, you’ll need a decent pH meter.

Fortunately, reasonably accurate pH meters can now be found starting at around $60. (You can find them cheaper, but buyer beware.) A quality pH meter will include an option to calibrate the meter with known solutions of pH 4 and pH 7. And, of course, a quality pH meter will return the same value when the same solution is measured twice in a row. A meter that never “settles down” and gives unrepeatable measurement is useless.

A pH meter also has uses beyond taking soil pH. If you are into canning, it can help you determine the pH of foods so you know whether they need to be pressure canned or not. They are also helpful to home brewers and home winemakers.

To measure soil pH, take a small sample. Only a few ounces are needed. Scrape the top layer of soil away and sample from within the next 6 inches in depth. Spread the sample out on paper towels or a newspaper somewhere dry and let the soil dry completely. Now, weigh out a few ounces of the soil sample. Likewise, weigh out the same weight in distilled water. (Tap water will not work; it needs to be distilled water.) The exact weight doesn’t matter, only that the weight of the soil and water are equal. Combine the two in a clean jar and shake for a few minutes to mix thoroughly. Then, let the container sit for at least a couple of hours to let the sample settle. Finally, calibrate your pH meter (per manufacturer instructions) and take the pH of the liquid standing above the soil.

An easier way to measure your soil’s pH is to send it away to a lab and have them do it. When you do this, you can also get a wide variety of other things tested for.

Nutrient levels

Finally, a gardener may wonder about the level of nutrients in his or her soil. Unless you’re a chemist, there’s no practical way to test for all the relevant elements yourself. As a gardener, you’d benefit most from knowing the levels of the macronutrients, including nitrogen, phosphorous, and potassium — the NPK of garden fertilizers — plus calcium, magnesium, sulfur, and sodium. In addition, there are a whole bunch of micronutrients — boron, manganese, iron, zinc, copper, molybdenum — that may be important for some crops. Luckily, there are several labs that you can send soil samples to in order to get them tested for these. And pH testing is also a part of the tests.

Soil testing can come through your county extension, local universities, nurseries, and independent labs. An internet search for “soil testing labs” yields a large number of labs, as well as lists of labs by region. Most labs’ websites list the various tests they perform, and many show sample test result sheets. Keep in mind that some labs are geared toward farmers, not gardeners, and do not process small numbers of samples. University extensions almost always will, however.

These tests are inexpensive — starting around $6 for the most basic, single sample test — and rarely exceed $40 for the number of samples a gardener would want. Most testing labs will mail you the bags or vials to package your soil in, along with instructions on how to collect it. Return your sampled soil to the company, and your test results will arrive in a short amount of time.

Check with the lab and understand that there is a spring rush every year.

Even in a fairly small garden, the levels of nutrients can vary quite a bit from one end to the other. Some garden books urge gardeners to take several samples of soil and mix them together to get an average soil sample. The problem with this is that no plants will live in that hypothetical average soil. If you have reason to believe your soil varies within your garden, test samples from individual spots. This will give you the amount of variation in your garden, and you can calculate an average, if that interests you, from the data.

A good soil lab report gives you not only the numbers — the concentration of various nutrients — but some analysis. Most will tell you the normal range for a given compound, so you can see how your soil compares. Many will point to remedies for various soil deficiencies. Examining the sample test sheets from various sites will let you pick a lab that gives you results you can use.

Of all the soil tests, nutrient levels can change the fastest. During the growing season, plants take in all the macronutrients in quantities great enough to be measurable in the soil. This is generally countered by the gardener’s application of fertilizers before the crops are planted and frequently also during the growing season. As such, a soil analysis is only a snapshot of a specific place in time.

Getting your soil analyzed is, of course, a first step. The next step is amending the soil to make it more hospitable for your crops. Often, remedies are suggested by the lab report. And, local nurseries should be well versed in local soils. They should be able to help you interpret your soil report and suggest the appropriate steps to take toward building better garden soil.

Building soil with the one and only Harvey Ussery. 

Worm Census

Worms help a garden with both drainage and aeration. And, a healthy population of earthworms is the sign of a garden with sufficient organic material to grow garden vegetables well.

A simple worm census is easy to do. Wait until the soil temperature is above 50 degrees Fahrenheit and choose a day when the soil is damp but not waterlogged. Dig a 1-cubic-foot sample of soil — a cube 1 foot on each side and 1 foot deep. Pile the soil onto spread-out newspapers or a light-colored tarp. Then, start looking for worms. Start at one end of the soil and search through it, pushing examined soil to the side. Work your way through the entire pile, counting every earthworm you encounter. If you have 10 or more, congratulations, your soil is healthy enough to support a reasonable population of earthworms. Low earthworm counts are usually seen in soil with too little organic matter.

Chris Colby is an avid gardener who lives in Bastrop, Texas, with his wife and cats. His academic background is in biology, but his main interest is in brewing beer.

  • Updated on May 9, 2022
  • Originally Published on Jul 31, 2017
Tagged with: gardening, soil, soil health, soil test