Testing Garden Soil in Your Vegetable Garden

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Take several subsamples from each area, then mix them and fill your containers to take to your extension office to be tested.
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A fall vegetable garden on an organic farm.
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If anything is wrong with that soil, ol' Doc Gritty will find it.
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Soil test results let you know what your soil needs in order for you to raise healthier plants and crops.

Understanding your soil’s chemistry and fertility is paramount to successful growing, yet most folks tend to skip this step and experiment with a little fertilizer here and a little acidifier there. While the hunt-and-peck approach works, why not save yourself some time and plenty of money and effort by pulling some cores, testing garden soil, and discovering what your garden really needs in order to be great.

Not even the best of gardeners can look at the soil and say, “Whoa, we have a problem with low potassium here.” And malnourished plants can’t pull up their roots and move to a more fertile patch. That is why testing the soil every three to five years is so important. Test results give insight into the soil’s needs and offer ways to amend or enrich the soil to raise healthier plants and crops. Results from soil testing will allow you to apply the right types of fertilizers or organic material – as well as the correct amounts – needed to bring the soil back into balance.

A soil sample can be taken any time, but it’s best to do it in the fall. Soil amended in the fall has all winter to adjust, and it’s ready to go just in time for spring planting.

Get ready to sample

To start, visit your local university extension office and pick up a box or bag for the soil sample. The extension service is cost-effective and includes recommendations on the best means to amend out-of-whack soils. A private soil lab also can test your soil – just be sure it’s not affiliated with a fertilizer company.

A soil test’s results are only as good as your record keeping, so before you do anything, label your sample container(s) with indelible ink. (Do this before you take your samples, because it’s hard to write on the containers once the soil’s inside.)

Rusty tools can alter the results, so always use a clean, chrome-plated or stainless steel trowel, shovel, corer or auger. You’ll also need a clean plastic bucket in which to collect and mix the soil. Now, check the soil for moisture – the soil should crumble in the hand and not be wet.  

Sample collection the easy way

Take one soil sample for each specific area – one for the garden, a separate one for the pasture, yet another one for the orchard, and so forth. If your yard has a healthy area and a not-so-healthy-looking area, sample both areas separately.

Within each area, take subsamples from six to eight different locations, spaced some distance apart for best results. In larger areas, such as pastures or fields, take 10 to 15 subsamples.

Set the shovel into the soil vertically, about 5 to 7 inches deep, and throw aside the first shovelful. Then, at the back of the hole (where the shovel’s back rested), use the shovel to cut a slice about an inch thick and 5 to 7 inches deep. Pull off the thin layer of grass and roots at the top, then place the soil in the bucket. Next, zigzag around the sample area taking subsamples.

Stay about 300 feet away from crushed lime roads when taking samples, or the soil test will show alkaline due to all the limestone dust. Also, don’t take samples from the garden where beans, peas or legumes have grown recently. (However, farmers sampling alfalfa or soybean fields can mention this to the soil lab, and the lab will adjust the results accordingly.)

Once you’ve finished taking subsamples for a given area, mix them in the bucket, breaking big clods into crumbs. Then fill the correctly labeled sample container (repeat if you have several areas to sample) and mail or deliver it to the soil lab. Be sure to enclose the processing fee. Results are generally returned in two to three weeks, except during busy seasons.  

Interpreting the results

The soil report will reveal the amount of organic material in your sample soil; the amounts of phosphorus, potassium, calcium and magnesium available to your plants; soil pH; cation exchange capacity; and fertilizer recommendations and suggestions.

Nutrients such as phosphorus, potassium, calcium and magnesium are listed from being “very low” to “very high.” Those that are listed as “low” are nutrients your soil is especially deficient in. These levels might take a year or more to bring back to normal. Nutrient levels listed as “medium” are adequate, but it wouldn’t hurt to add a little more. A “high” amount of a nutrient needs no amending. A “very high” amount also should not be amended; otherwise, that nutrient may reach levels that are toxic to the plant. More is not always better.

The pH level of the soil shows whether the soil is acidic (below 6.0), normal (6.0 to 7.0), or alkaline (8.0 to 12.0). In excessively acidic or alkaline soil, nutrients are locked away and plants can’t absorb them. Lime is used to neutralize an acidic soil, unlocking nutrients. Lime is also recommended if the soil needs phosphorus, calcium or magnesium (the last of which is available in dolomitic limestone).

Organic material (OM) is the plant or animal residue in the soil, and more is generally better. Organic material provides nitrogen, nutrients and trace minerals; helps the soil retain nutrients; and improves soil structure and water retention. A soil high in organic material can have an active biomass, which benefits soil health greatly.

Cation (pronounced cat-I-en) exchange capacity, or CEC, indicates your soil’s capacity to hold nutrients. A higher number often indicates a more fertile soil. Levels above 20 or 25 mean the soil has more nutrients than plants can use in a year, while levels below 10 mean that nutrients will be depleted very quickly and the soil needs amending. Clay soils often have a higher CEC because negatively charged clay particles hold on to positively charged ions of calcium, magnesium, potassium and nitrogen, and keep them from leaching away. Soils high in organic matter also have a higher CEC.

The report also will list recommendations on how much fertilizer to apply for different cropping options; how much lime to apply, if any; and how much nitrogen to add, as well as when to add it. 

Implementing an action plan

When you start amending your soil, use organic methods for good, long-term results. Remember, soil is complex and can take time to change, but you can coax it along. When adding lime, for instance, expect to see complete results a year after putting the lime down. This is why it’s a good idea to be patient, and to pile on the compost while you’re being patient.

Building good soil won’t take place overnight. It is an ongoing process. Working with the soil means creating the most nourishing and healthy home where your plants can thrive – and where you get the best yields from healthy crops. 

Melinda R. Cordell, a former horticulturist who is now working on an MFA in writing for children from Hamline University, lives in northwest Missouri with her family and five hens.