What To Know About Soil Testing

If you grow food in one part of your yard and your kids play in another, it’s important to know what’s in your soil. Soil testing, whether DIY or mail-in, is the best way to find out.

Optimal soil is rich in micro-nutrients, organic matter and certain minerals that interact to provide plants with the right ingredients for growth. You can’t see if your soil is deficient, but you’ll know it when your plants fail to thrive or don’t produce fruit or flowers as expected.

Why Get Your Garden Soil Tested?

Soil testing, whether you use an at-home test kit or send a soil sample to a commercial service, will tell you if your soil is too acidic or alkaline (called the pH level), as well as whether it’s missing other important components. This simple test will point you to solutions, such as adding lime, sulfur, compost or fertilizers.

Lab tests typically go a step further than DIY tests. Labs review your soil’s organic matter level, which indicates its nutrient- and water-holding capacity, among other things.

The best time to test is early spring or late fall, which gives you time to make adjustments before the next growing season.

What Are You Testing For?

Most soil tests evaluate:

• The amount of alkaline, or excess lime, which occurs naturally in some parts of the country;

• Whether there is high acidity, which comes from natural components of the soil (such as granite rock) and climate (areas with lots of rain and snow often have acidic soil);

• Nitrogen, which is important to healthy plant structure like leaves and stems;

• Phosphorus, which contributes to blossoms and fruit;

• Potassium, which helps plants resist disease.

Sampling Your Soil

For soil testing, collect a sample from the area where you wish to plant. For best results, the University of Massachusetts Soil and Plant Testing Laboratory suggests:

• Take a scoop of soil, at least four to eight inches deep, from about a dozen different locations within the area, avoiding edges.

• Mix the scoops of soil in a bucket and allow it to dry out.

At-Home Soil Testing

Many kinds of soil tests can be done at home. Some test for soil pH, such as this simple-to-use meter that also reads the amount of sunlight and moisture content.

Other kits provide more in-depth results, like approximate values for nitrogen, phosphorus and potassium as well as the pH level. These products, which resemble chemistry sets, ask you to mix small amounts of soil with water and drop capsules of chemicals into test tubes. Results are tested against color swatches to determine the mineral content.

Or try this DIY test for pH, which is simple if less precise:

• Mix one-half cup vinegar with a scoop of soil. If it fizzes or bubbles, your soil is alkaline.

• Mix one-half cup water with a scoop of soil, then add baking soda. If it fizzes or bubbles, your soil is acidic.

• If neither reaction occurs, your soil is balanced, or neutral.

Soil Testing Lab

In 1914 the U.S. Department of Agriculture established local units, called extension offices, at land-grant colleges and universities to aid small farmers. Soil testing is usually available in every state for a fee ($10 to $20, plus postage, depending on the number of tests you request). These generally return a more accurate and thoroughly detailed analysis than self-testing provides.

You’ll also likely receive instruction on the types and amount of fertilizer to use for different plants. Use this list to locate the extension office closest to you and inquire about their soil testing procedure.

Garden centers and home improvement stores also sell mail-in soil test packages. These come with instructions and prepaid mailing envelopes that return professional results and recommendations in less than two weeks — faster in winter, when they’re less busy — for about $30.

Testing for Lead

Professional soil tests can detect the presence of lead and other toxic metals. Experts say that this is particularly important for vegetable gardens and soil near children’s play areas.

The Environmental Protection Agency (EPA) recommends caution when soil exceeds 200 parts per million of lead content, particularly when growing green leafy vegetables, or when soil can be carried into homes and inhaled or ingested by children.

The EPA recommends:

• From 200 to 400 parts per million of lead, no root crops (carrots, beets, potatoes).
• From 400 to 1,000 parts per million, it’s okay to grow legumes and fruit plants, like beans and tomatoes. But children should not be allowed to come into contact with the soil.
• Above 1,000 parts per million, do not disturb the soil. Create container or raised bed gardens instead.