University of Maryland Extension University of Maryland

College of Agriculture & Natural Resources

Soil Testing

University of Maryland Extension recommends testing turf and garden soil every 3 - 4 years. A basic soil test that gives readings for soil pH, phosphate, potassium and magnesium levels is sufficient for most home gardeners. Watch the video below to learn how to collect a soil sample.

Resources:

  • HG110a, List of regional soil testing labs.
  • HG 110, Selecting and Using a Soil Testing Laboratory
  • HG 42, Soil Amendments and Fertilizers
  • HG 18, Lead in Garden Soils

The importance of soil to plant growth can be summed up in the aphorism “it’s better to plant a $2 tree in a $25 hole, than a $25 tree in a $2 hole.” Soils contain the nutrients, water, and living organisms that help create healthy and sustainable gardens and landscapes. The first step to improving your soil is to invest a small amount of time and money to have your soil tested.

A basic soil test will tell you some important things about your soil that you cannot determine just by looking at it, smelling it, and feeling! And, it can save you money- less fertilizer used based on soil test results, and increased fertilizer efficiency by getting soil pH in the correct range. We receive hundreds of soil-related questions each year and here are some that are most frequently asked:

What kind of test should I do?
Please read our soil testing fact sheet. It covers the following important topics: Taking a Soil Sample, Mailing in a Soil Sample, Interpreting Test Results, Abbreviations and Terms Found in Soil Test Reports, Fertilizing Responsibly for a Healthy Chesapeake Bay. HG 110a contains a list of regional soil testing labs that we recommend. The video above shows how to collect a soil sample for testing.

Select the basic test offered by the lab that you choose. This typically includes pH (a measure of the alkalinity or acidity of your soil), phosphorus, potassium, calcium, and magnesium. The chemical symbols for these four nutrients are P, K, Ca, and Mg, respectively. These are important nutrients required by plants in large quantities. The basic soil test will probably also include other nutrients- sulfur (S), manganese (Mn), zinc (Zn), copper (Cu), iron (Fe), and boron (B), and some include a textural analysis (e.g. silty clay loam), % of organic matter, and cation exchange capacity (CEC).

Soil pH is one of the most important measurements. It plays a big role in the availability of nutrients to plant roots, nutrient run-off and leaching and microbial efficiency. Don’t pay for extra tests, such as soluble salts, or specific micronutrients unless you have a very good reason.

Soil tests should be done every 3 years for lawns and vegetable gardens. Problem sites can be tested more frequently. Fall is a good time to test soil because any soil amendments that you add in fall, like lime and compost, will have time to improve the soil before spring. Use the same lab each time you have your soil tested because there are no set standards followed by all testing labs. They use different chemical extractants to determine nutrient levels which leads to different test results, and they use different units (e.g., lbs./acre vs. ppm) and bases (e.g., P vs. P2O5) for expressing those results.

Frequently Asked Questions

I think I have clay soil like most everyone else in my county. What can I do about it?
Most Maryland soils are made up of mineral particles- sand, silt, and clay (about 45%); organic matter (about 1-5%); and air and water- pore spaces (about 50%). Soils are classified largely by their texture and that is determined by the relative amounts of sand, silt, and clay. Clay gets a bad rap, but it is an important constituent of soil because it holds nutrients and water. But too much clay can cause problems. Soils high in clay (more than 50%) feel sticky, don’t drain well, and become rock hard when dry. You can’t change the texture of your soil (the percentage of clay) but you can improve soil structure (the arrangement of individual soil particles). Adding lots of organic matter such as compost, farm manure, or shredded leaves to clayey soil will allow it to drain more easily and hold the right amounts of water and air for better plant growth and increased biological activity.

Should I add gypsum? The bag says it will break up clay soil.
Gypsum is calcium sulfate. Despite what the bag proclaims, there is no scientific evidence to support it for the kind of soils we have in Maryland. It is a good source of soluble sulfur and calcium and can be useful to add to soils that are low in these two nutrients and are in the correct pH range (6.0-7.0 for most garden and landscape plants).

Shouldn’t I send a 12 inch deep slice of soil to the lab since roots go at least that deep?
Actually, the top four to six inches of your soil contains most of the nutrients available to your plants. That is also the zone of greatest biological activity, where huge populations of soil critters consume organic matter and recycle nutrients. It’s also where most plant roots are located.

Is lead a problem outside of Baltimore?
Vegetable gardens should be tested for lead levels regardless of their location. Although it is true that lead is more prevalent in cities, high levels have been measured in suburban and rural areas. Lead levels may be especially high in gardens next to busy roads from vehicle exhaust. Learn more about this issue by reading HG 18 - Lead in Garden Soils. The University of Massachusetts lab includes lead testing in their basic soil test.

I read that phosphorus is one of the big Chesapeake Bay pollutants. My soil test came back very high in phosphorus. Will this hurt my plants? I want to do my part for the Bay so tell me how I can get the extra phosphorus out of my soil.
High levels of nutrients will usually not harm plants. There are exceptions, such as when soils are very low in pH and high in aluminum or manganese.

Excessive amounts of nutrients like phosphorus cannot be physically removed. Plants remove some phosphorus each year through their growth processes. Phosphorus can move into waterways when soil washes off your property. Keeping soil covered at all times with plants, grass, or mulch is essential.

How come my soil test results don’t have a measure of my nitrogen levels? I thought nitrogen was important.
Nitrogen is needed in relatively large quantities and is often the nutrient that limits or determines maximum plant growth. It is not measured because it moves back and forth between organic (not available for plant uptake) and inorganic forms (available for plant uptake). This is affected by temperature, rainfall, soil texture and structure, biological activity and many other factors. Organic matter provides a slow release of nitrogen during the growing season.

My soil test results gave two different pH readings- “5.8 pH” and “7.4 buffer pH”- how come?
The 5.8 pH reading is the actual pH reading and measures “active” soil acidity. The buffer pH is a measure of the “stored” acidity. The buffer pH is important because it determines how much lime needs to be added to a soil to make a change in pH. Soils high in clay and organic matter have a lot of “stored” acidity because these particles have many negatively charged sites that hold positively charged hydrogen ions (cations). Hydrogen cations make the soil more acidic. Therefore it will take more calcium (lime) to raise the pH of a clayey soil than it would a sandy soil. Buffer pH is measured by adding a weak 8.0 pH base to low pH soil samples.

My soil test results came back with a recommendation to apply 4 lbs. of nitrogen per 1,000 sq. ft. of lawn area. I thought you guys recommended less?
Right you are! The UME recommendation for cool-season turf (fescue and bluegrass) is as low as .7- .9 lb. of nitrogen per 1,000 sq. ft. Always follow UME lawn fertilization guidelines, regardless of who tests your soil.

I want to be more self-sufficent and knowledgeable so shouldn’t I be doing my own soil testing with one of those store-bought kits?
Using a professional lab is recommended because they can test wider range of nutrients with greater accuracy than a store-bought kit. These kits typically test soil pH, phosphorous, and potassium. Here is a brief 2007 summary of a comparison of the accuracy of some commercially available home gardener soil test kits.

Please send us a question at Ask the Experts if you have soil testing questions or need a soil test bag. We can also help you interpret soil test results and recommendations. HGIC challenges YOU to have your soil tested this year!

 

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IET Departmentof the College of Agriculture and Natural Resources. © 2014.