Why is soil testing important?
Soil's importance for 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."
- Have your soil tested before planting a vegetable or flower garden, trees, and shrubs, or starting or renovating a lawn.
- Soil testing labs provide more complete and accurate results than do-it-yourself soil test kits.
- Soil test results give you baseline information on soil pH, nutrient levels, and organic matter content and recommendations for fertilizing and adjusting soil pH. Soil testing is an essential tool for improving soil health.
- Soil testing helps reduce overfertilizing, keeping excess nutrients (nitrogen and phosphorus) out of Maryland's groundwater and surface waters.
- Vegetable gardens, regardless of location, should be tested for lead (Pb).
- Need help understanding the report and recommendations from a soil testing lab? Send us your questions and the report through our Ask Extension service.
Selecting a lab
- The University of Maryland no longer has a soil testing lab but HGIC maintains a list of recommended soil testing labs for you to choose from. The University of Maryland does not endorse particular businesses and the inclusion or exclusion of specific labs does not reflect a bias. All of the listed labs competently test soils and provide reports that include liming and fertilizing recommendations.
- The labs listed below offer lawn/garden and commercial soil testing. Select the appropriate category.
- Test results are usually available online for fast, efficient reporting.
Select the "Basic Test"
- 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 may also include other nutrients like sulfur (S), manganese (Mn), zinc (Zn), copper (Cu), iron (Fe), and boron (B), as well as % of organic matter (OM), and cation exchange capacity (CEC). A couple of the labs also include lead testing (Pb) as part of a basic test.
- 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.
- Nitrogen is needed in relatively large quantities but it is not measured because it continuously moves between organic forms (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. Nitrogen recommendations are provided, but they are based on the need of the particular plant you are growing for the upcoming season, not the amount in your soil.
- Boron, zinc, and manganese are trace elements that can sometimes be deficient in Maryland, especially in Coastal Plain soils. If a soil test report shows a low Boron level dissolve 1 tablespoon of Borax in 1 gallon of water and apply the solution evenly from a sprinkling can over 100 sq. ft.
- Don't pay for extra tests, such as soluble salts, or specific micronutrients unless you have a very good reason.
- In addition to testing for lead (Pb), some labs also test for arsenic (As), cadmium (Cd), and chromium (Cr). Soils on old industrial sites are sometimes contaminated with these heavy metals. Soil testing labs do not generally test for other types of soil contaminants, such as human pathogens and pesticides.
- Some labs offer a textural analysis for an additional fee. Your soil's texture is the relative percentages of sand, silt, and clay particles that make up the mineral portion of the soil.
- Perform a soil test 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, like lime and compost, will have time to improve the soil before spring in time for planting.
- Use the same lab each time you have your soil tested. Labs can 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-phosphorus vs. P2O5,-phosphate) for expressing those results.
Table of soil testing laboratories
Basic soil tests typically cost $11 - $20, plus mailing costs. Any additional testing will increase the cost. Please contact the lab for current pricing before submitting your sample. Include payment with your sample.
| Soil testing labs | Soil testing labs |
|---|---|
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AgroLab, Inc. Basic test includes pH, P, K, Mg, Ca,
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University of Delaware Soil Zn, Fe, B, S, Al, Pb, OM (lead testing included). |
|
Pennsylvania Agricultural Analytical |
UMass Soil and Plant Nutrient Testing Lab Basic test includes pH, P, K, Ca, Mg, Mn, Cu, S, Commercial growers who must comply with
|
|
Spectrum Analytic, Inc. |
Waypoint Analytical Click on Home Owners & Landscapers, then |
Getting your soil tested
- Follow the specific instructions provided by the soil testing lab you select.
- General guidelines
- Separate samples should be taken for distinct areas like the front yard, back yard, vegetable garden, etc. The sample should represent the soil in which the plants are or will be growing in.
- Use a spade or trowel to take 10-12 random sub-samples per sampling area. The samples are thin slices taken to a depth that contains or will contain the bulk of the plant's roots- 4 inches for turf; 6-8 inches for garden and landscape beds.
- Mix together all of the slices in a clean bucket removing all rocks, debris, and plant parts.
- Labs typically require 1 cup of soil per sample. Mail the soil in a zipper-sealed bag or a clean plastic bag.
- Don't mail wet soil; you should not be able to squeeze water from the sample.
- Download the soil test submission forms from the lab's websites. Fill out the form including your contact information and answers to the questions about the soil you are testing. Mail the completed form back to the lab in a small box or padded envelope with the sample and payment for the correct amount.
Understanding a soil test report
(PDF) Understanding soil test results - infographic
- Lab reports provide a graphical representation of results that indicate the level of various nutrients from your soil (low medium, high, excessive). Especially important are soil pH, P, K, Mg, Ca, organic matter (OM), and fertilizer and lime recommendations.
- "Optimal" and "excessive" levels mean that the nutrient concentration in the soil is more than adequate for optimum plant growth. Adding more of that nutrient will not improve plant growth.
- Excessive amounts of nutrients do not harm plants and cannot be physically removed from the soil. There are exceptions, such as when soils are very low in pH and high in aluminum or manganese, making those nutrients available to plants at toxic levels.
- Phosphorus (P), in particular, should not be added if levels are in the "medium" to "optimum" range. It can move into waterways when soil is moved off-site by stormwater and gravity. Along with nitrogen run-off, phosphorus feeds algal blooms leading to eutrophication- low oxygen dead zones.
- Cation Exchange Capacity (CEC) measures the capacity of a soil to hold and release nutrient ions. Negative charges on clay and organic matter particles attract positively charged ions (cations) like potassium and calcium. Soils with higher percentages of clay and/or organic matter generally have higher CEC. This measurement will vary across Maryland soils. Add organic matter if your soil's CEC is less than 10.
- Soil organic matter (OM) is made up of living, dead, and decomposing plants, small animals, and microorganisms. Soils high in OM retain more moisture, have a crumbly structure that resists soil compaction, and contain a reservoir of nutrients that are slowly released over time.
- OM is reported on a percentage by weight basis in the lab report. Usually, a sample is weighed in the lab and then ignited to burn off the carbon compounds, leaving only the mineral soil. The sample is reweighed to determine the OM%. Gardeners who add lots of organic matter to their soils may be surprised that the OM content is less than 5%. This is because OM is less dense than mineral soil and the measurement is based on weight, not volume.
- Healthy soils have a 2% or greater OM content. Regular additions of organic matter will gradually increase the OM level. Where the topsoil is thin and OM is low you may need to add 2-4 inches of compost for a few years. Yearly additions of 1 inch of compost can help maintain high productivity in vegetable and flower gardens.
Soil pH
- Soil pH is a measure of how acidic (sour) or basic (sweet) your soil is. Soil pH directly affects nutrient availability.
- The pH scale ranges from 0-14 with 7 as neutral. Numbers less than 7 indicate acidity, while numbers greater than 7 are basic.
- Plants have different soil pH preferences. Azaleas, rhododendrons, blueberries, and conifers prefer acidic soils (pH 4.5-5.5) and are known as ericaceous plants. Vegetables, grasses, and most ornamentals grow best in the 5.5-7.0 pH range.
- Soil pH values significantly above or below these ranges may result in less vigorous growth or symptoms of nutrient deficiency or toxicity.
- Most soil test results include soil pH and buffer pH. The soil pH number 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 change soil pH. Soils high in clay and organic matter have a lot of "stored" acidity because these particles have many negatively charged sites that hold cations (positive charge). Hydrogen ions (H+) and aluminum ions (Al+++) are the principal sources of stored acidity. It takes more calcium (lime) to raise the pH of a clayey soil than it would a sandy soil. Such soil is more highly buffered, meaning it resists pH changes. Buffer pH is measured by adding a weak 8.0 pH base to low pH soil samples. The lower the buffer pH (the closer it is to the actual soil pH), the more lime required to raise soil pH.
Soil pH recommendations
Raising soil pH
- Lime raises the pH of acidic soil and supplies calcium and magnesium.
- Liming recommendations are usually given as pounds of ground limestone per thousand square feet. Don't spread more than 50 lbs of ground limestone or its equivalent in a single application to the soil or lawn surface. For example: if the recommended amount is 85 lbs, apply the first 50 lbs now and the remaining 35 lbs roughly six months from now.
- Up to 70 lbs of limestone per thousand square feet can be applied in a single application if it is tilled into the soil.
- Dolomitic ground limestone contains calcium carbonate and magnesium carbonate as is recommended to raise a soil's pH and magnesium levels.
- Common limestone products:
- Agricultural lime is pulverized calcitic limestone. The finer the grind or mesh size, the more readily it will act to raise soil pH. Powdered lime is faster acting.
- Granular and pelletized lime is less likely to clog when spread with a fertilizer spreader over turf areas and can be substituted for ground or pulverized limestone.
- Hydrated lime (calcium hydroxide) is quick-acting. You need only 75% of the calcitic lime recommendation.
Lowering soil pH
- Sulfur is a highly mobile and soluble nutrient prevalent in the soil as sulfate anions (negative charge). It is difficult to measure accurately and may be omitted from basic soil tests.
- Soil sulfur deficiencies may be increasing in Maryland as less sulfur is deposited as a result of coal-burning power plants.
- Elemental sulfur (90% S), sold as flowers of sulfur, micro-fine sulfur, and granular sulfur, and iron sulfate (12% S) are used to lower soil pH. Both are widely available at garden centers and hardware stores.
- Avoid using aluminum sulfate because aluminum becomes more available with declining pH values and may lead to plant toxicity problems.
- Use this chart for reducing soil pH if the lab does not provide a specific recommendation.
Soil test report fertilizer recommendations
- Detailed information on fertilizers.
- For lawns, always follow UME lawn fertilization guidelines regardless of the fertilizer recommendations given in the soil test report.
- Labs report levels of plant-available P and K but fertilizer recommendations are presented as pounds of phosphate (P2O5) and potash (K2O).
- To convert pounds per acre recommendations to pounds per square feet, do the following math calculation (there are 43,560 sq. ft, in an acre):
lbs. per acre/ 43.56 = lbs. / 1,000 sq. ft.
lbs. per acre/435.6 = lbs. / 100 sq. ft.
What if I can't find the fertilizer recommended in my soil test report?
Example: Your soil test report recommends applying 10 lbs. of 10-5-4 fertilizer per 1,000 sq. ft. but the local garden center only has 4-5-3 fertilizer and 6-2-1 fertilizer.
The 10-5-4 recommendation shows that the lab found your soil to be deficient in P and K.
Select an alternative fertilizer that has a similar analysis to 10-5-4 and calculate how much fertilizer to apply based on the N content. Either of the alternative fertilizers would be suitable.
Alternative fertilizer #1: divide the percentage of N in the 10-5-4 fertilizer by the percentage of N in the 4-5-3 fertilizer:
0.10 ÷ by 0.04 = 2.5
The recommendation is to apply 10 lbs. of 10-5-4 so you would need to apply 25 lbs. (10 X 2.5) of 4-5-3 fertilizer to deliver the same amount of N.
This will more than double the amount of phosphate (P2O5) and potash (K2O) applied compared to using the 10-5-4 fertilizer. Excess P carried off-site in soil washed away during storms can contribute to pollution of waterways. However, the risk is reduced where the ground is level and soil is protected by plants or mulch.
Alternative fertilizer #2: divide the percentage of N in the 10-5-4 fertilizer by the percentage of N in the 6-2-1 fertilizer:
0.10 ÷ 0.06 = 1.66
The recommendation is to apply 10 lbs. of 10-5-4 so you would need to apply 16.6 lbs. (10 X 1.66) of 6-2-1 fertilizer to deliver the same amount of N.
Applying 16.6 lbs. of 6-2-1 fertilizer will reduce the amount of P2O5 and K2O applied compared to using the 10-5-4 fertilizer. In this case, you could apply a small amount of superphosphate and potassium chloride fertilizer to make up the difference.
How to convert a synthetic (chemical) fertilizer recommendation for an organic fertilizer?
Example: the recommendation is to apply 2 lbs. of N per1,000 sq. ft. by applying 4.35 lbs. of urea fertilizer (46-0-0). You want to substitute cottonseed meal (6-2-1).
Solution #1:
Divide the percentage of N in the synthetic fertilizer by the percentage of N in the organic fertilizer and multiply by 4.35 (amount of urea):
0.46 ÷ by 0.06 X 4.35 = 33.3 lbs. of cottonseed meal
Solution #2:
Calculate the amount of the fertilizer product needed by dividing the pounds of N needed by the percentage of N in the product.
2.0 ÷ by 0.06 = 33.3 lbs. of cottonseed meal fertilizer per 1,000 sq. ft. If your garden is 500 sq. ft. you would apply ½ the amount.
- University of Georgia - How to Convert an Inorganic Fertilizer Recommendation to an Organic One
- Send soil testing questions to "Ask Extension". Please upload a copy of your soil test results with your question.