Information you will find on a soil test report
A soil test report provided by a laboratory will indicate the levels of nutrients in your soil as well as other components. Soil pH, Phosphorus (P), Potassium (K), Magnesium (Mg), Calcium (Ca), organic matter (OM), and fertilizer and lime recommendations are especially important for maintaining a garden or lawn.
Download sample soil report
Nutrients
- An "optimal" or "excessive" or “very high” level means that a nutrient in the soil is more than adequate. 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" to “high” range. It can get into waterways when soil is moved off-site by water and gravity. Along with nitrogen, phosphorus feeds the growth of algae in bodies of water like the Chesapeake Bay. When the algae die, it causes a condition called eutrophication - excess nutrients in the water - which leads to low oxygen levels that kill aquatic plants and wildlife
Cation Exchange Capacity
Cation Exchange Capacity (CEC) measures the capacity of 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.
Organic Matter
- 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. It is not uncommon for well-established vegetable gardens to have an OM content >12%.
Soil pH
- Soil pH is a measure of how acidic (sour) or basic (sweet) your soil is. Soil pH directly affects nutrient availability. For example, phosphorous (P) combines with iron and aluminum when the soil pH is low (below 6.0), and with calcium when the soil pH is high (above 7.3). In both cases, less P is available for plant uptake. 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.
- Vegetables, grasses, and most native and ornamentals grow best in the 5.5-7.0 pH range.
- Conifers and ericaceous plants (azaleas, rhododendrons, blueberries, and their relatives) prefer acidic soils (pH 4.5-5.5)
- 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 charges). Hydrogen ions (H+) and aluminum ions (Al+++) are the principal sources of stored acidity. It takes more calcium (lime) to raise the pH of clayey soil than it would 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 is required to raise soil pH.