How to Read a Soil Test Report (And What to Do Next)

What a Soil Test Report Tells You

Read a soil test report in three passes. First check the pH against the range for your plants, which is 5.5 to 7.0 for most of the garden. Next look at the rating beside each nutrient and plan to add only those marked low or below optimum. Last, use the organic matter and CEC lines to judge how well the soil holds water and nutrients. The lab’s lime and fertilizer recommendations at the bottom already combine these, so the report can be followed as written once you know what each line refers to.

Most university labs report the same core items. University of Maryland Extension lists soil pH, phosphorus, potassium, magnesium, calcium, organic matter, and the lime and fertilizer recommendations as the parts that matter for a garden or lawn. Clemson’s report shows the pH as a number, the nutrients as a bar graph, and the lime needed for each 1,000 square feet. Layouts differ between labs, but the lines carry the same meaning.

Start With pH

Soil pH is a measure of acidity. Penn State Extension gives the scale as 0 to 14, with values under 7 acidic, 7 neutral and values over 7 basic. Compare your number with the range for what you grow:

Plants	Preferred soil pH	Source
Vegetables, grasses, most native and ornamental plants	5.5 to 7.0	University of Maryland
Vegetables, most turfgrasses, most ornamentals	6.0 to 6.5	Clemson
Azaleas, rhododendrons, blueberries, conifers	4.5 to 5.5	University of Maryland

The first two rows are compatible. Maryland gives the range in which these plants grow well, and Clemson and the University of New Hampshire give the narrower band of 6.0 to 6.5 that plants generally prefer. Penn State’s lab sets its limestone recommendation to reach a target pH of 6.5. For crop-by-crop targets, see our guide to soil pH for a vegetable garden.

pH comes first because it governs how available other nutrients are. Oregon State University Extension notes that the availability of most micronutrients falls as pH rises, molybdenum excepted, and that micronutrient deficiencies rarely occur when soil pH is below 6.5.

Buffer pH and the Lime Recommendation

Many reports carry a second pH figure labeled buffer pH. Maryland Extension explains the difference: soil pH measures the active acidity, and buffer pH measures the stored acidity. The buffer value determines how much lime is needed to shift the pH. You do not need to interpret it yourself. The lab has already turned it into the lime line on the report.

Three practical points apply to that line.

1. Lime is slow. Clemson notes that pelletized dolomitic limestone takes several months to dissolve fully and change the soil pH, and recommends sampling a couple of months before planting for that reason.
2. Large amounts go on in stages. Maryland advises spreading no more than 50 pounds of lime per 1,000 square feet on a lawn in a single application, and up to 70 pounds per 1,000 square feet where the limestone is worked into the soil.
3. The type of lime matters when magnesium is low. Dolomitic lime contains calcium and magnesium, and labs recommend it when both pH and magnesium need raising.

If the pH is too high, the remedy is sulfur. New Hampshire’s summary is that lime raises pH and sulfur lowers it. Use the amount the lab gives for your soil. Acid-loving shrubs are the case where a gardener lowers pH on purpose, and our guide to soil acidifiers for blueberries compares the materials.

Phosphorus and Potassium Ratings

Each nutrient comes with a number and a rating word. Penn State’s reports use below optimum, optimum and above optimum. Other labs use low, medium, high and very high, or add an excessive category. Maryland’s guide explains the top end: a rating of optimal, excessive or very high means the nutrient in the soil is more than adequate.

The rating matters more than the raw number, because numbers are not comparable between labs. Oregon State gives an example from its own state, where one phosphorus extraction method is used for soils west of the Cascade Range and a different one for soils to the east. The same soil would return different figures under each. Read the number against the scale printed on your own report.

Phosphorus is the nutrient most often oversupplied in gardens. New Hampshire Extension observes that phosphorus tends to run high in that state, particularly where a garden has had a lot of compost or manure. Maryland’s rule is that phosphorus should not be added if the level is medium, optimum or high. Choose a fertilizer with no phosphorus in that case.

Potassium, which New Hampshire notes helps plants resist drought, may be low depending on how the land was used before. Add it when the rating says so, at the rate the lab gives.

Why Nitrogen Is Usually Missing

New readers often look for a nitrogen result and find none. Clemson states that nitrogen testing is not recommended because available nitrogen is too variable, owing to its mobility in the soil. Penn State describes nitrogen as a nutrient that moves quickly through the environment and changes form. Oregon State adds that a test can measure nitrate and ammonium at the moment of sampling but does not reflect future conditions.

The nitrogen recommendation on a report is therefore based on the crop you named on the submission form. New Hampshire’s lab also credits a certain amount of nitrogen for the organic matter in the soil, so a soil rich in organic matter gets a smaller nitrogen recommendation.

Calcium, Magnesium and Micronutrients

Calcium and magnesium are closely tied to liming. Maryland explains that garden lime, made from calcitic limestone, provides calcium, and that dolomitic lime contains calcium and magnesium. A low magnesium reading alongside a low pH is the case for dolomitic lime.

Micronutrients such as iron, manganese and zinc appear on some reports and not others. Given Oregon State’s point that deficiencies are rare below pH 6.5, it makes sense to look at the pH line first when a micronutrient reads low in a high-pH soil. Some labs also report lead. New Hampshire’s report gives a lead value and states whether that level is potentially harmful, which is worth reading closely for any bed that grows food.

Organic Matter and CEC

Organic matter is reported as a percentage. Maryland describes it as the living, dead and decomposing plants, animals and microorganisms in the soil, and gives 2 percent or greater as the mark of a healthy soil. Soils high in organic matter hold more moisture, resist compaction and release nutrients slowly. The figure moves slowly: Oregon State considers a rise of 2 percentage points, such as from 1 to 3 percent, a large change.

CEC stands for cation exchange capacity. It measures the soil’s capacity to hold and release nutrient ions. Oregon State’s guide explains the pattern: soils high in clay or organic matter have a high CEC, and sandy soils low in organic matter have a low one. Maryland gives a working threshold, which is to add organic matter if the CEC is less than 10. You cannot change the clay content of a garden, so compost is the tool for raising a low figure, and how much compost to add to a garden bed gives the rates.

Turn the Report Into a Plan

Work through the report in a fixed order.

1. Correct pH first, using the lab’s lime or sulfur amount. Apply it months ahead of planting where you can.
2. Add phosphorus and potassium only where the rating is below optimum.
3. Add organic matter if it is under 2 percent or the CEC is under 10. Where phosphorus is already high, go lightly with manure and compost, since both add more.
4. Supply nitrogen according to the crop recommendation.

Lab rates are often printed in pounds per acre. Maryland gives the conversion for garden use: divide pounds per acre by 43.56 to get pounds per 1,000 square feet. A recommendation of 87 pounds per acre therefore comes to about 2 pounds per 1,000 square feet.

Retest to see whether the changes worked. Clemson recommends sampling every year, and University of Maryland Extension asks for a test every 3 years for lawns and vegetable gardens. Keep the old reports so that each new pH and organic matter figure can be set beside the last one. Between lab tests, the options in our comparison of soil test kits can track pH at home.