Regular soil testing is beneficial for your lawn or garden. After selecting a lab, collecting a sample, and sending it for analysis, you'll receive a report. Now what?
Soil test reports include key factors that help determine which fertilizers or amendments to use and in what amounts. Most reports also provide treatment recommendations tailored to the specific area tested (e.g., lawn, vegetable garden). If you trust the source, you can follow these. You may also compare them with general guidelines—if they align, proceed with the lab’s plan; if not, consider calculating your own or using another source.
The guidelines below apply broadly to landscape areas. Most lawn and garden plants grow well when these specifications are met, though specific crops (like fescue or tomatoes) may have more tailored recommendations to get the best production and growth.
pH | Buffer pH | CEC | Organic Matter | Soluble Salts | Phosphorous (P) | Potassium (K) | Nitrogen (N) | Calcium (Ca), Magnesium (Mg), & Sulfur (S) | Micronutrients | Heavy Metals | More Information
pH
Soil pH measures acidity or alkalinity on a scale from 0 to 14; below 7 is acidic, and above 7 is alkaline. It affects nutrient availability, making it important for plant health.
Most horticultural crops grow well in soil with a pH between 6.0 and 7.5. In Iowa, pH typically ranges from 5.5 to 7.5, often leaning toward the higher end.
If your soil pH is outside the ideal range, you can adjust it using soil amendments. Lime is used to raise pH, and elemental sulfur to lower it..
Learn more in this article: The Importance of Soil pH.
Buffer pH
Buffer pH measures the soil’s resistance to pH change and is different from the soil's pH (do not confuse the two!). It’s used only to determine how much lime to apply when raising pH. This value is typically reported when liming is necessary or to be considered.
CEC
Cation exchange capacity (CEC) indicates how effectively a soil retains nutrients. High CEC indicates better nutrient retention (making them less likely to wash or leach away), a characteristic common in soils rich in clay or organic matter.
The ideal CEC is 15–20. Values below 6 need remediation or close monitoring. Adding organic matter is a simple way to boost CEC.
Organic Matter
Organic matter enhances soil productivity by improving water retention, nutrient availability, structure, drainage, and aeration, all of which are key for root growth.
In Iowa, unamended soils typically have 2–4% organic matter. Levels between 3% and 6% are ideal; below 3% requires action. To boost organic matter, add compost, peat, manure, fallen leaves, or plant residues. Many gardeners aim for around 5%.
Soluble Salts
High soluble salts in soil can harm plants by reducing water uptake which leads to foliage burn, restricted root growth, and fewer blooms. Sensitivity varies by species, with seeds and seedlings being most vulnerable.
Salt levels become elevated due to excess fertilizer, pet urine, de-icing salts, and poor-quality irrigation water, among other factors. To lower high levels, irrigate heavily to leach salts.
Soluble salts at 2 mmhos/cm or less require no action. Sensitive plants will begin to see damage above 2 mmhos/cm, moderately tolerant plants above 4 mmhos/cm, and tolerant plants above 8 mmhos/cm.
Phosphorus (P)
Phosphorus (P), a key macronutrient alongside nitrogen and potassium, is shown as the middle number on fertilizer labels. Low levels cause stunting and reddish growth. In excess, it tends to move out of the soil into streams, rivers, lakes, and ponds, where it causes issues.
Two different tests are used depending on the soil pH. The Bray-P1 test is used when the soil pH is less than 7.4, and the Olsen-P test is used when the soil pH is greater than 7.4. Each test extracts phosphorus differently, so ideal parts per million (ppm) levels vary depending on the method. The relative levels of phosphorous based on the test used are outlined in the table below.
Many Iowa soils, especially those in established lawns, have abundant levels of phosphorus, and additional phosphorus is typically not needed. This means that fertilizers in which the middle number of the fertilizer analysis is zero, or nearly so (for example, 36-0-9 or 5-1-3), are often recommended.
| Relative Level | Bray-P1 (for low pH sols) | Olsen-P (used for high pH soils) |
|---|---|---|
| Low | 0-5 ppm | 0-3 ppm |
| Medium | 6-10 ppm | 4-7 ppm |
| High | 11-25 ppm | 8-18 ppm |
| Very High | 25+ ppm | 18+ ppm |
Potassium (K)
Potassium (K) is a major macronutrient, along with nitrogen and phosphorus, and is listed as the third number on fertilizer labels. A deficiency can lead to stunted development and brown leaf edges.
Soil type affects potassium levels. Sandy soils lose potassium easily through leaching, making deficiency more common. In contrast, clay and organic soils retain potassium better, but because it’s relatively immobile, potassium should be incorporated into the soil, not just surface-applied, to reach plant roots.
Potassium levels above 125 ppm are sufficient and don’t require fertilization. Levels below 50 ppm suggest a likely deficiency and should be corrected.
Why Don't They Measure Nitrogen?
Soil tests don’t measure nitrogen, even though it’s the most-used plant nutrient. Plants absorb nitrogen as nitrate and ammonium (which is quickly converted to nitrate in the soil). Nitrates are not retained by the soil and are easily dissolved in water, allowing them to be readily leached.
Due to nitrogen’s mobility and complex behavior, soil tests for it are unreliable. Instead, nitrogen fertilizer recommendations are based on organic matter levels and the type of crop or plant being grown. Low-organic soils (0–3%) growing plants that require higher nitrogen levels, like sweet corn and turfgrass, require more nitrogen than high-organic soils (4.5–18%) growing plants that require lower nitrogen levels, like shade trees.
Nitrogen is released from organic matter through the action of microorganisms in the soil. Over a growing season, approximately 0.7 pounds of nitrogen per 1,000 square feet is provided by each 1% of organic matter present in the soil. This means that low-demand plants, such as shrubs and trees, may not require any supplemental nitrogen fertilizer if adequate organic matter is present. Most high-demand plants, like turf, will require additional nitrogen from fertilizer even when organic matter percentages are high (just not as much!).
Learn more about recommended fertilization rates in this article: Fertilizing in the Home Garden.
Calcium (Ca), Magnesium (Mg), & Sulfur (S)
Calcium (Ca), Magnesium (Mg), and Sulfur (S) are rarely deficient in Iowa soils, except in sandy, acidic, or low-organic soils. Soil tests may report their levels, which are usually adequate for plant growth. If not included in the basic soil test, these nutrients can be added, but only if the plants are showing deficiency symptoms.
Calcium levels are considered low when they are below 300 ppm (which is rare if soils are limed). Magnesium levels less than 35 ppm and sulfur levels under 15 ppm are considered low. If deficient, the soil test report recommendations will provide guidance on amendments or fertilizers to use to bring them into the appropriate range. These recommendations often advise adjusting the soil pH or modifying the calcium-to-magnesium ratio rather than simply adding these nutrients to the soil.
Micronutrients
Micronutrients like boron, chlorine, copper, iron, manganese, molybdenum, nickel, and zinc are needed in small amounts, so they’re rarely tested or deficient. Their availability is strongly affected by soil pH. If plants are exhibiting deficiency symptoms, it is likely due to an improper pH level rather than a nutrient deficiency. Adjusting soil pH to the ideal range of 6.0 to 7.5 ensures that all micronutrients are available for plant use.
In Iowa, iron deficiency symptoms often develop on sensitive species, such as pin oak. Soil tests will show adequate levels of the nutrient in the soil. Rather than adding iron through fertilization or amendments, adjusting the soil pH to the appropriate range enables the soil's existing iron to be readily taken up by the plants. Learn more in these articles: Micronutrient Deficiencies of Trees and Pin Oak Chlorosis.
Lead & Other Heavy Metals
Lead and other heavy metals, such as arsenic, are concerns for human health. They are not plant nutrients and therefore are not used by plants. Most plants will not absorb and accumulate heavy metals to a level that causes a health concern. The main concern is ingesting contaminated soil or dust on produce that’s touched the ground or been splashed by rain.
Tests for heavy metals, such as lead, are not typically included in a standard soil test. Only if this condition is suspected, is it necessary to test for these elements. The statewide standards for heavy metals in soil, as outlined by the Iowa Department of Natural Resources, are listed below:
- Lead - 400 ppm
- Arsenic - 1.9 ppm
- Cadmium - 70 ppm
- Chromium - 190 ppm
- Mercury - 23 ppm
- Copper - 3,100 ppm
- Nickel - 1,500 ppm
- Zinc - 23,000 ppm
More Information
- Soil Testing Resources for Home Gardeners
- Managing Garden Soil
- How to Change Your Soil's pH
- Fertilizing in the Home Garden
- Lawn Fertilization
- How to Calculate Fertilizer Rates for Lawn and Garden
- How to Understand Soil Test Results, University of Illinois (Video)
- Soil Test Interpretations and Fertilizer Management for Lawns, Turf, Gardens, and Landscape Plants, University of Minnesota
Photo credits: 1: Anton Skripačev AdobeStock; 2: encierro AdobeStock; 3: The Toidi AdobeStock; 4: Iowa State University Plant and Insect Diagnostic Clinic