Soil Testing: A Quick Review
By Jennifer Johnson, Anoka County Extension Master Gardener
The two most important factors in successful vegetable gardening are water and soil. The need for water is obvious for the health of plants because you can see them wither without sufficient water. But the soil, the habitat in which plants grow, is often taken for granted because you can’t see what you’ve got.
When starting a new garden plot, gardeners should always test the soil to find out what’s there. The Minnesota Extension Soil Testing Laboratory provides this service. The Extension does not recommend home tests because they are not calibrated for Minnesota soils. The form for requesting soil analysis, found at z.umn.edu/soiltest, provides information on how to conduct the sampling. Use a clean plastic container to mix the samples. I once used a cardboard box because it was handy, but cardboard is recycled and can contain various chemicals that contaminate the sample. A regular test, which costs $22 in 2026, evaluates the most critical factors for healthy garden soil: Texture, organic matter, pH, phosphorus, and potassium. The test report, which is available a couple of weeks after submission, provides helpful information and recommendations.
With the test results you can make more informed decisions about your garden and whether and/or how to amend your soil. Knowing your soil’s texture helps estimate drainage and the frequency of watering. Coarse soil contains more sand and is less able to hold water, thus it requires more watering. Fine soil has more clay that stays wet longer. The best pH for gardens is 6.5 with an acceptable range of 5.5-7 (slightly acidic to neutral). Although soil pH changes slowly, it can have a significant impact on your plants’ intake of nutrients. Acidic soil is lower than 7 (lime raises pH for acidic soil). Basic soil is higher than 7 (sulfur lowers pH for basic soil). Some think coffee grounds raise pH, but the grounds are not acidic enough to alter pH. Coffee grounds may add a small amount of nitrogen.
Organic matter is an estimate of available nitrogen, which supports plant growth, and this estimate provides a basis for fertilizer recommendations. Phosphorus supports photosynthesis and root and seed development. The tests (Olsen and Bray) will depend on the pH of the soil. Potassium also supports photosynthesis among other things. The Lab will make recommendations concerning both phosphorus and potassium.
Urban soil can be contaminated due to proximity to industry and transportation. One example is lead, which is potentially toxic. Soil testing for lead can be requested at an additional cost. Soils with lead levels above 7-20 parts per million are considered contaminated, resulting primarily from auto emissions and paint chips from older fences and buildings. Lead persists for a long time. The greatest risk to gardeners is through contact with the soil, (ingestion or inhalation) rather than lead concentrations in the vegetables/fruits grown in that soil. The Extension and the Department of Health offer tips on what to do to reduce the risk.
Be informed about your soil to safely grow healthy plants.
Resources:
1. Healthy Soil, Healthy Plants. Dr. Anne Sawyer, https://swac.umn.edu/healthysoilplants
2. Soil Testing Laboratory: How to Sample-Lawn & Garden. This site includes links/tabs to the Soil Test Request Form and Information on Interpreting the Test Results, Nutrients (nitrogen, phosphorus, potassium), and an article under Resources about Urban Gardens and Soil Contaminants, https://soiltest.cfans.umn.edu/how-sample-lawn-garden
3. What does Phosphorus do for soil and plants. Science Insights, https://scienceinsights.org/what-does-phosphorus-do-for-soil-and-plants
4. MDH Lead page, https://www.health.state.mn.us/communities/environment/lead/index.html
5. Busting Common Soil & Garden Myths, Dr. Joe Storlien, Professor at St. John’s University, Lecture at HGLF, 3-28-26