How, Why and When to Test Your Soil

A picture of someone taking a soil sample

Soil testing is an important practice for anyone trying to grow healthy and productive plants.

By Justin Keay

[This article was first published in The Gateway Gardener Winter 2019 issue.]

MU Extension offices across the state processes thousands of soil samples every year and MU Extension Specialists are always happy to help you understand your soil test results. Soil testing is an important practice for anyone trying to grow healthy and productive plants. Blindly applying fertilizer can result in excess salt build up in soils, as well as nutrient excesses that can inhibit plant growth. Once you add fertilizer to your soil, it cannot be removed. Soil testing helps guide fertilizer usage so that plants receive only the amount of nutrients they need to thrive. Applying appropriate amounts of fertilizer is also an important part of environmental stewardship. Fertilizer runoff from urban landscapes contributes to algal blooms, which can cause fish kills in our rivers, ponds, lakes, streams and oceans. The most notable example of the effects of fertilizer runoff is the massive “dead zone” in the Gulf of Mexico where no fish can survive. Everyone lives in a watershed, and in the St. Louis metro area every drop of runoff ends up in the Missouri or Mississippi river.

People often ask me, when is the best time to take a soil sample? The simple answer is, whenever you have the time and remember to do it. If you take multiple samples over the years, it is optimal to take them at the same time of the year, but the most important thing is to get started with your first sample. Taking a soil sample in the correct way will ensure accurate soil test results. Sample your lawn and garden separately as they are likely fertilized differently and have different nutrient requirements. If your front lawn is in full sun with no trees and your back lawn is mostly shaded and stays wetter, you should sample them separately as well. It is best to take 6-8 subsamples and combine them in a bucket. Simply use a small trowel or shovel, insert it 4-6 inches deep and pull up a small slice of soil. Combine subsamples into a bucket and mix thoroughly. Remove any grass or leaves and let the sample air dry. Take at least 3 cups to your local  Extension office for testing. In the St. Louis, Missouri area, MU Extension in St. Louis County is located at 132 E. Monroe in Kirkwood, in St. Charles County at 260 Brown Rd. in St. Peters, and in Jefferson County at 301 3rd Street in Hillsboro. Virtually every county in the US has an Extension office, just search the web for your state and “Extension” followed by your county of residence.

When you receive your soil report (analysis), you’ll likely have some questions. You may find your answers in one of the articles below:

A Basic Soil Chemistry Review

How to Interpret A Soil Test

How to Choose And Apply an Appropriate Fertilizer

Justin Keay is the MU Extension Field Specialist in Horticulture for St. Louis City/County and St. Charles County.