Soil Testing
By: Ed Browning, Master Gardener Coordinator

What makes a garden grow? Ample water, tillable soil and sufficient nutrients among other things. How does one know if there are sufficient nutrients? If your car is low on oil, you can tell just by looking at the car, right? Not! If your computer crashes you can tell what caused it just by looking at the monitor. Isn't that right? I don't think so! Why then would you expect to be able to guess how much fertilizer to add to a lawn or garden soil? Nutrient levels can't be determined by looking at, feeling, or smelling the soil. That's why you should soil test. How do you get a good sample and what do you do with the analysis once you get it back?

Testing the soil is a very important tool for gardening, lawn care or any other horticultural practice. Tools that can be used for collecting a soil test include a soil probe, soil auger or a shovel. In the Extension Center, we have a soil probe that we loan to anyone wishing to test their soil, but a shovel will do just as well.

We normally recommend taking a sample of the soil profile to the depth of tillage. For example, if the garden is usually tilled six inches deep, that would be the depth to dig for a good sample. If using a shovel, dig a hole about six inches in diameter and six inches deep. Then take a ½ inch to 1 inch slice of soil from the side of this hole. Do this around the garden three or four times and mix these together, saving about a pint of this mix of soil for the test. We can send the sample to a University of Missouri lab for analysis and it takes about two weeks to get the results back. The results are sent to our agronomy specialist for recommendations of nutrient amendments. Once the analysis has been determined, recommendations are supplied in terms of pounds of nutrients needed per 1000 square feet of garden or lawn area.

Information provided by our lab include levels of phosphorus, potassium, calcium, magnesium, organic matter and pH. Nitrogen is the only nutrient that is not determined by a specific test but recommendations are based on the organic matter level. Other labs in the region may provide a similar service but may not make recommendations for which, if any, nutrients to add to the soil.

There are three information sheets available in the Extension Center to help with sampling the soil and interpreting the results. MU Guide sheets 6954, "Soil testing for lawns"; and 9112, "Interpreting Missouri soil test reports" were writen primarily for field crops and pastures but apply just as well to gardens and lawns.