Howard F. Schwartz, Colorado State University, Bugwood.org
Collect a good soil sample
Accurate soil test results start with good sampling. Remember that a teaspoon of soil in the lab should represent the 20 million pounds of soil from a ten acre field! Follow the directions from your lab. The lab assumes you took the sample the way their directions indicate.
The next step is sending your sample to the lab. A couple things to know about soil testing labs--the tests are pretty standard and certified labs are generally very accurate in the analysis they do. Why then do we sometimes get different results from different labs? Well, that may go back to sampling error. But it is also good to know that different labs use different extractants. What we want to know with a soil test is how much of a certain nutrient is available to the crop over the growing season. Unfortunately, there is no way to look in the microscope and know what that amount is. There is a spectrum of unavailable to available nutrients in the soil. So scientists have developed different extractants that take a slice of the more available nutrients. These extractants are based on specific soil properties. For example, in Pennsylvania the standard test for phosphorus is called a Mehlich 3. In alkaline soils, like they have in California, they use a different test. It extracts a different amount of phosphorus. Make sure you know which tests your lab is using and that they are appropriate for the soils in your region.
What do the soil test levels mean?
Soil test levels don't mean a lot by themselves. A soil test level must be related to plant response. Soil tests are calibrated by growing crops in soils with different nutrient levels. They chart how yield goes up with increased fertility. Where the yield levels out is considered optimum (see plant yield response chart below). But be careful. Different labs use different terminology to mean the same thing. For example, the level where plants have adequate nutrients and adding more is unlikely to create a response is called "optimum" by the Penn State Ag Analytical Lab. But other labs call that same zone "medium" or "high."
Soil test recommendations
Soil management is more than just applying nutrients. We are trying to manage the soil's ability to supply nutrients over time. We can do this by managing organic matter, pH, soil physical properties and building up soil nutrient levels. A really good way to see if you are building or mining your soil is to keep track of (and even chart out) your soil test trends over time. Nutrient levels will fluctuate, but they should not plummet (mining the soil) or skyrocket (excess nutrients can be an environmental hazard). Building and maintaining the soil requires gradual build-up of fertility and organic matter as well as knowledge of how much you will be removing with your crops.
Soil tests are a good tool. But it is important to understand how they work to get accurate results and a relevant interpretation. If you are new, your local Extension Educator can help you interpret your test and make sure you have sufficient nutrients.
The Pennsylvania State University provides soil testing through the Agricultural Analytical Services Lab for a nominal fee. Organic matter and other tests are available for an additional fee.