Results from soil tests continue to be one of the most important indicators of current soil fertility and a great source of information when determining nutrient recommendations for an upcoming crop. Recently, soil test results are being used as a way for a renter to show the landlord positive change or maintenance in soil fertility levels throughout the life of a land lease. It’s important to be very careful when using soil tests for these situations. An acre of soil to a 6-inch depth weighs about 1,000 tons, yet less than 1 ounce of soil is used for each test in the laboratory.  Therefore, it is very important that the soil sample is representative of the entire field.  Variability can result from a number of factors including: from the number of cores taken, the depth cores are taken from, time of year, and field moisture conditions.

Even though soil tests remain one of the most useful and basic crop and soil management tools we have, it is important to understand the limitations of the results for both accuracy and potential uses. Soil tests effectively distinguish soils with low and high probabilities of crop response for most nutrients (Bruulsema, 2004). The actual number presented to you on your soil test results sheet should be used to gauge that probability of crop response, and not necessarily as a finite value where one number is tremendously better or worse than another.

Here are some links to helpful resources with further information on the variability that might be present in your soil test results.

Effect of sampling time on soil test potassium levels, presented at 2010 Wisconsin Crop Management Conference, Vitko, Laboski, Andraski

Why are soil test potassium levels so variable over time in the Corn Belt?, International Plant Nutrition Institute website, Murrell

Seasonal variability in soil test potassium, presented at 2005 Wisconsin Crop Management Conference, Laboski

Understanding the science behind fertilizer recommendations, International Plant Nutrition Institute website, Bruulsema

An excerpt from “Agronomic and Environmental Implication of Phosphorus Management Practices”, Mallarino, Bundy

Sampling soils for testing, UW Extension Publication #A2100, Peters, Laboski, Bundy