Soil Testing:  Why and How to Do It &
What Does It All Mean?

Introduction

Before you landscape your yard or apply fertilizers to your yard, garden, or crops, you should learn the answers to two questions:

The answers to these questions will help determine nutrient and organic additive needs, as well as what types of vegetation to plant.  The only way to answer these questions is to have your soil tested by taking a sample to your County Extension Office or sending a sample to a soil testing laboratory.

Back to Top

Basic Soil Testing Procedure

To obtain a good soil sample make sure the sample is characteristic of your yard or field.  This requires taking a minimum of 5 core soil samples for an area and mixing them together to form a composite.  If the soil is different in your garden or lawn or fields, then a different composite soil sample should be taken for each area.  Soil samples may be taken at any time of the year.  However, if you want to plan for the next year's growing season, then fall is best.  Fall testing will allow you to take advantage of "end of the year" sales, as well as plan for the upcoming spring.  

For field crops, sample every 2-3 years or once in a rotation.  Soil for high value crops like potatoes should be tested annually.  For lawns and gardens, the frequency for soil sampling varies from individual to individual.

Back to Top

Results and Recommendations

The basic soil test results will tell you the pH of the soil, organic matter (nutrient holding capacity), available phosphorous (P), and available potassium (K).  Although nitrate (N) is not tested for, a recommendation will be made based on the need of the crop.  For an additional fee, the soil can be tested for magnesium (Mg), calcium (Ca), sulfur (S), boron (B), manganese (Mn),  and zinc (Zn).  

The most important values are the pH and following macronutrients:

The recommendations for soil management will be based on the types of plants you plan to grow.  Suggestions may include increasing or decreasing the pH of your soil, or adding some of the above nutrients, usually in the form of fertilizers.  The recommendations will specify how many pounds of a nutrient to add per 1,000 square feet.

Back to Top

Soil pH

The pH of your soil indicates whether the soil is more acidic or more basic. The pH scale ranges from 0-14, with 7 being neutral, 1-6 being acidic, and 8-14 being basic.  The pH helps determine what kinds of plants can grow in the soil.  Most plants can grow in soil with a pH between 5.2-7.8.  But some plants prefer smaller ranges or more extreme conditions.  For example, lawn grasses prefer a range of 5.5-6.5, and blueberries, azaleas, and rhododendrons prefer acidic soil between 4.5-5.2.  ⁶⁴

In general, soils usually do not have to be made more acidic, unless you plan to grow acid-loving plants, such as those mentioned above. However, to increase the acidity of your soil (decrease the pH), add sulfur or acid peat.  On the other hand to reduce the acidity of your soil (increase the pH), add limestone.

Back to Top

Soil Nutrients

When fertilizing, you only want to apply enough to raise the soil nutrient content to meet the needs of the plant.  Excessive fertilizer or inappropriately applied fertilizer can easily leach through the sandy soils into the groundwater. 

Fertilizers are labeled with three numbers that indicate the percentage by weight of the three major nutrients necessary for healthy plants.  For example, a 20 pound bag of 10-10-10 fertilizer contains 10% (2 lb.) nitrogen (N), 10% (2 lb.) phosphate (P₂O₅), and 10% (2 lb.) potash (K₂0).  The first number represents nitrogen (N), which helps makes lawns and plants green.  The second number represents phosphorus (P), which helps develop strong roots, bulbs, and flowers.  The third number represents potassium (K), which helps develop strong stems.  The recommendation for the amount and type of fertilizer to use will depend on your soil test results.

Although organic matter contains nutrients, it mainly helps soil structure by increasing soil drainage, aeration, water holding capacity, and the ability of soil to hold nutrients.  To increase the organic matter in your soil, add compost soil, old manure, or peat moss. 

Back to Top

Conclusion

An evaluation of soil fertility and pH is an important step in landscape and crop planning, and in determining problems with unhealthy plants.  However, problems with plants may be caused by other factors such as disease, insects, light availability, soil moisture or compaction, or climatic conditions.  Contact your County Extension Office for help in solving landscape problems, more information, or literature.

Back to Top

The University of Wisconsin Soil & Plant Analysis Laboratory contains more information on soil testing.

The University of Minnesota County Extension site contains a more detailed explanation of 
soil testing, soil interpretations, and fertilizer management.

For more soil testing information, read "Sampling Soils for Testing" from your Wisconsin County Extension Office.

Back to Taking Action
Back to Teaching and Learning