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Master Gardeners of Douglas County

Master Gardeners of Douglas County

Soil Testing

In the Garden with Extension Educator Larry Zilliox

April 7, 2008

One of the springtime questions I often get is should I fertilize my garden and with what kind of fertilizer mixture. It is a difficult question to answer without information on the soils in the garden or lawn. An additional factor this year will be the cost of the fertilizer. The price of fertilizer has sky rocketed making it an economical decision in addition to nutrient need. Here are some guidelines you might consider this spring.

The first thing one should do is take a soil test. Most home yards are what we called disturbed soils, a mixture of top and subsoil because of the construction of our home. This mixing of soil means that what you are working with in your yard may be different from your neighbors. In the construction of their house, the contractor may have removed the top soil while construction was taking place and then replaced the top soil after the home was built while your contractor may have dug the hole for your basement and piled the dirt nearby and then spread that soil over the top of your new yard. As a result, your yard would react very differently than the neighbors.

Therefore, the first step is to learn what you have in your yard. Taking a soil test will help you learn what nutrients are available to plants. My advice would be to do several tests, one in the front yard and another in back. A third area to test would be garden areas where you are growing flowers or vegetables. Normally doing a test every five years should be adequate, unless you see nutrient problems developing in areas.

Soil sampling begins by getting a clean container in which to put your soil samples. Don't use a container that has been washed with soap as it could contain some phosphorous residue altering the test values. If you are sampling an established lawn, your sample needs to be from the top three inches of soil. If it is a garden area then sample to a depth of six inches. Generally, we would recommend taking seven to ten samples from the testing area to get a good representation of the soil in the area. These samples are then thoroughly mixed and a subsample of two cups of soil is sent to the testing lab. There are a number of private testing labs in the state or you can use the University of Minnesota Soil Testing Lab. Sample bags and forms are available from many County Extension Offices.

Results are normally returned in about a week and will have a recommendation for fertilizer needs of the tested area. Then it is a matter of finding a fertilizer mixture that will approximate the recommendation. Remember that state law states that you cannot apply phosphorous containing fertilizer to a lawn without a fertilizer test showing a need for it.