Soil Sampling, the Acid Test

Now that we have about a month to go before setting out transplants and the lawn is greening up nicely, it is time to think about soil testing your garden, flower beds, or lawn.  This test will tell you if your soil is too acid and whether or not it contains the nutrients necessary for proper plant growth. It can save you money in the long term by telling you what is necessary and what isn’t.  Adding a nutrient that is already present is unnecessary and adds to run-off pollution. Taking a soil sample is easy.  Sample bags are available from the Iron County University of Wisconsin Extension Service Office in the Iron County Courthouse.  They also have a sheet which will instruct you in how to physically take the sample.  The cost is $15 for a garden or lawn sample and the results are mailed directly to you in about two weeks.

Soil sampling measures four critical factors in the composition of the soil in your garden or lawn: Acidity, Nitrogen, Phosphorous, and Potassium (Potash).  These four are like the legs of a chair – you need all four present in the correct amounts in order to have stability.  While nitrogen, phosphorous, and potassium are essential nutrients for plants, proper acidity is necessary in order for plants to utilize those nutrients. If your soil is too acid or too alkaline, plants can not use some of the soil nutrients even if they are available.

Soil acidity is measured in pH where a pH of 7.0 means the soil is neutral, neither acid nor alkaline. A pH of less than 7 indicates an acid soil and the lower the number the more acid the soil.  Acidic soils are sometimes referred to as “sour” because they can taste like vinegar if touched to the tongue.  Most plants and lawns need a pH in the range of 6.2 to 7.0.  Soils in this area tend to be quite acidic. An area that I am clearing for a hayfield tests at a pH of 5.8.  If you are gardening in town, your site may be on fill dirt so there is no predicting. Soil pH can be raised by using garden lime or wood ash and your soil sample report will give a specific recommendation for the number of pounds of garden lime to add per 1000 square feet of lawn or garden.  Wood ash will increase the pH and add some nutrients, especially potassium, but it is difficult to know how much to use.

A few plants thrive in an acid soil, notably azaleas and blueberries. You can buy soil acidifiers for these plants or use a special fertilizer for acid-loving plants. These plants should not be interspersed in gardens or flower beds with other plants that require a more neutral pH.

There are actually thirteen essential plant nutrients, of which three are considered primary nutrients.  These three are nitrogen, phosphorous, and potassium and are usually referred to by their chemical symbols N, P, and K.  Commercial fertilizers express their percentages of N-P-K using numbers such as 5-10-12.  A fertilizer with a 5-10-12 composition would contain 5 percent nitrogen, 10 percent phosphorous, and 12 percent potassium, with the remaining 73 percent consisting of inert material.  All soil amendments classified as fertilizers sold in Wisconsin must have these percentages established by laboratory test, whether or not they are classified as organic.  The soil sample report will give you recommendations for applying these three nutrients to your garden, lawn, or flower beds.

The remaining 10 plant nutrients consist of the secondary nutrients calcium, magnesium, and sulfur, and the trace nutrients zinc, copper, boron, iron, manganese, chlorine, and molybdenum.  (Try to say that last one three times).  These secondary and trace nutrients are usually not a problem in this region of the country and are not routinely tested for.

For additional information, the Extension Office has or can get the pamphlets “Tree and Shrub Fertilization” (A2308), “Garden Fertilization” (A2304), “Sampling Lawn and Garden Soils for Soil Testing” (A2166), and “Calibrating and Using Lawn Fertilizer and Lime Spreaders” (A2306).  Lynn Adams, a Wisconsin Master Gardener, is available at the Extension Office through the summer as an additional resource.