Why Do a Soil Test?

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The cost of soil testing is minor in comparison to the cost of plant materials and labor. Correcting a problem before planting is much simpler and cheaper than afterwards.

Once your yard is established, continue to take periodic soil samples. While many people routinely lime their lawns, this can result in raising the pH too high.

However, since many fertilizers tend to lower the pH, the pH may drop below desirable levels after several years, depending on fertilization and other soil factors.

Home tests for pH, nitrogen, phosphorus, and potassium are available from garden centers. While these may give you a general idea of the nutrients in your soil, they are not as reliable as tests performed by the Cooperative Extension Service at land grant universities.

University and other commercial testing services will provide more detail and you can request special tests for micronutrients if you suspect a problem. In addition to the analysis of nutrients in your soil, they often provide recommendations for the application of nutrients or on adjusting the pH.

The test for soil pH is very simple—pH is a measure of how acidic or alkaline your soil is. A pH of 7 is considered neutral. Below 7 is acidic and above 7 is alkaline. Since pH greatly influences plant nutrient availability, adjusting the pH will often correct a nutrient problem.

At a very acidic pH, several of the micronutrients become less available for plant uptake. Iron deficiency is a common problem even at a neutral pH on such plants as rhododendrons and blueberries. At a very alkaline pH, other micronutrients may be too available, resulting in a plant toxicity.

Phosphorus and potassium are tested regularly by commercial testing labs. While there are soil tests for nitrogen, these may be less reliable. Nitrogen is present in the soil in several forms and the forms can change rapidly. Therefore, a precise analysis of nitrogen is more difficult to obtain.

Most university soil test labs do not routinely test for nitrogen. Home testing kits often contain a test for nitrogen, which may give you a general idea of the presence of nitrogen, but again, due to the various transformations of nitrogen, the reading may not be reliable.

Organic matter (soil tilth) is often part of a soil test. High levels of soil organic matter are highly desirable. Organic matter has a large influence on soil structure. Good soil structure improves aeration and water movement and retention. This encourages increased microbial activity and root growth, both of which influence the availability of nutrients for plant growth.

Soil organic matter also directly affects the availability of plant nutrients, and affects how pesticides react in the soil. Soils high in organic matter tend to have a greater supply of plant nutrients compared to many soils low in organic matter. Organic matter tends to bind up some soil pesticides, reducing their effectiveness.

Tests for micronutrients are usually not performed unless there is reason to suspect a problem. Certain plants have greater requirements for specific micronutrients and may show deficiency symptoms.

Iron deficiency is common on blueberries, rhododendrons, and pin oaks unless the soil is quite acidic. On these plants, the younger leaves will usually show signs of the deficiency first. The areas between the veins will be yellowish while the veins remain green.

Other plants growing in the same soil will show no signs of a deficiency. In this case, altering the pH will often correct the problem.

Most of this article comes courtesy of the USDA Natural Resource Conservation Service.