Soil testing

By Jennifer Fishburn, University of Illinois Extension

[September 24, 2012]  SPRINGFIELD -- The most important test a gardener can take is a soil test. A soil test is used to determine the level of nutrients in the soil and the soil's pH. Soil tests can reveal why some plants aren't growing well in a particular area.

Soil sampling can be done any time of the year; however, the ideal time is when the garden season has ended in the late summer to early fall. It is desirable to take samples before soil temperatures drop below 50 degrees F. Be sure to wait six to eight weeks before testing a recently fertilized area.

The pH value is the measure of acidity or alkalinity of the soil. The pH scale ranges from 0 to 14, with 7 being neutral. Numbers less than 7 are considered acid, and numbers greater than 7 are alkaline.

Soil pH directly affects nutrient availability. Most soil nutrients are readily available when soil pH is at 6.0 to 7.5. When pH rises above this value, nutrient elements such as phosphorous, iron, manganese, copper and zinc become less available. This explains why pin oak trees suffer from iron chlorosis.

Different plants thrive best in different soil pH ranges. For example blueberries and rhododendrons need a soil that is acidic, about 4.5 pH. Soil pH values above or below the optimum range may result in plant symptoms of nutrient deficiencies.

Soil test results are only as good as the sample provided to the testing lab. Here are a few steps to follow:

Divide your property into sections according to use and soil type. Submit a sample for each area of the yard -- vegetable gardens, flower gardens, lawn. In addition, each sample should represent only one soil type.

Use a clean soil probe, spade, hand trowel or shovel to collect samples. Avoid using brass, bronze or galvanized tools because they can contaminate samples.

A soil sample must be representative of the area being tested. So a soil sample is a composite of numerous subsamples. Randomly select and evenly space subsamples. Collect at least eight subsamples from a 100-square-foot area.

Soil sampling depths depends on what plants are growing in that location. Recommended depths are 4 inches for established lawns, 6 to 8 inches for vegetable and flower gardens, and 6 to 12 inches for trees and shrubs.

When testing the area under a tree, take subsamples from the trunk to the outer edges of the branches.

If using a shovel, spade or hand trowel, dig a hole and set the soil aside. Then cut a half-inch to 1-inch slice of soil from one side of the hole. Place the slice in a clean, plastic container. Remove leaves, roots, thatch and debris from the sample.

Repeat this process within the sampling area. Mix subsamples. Spread the sample on a clean paper and allow the soil to air dry. Place about 1 pint of thoroughly mixed soil in a bag or box. Then place packaged sample in a sturdy mailing carton.

Be sure to label each sample to indicate the area being sampled: i.e., lawn, vegetable garden, flower garden, rose garden, fruits, pin oak tree, etc. Also include your name and address. If the soil is from a bare area, indicate what will be grown in the area.

Mail sample to a soil testing lab. Expect soil test results within two weeks. Results should indicate the amount and type of fertilizer or other soil amendments to apply to the soil.

While soil test results provide a great deal of information, a standard soil test will not identify poor soil drainage, soil compaction, over-watering or under-watering, or environmental disorders.

For a list of soil labs, visit the University of Illinois Extension Soil Testing Labs website at http://urbanext.illinois.edu/soiltest/.

[By JENNIFER FISHBURN, horticulture educator, University of Illinois Extension, Logan-Menard-Sangamon Unit]