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.
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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
For a list of soil labs, visit the University of Illinois
Extension Soil Testing Labs website at
[By JENNIFER FISHBURN, horticulture
University of Illinois Extension, Logan-Menard-Sangamon Unit]