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Missouri Environment & Garden


Manjula Nathan
University of Missouri
Plant Science & Technology
(573) 882-3250

Soil Testing Takes the Guesswork Out of Growing Healthy Lawns and Gardens

Manjula Nathan
University of Missouri
(573) 882-3250

Published: March 1, 2008

Soil testing is a gardener’s best guide to the wise and efficient use of fertilizer and soil amendments. We frequently get questions from customers like “I apply excess fertilizer every year, how come my plants are not doing well?” Then we ask questions to identify the source of the problem. Most of the time the answer is they have never done a soil test but have been guessing on their fertilizer requirements. They do not realize that by guessing the fertilizer requirement they are not only wasting money by over or under application, the excess fertilizer can end up in streams, ponds and under ground water and pollute the environment. A soil test is like taking an inventory of the nutrients available to plants, which are too high, too low or just right. While plant growth may offer clues to nutrient availability, a gardener won’t precisely know until they test their soil. Although soil-testing kits are available in garden centers, laboratory testing is more reliable, and the results from laboratories are accompanied with interpretations and recommendations.

Why Do a Soil Test?

A soil test is the best way to know the needs of your garden and lawn. A soil test indicates nutrient levels already in your soil – a first step in determining how much fertilizer and lime you need for the plant you are trying to grow. The nutrient status of the soil fluctuates throughout the growing season each year. The availability of nutrients in the soil is altered by the addition of fertilizers, manure, compost, mulch, and lime or sulfur, in addition to leaching. In addition nutrients are removed from soils as a result of plant growth and development, and by the harvesting of crops. The soil test will provide the nutrient status of the soil and the fertilizer recommendations.

Some plants grow well over a wide range of soil pH, while others grow best within a narrow range of pH. Most turf grasses, flowers, ornamental shrubs, vegetables, and fruits grow best in slightly acid soils which represent a pH of 6.0 to 7.0. Plants such as rhododendron, azalea, and blueberries require a more acidic soil to grow well. A soil test is the only precise way to determine whether the soil is acidic, neutral, or alkaline. The soil test takes the guesswork out of fertilization and is extremely cost effective. It not only eliminates the waste of money spent on unnecessary fertilizers, but also eliminates over-usage of fertilizers, hence helping to protect the environment.

When Do I Soil Test?

soil sampling tools

Soil samples can be taken in the spring or fall for established sites. Although fall and early spring are typical times to test soil, one can really do it any time the soil is not frozen, but don’t sample after recent fertilizer or lime applications. For new sites, soil samples can be taken anytime when the soil is workable. Most people conduct their soil tests in the spring. However, fall is a preferred time to take soil tests if one wants to avoid the spring rush and suspects a soil pH problem. Fall soil testing will allow you ample time to apply lime to raise the soil pH. Sulfur should be applied in the spring if the soil pH needs to be lowered. Garden soils should be tested every two to three years.

Soil testing is strongly recommended when establishing a new lawn, renovation of an existing lawn, or landscaping. The cost of soil testing is minor in comparison to the cost of seed and plants 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. Routine fertilizer or lime applications can result in excessive soil nutrient levels or a deleterious soil pH. For example many fertilizers tend to lower soil pH, and after several years of fertilization the pH may drop below a desirable level.

How to Take a Representative Sample?

six inch soil core

The test results are only as good as the sample taken. It is extremely important to provide a representative sample to the testing lab so that a reliable test and recommendations can be made for the entire area. This can be accomplished by submitting a composite sample. A good representative composite sample from a garden or lawn should contain 8 to 10 cores or slices. Each core or slice should be taken at the same depth (0 to 6 inches) and volume at each site. Sample at random in a zigzag pattern over the area and mix the sample together in a clean plastic bucket. More samples need to be taken if the area was recently limed or fertilized. Separate samples need to be taken from lawns, gardens, flower beds or shrub borders. Separate samples should be taken from areas with distinctive soil types or plant performances.

What Tests to Run?

Testing your soil for nutrients and pH is important to provide balanced application of nutrients, while avoiding over application. At University of Missouri Soil Testing Laboratory we offer a regular fertility test that includes measurements of pH, lime requirement, organic matter, available phosphorus, potassium, calcium, magnesium, and cation exchange capacity. Soil pH greatly influences plant nutrient availability. Adjusting pH often corrects the nutrient problem for most plants. The optimum pH for most plants is between 6.0 and 7.0. The lime requirement measurement indicates the amount of amendment (usually lime) necessary to correct a pH problem. Organic matter has several roles in the soil; generally the more organic matter the better. Nitrogen recommendations are based on the organic matter level. Phosphorus, potassium, calcium, and magnesium are all essential plant nutrients. For the layman a cation exchange capacity (CEC) value has no meaning, but it is a measure of the soil’s ability to hold nutrients.

Test costs vary according to the number of nutrients tested. The University of Missouri Soil Testing Laboratory charges $10.00 (when submitting directly to the lab) for a regular fertility test. Several other specific analyses are available. These include but are not limited to soil analysis for sulfur, micro-nutrients (Zinc, Iron, Copper, Manganese, Boron), salt content (electrical conductivity), and soil texture. Test reports provide interpretation and nutrient recommendations. The turnaround time for a soil test is 24 hours. Customers have to add mailing time to get the reports by regular mail services. For plant, water, compost, manure and greenhouse media tests the turnaround time is within 5-7 working days.

How and Where to Submit Samples?

You can contact your Regional Agronomy/Horticulture/ Natural Resources Specialist, or local Extension Office to obtain Sample Information Forms and sample boxes, and can submit samples through their offices. The Regional Specialists at your local Extension Offices can be a source of information for interpreting and personalizing your soil test reports and recommendations. Samples can be submitted directly to the University of Missouri Soil Testing labs at 23 Mumford Hall, Columbia, MO 65211, (573)882-0623 or Delta Soil Testing Lab at PO BOX 160, Portageville, MO 63873.

The lab maintains a comprehensive Web site at http://soilplantlab.missouri.edu/soil. The Web site provides a list of services provided by the lab, costs of tests, sample information forms, location of the lab and other relevant information. Now the lab also provides Web access of soil test results with a specifically assigned password to clients upon request. We also have the option for electronic mailing of data if required. Customers can drop off their samples in person at 23 Mumford Hall, mail them in, or drop them off at their County University Extension offices.

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REVISED: September 30, 2015