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Soil Testing is an Excellent Investment for Garden Plants and Commercial Crops

by Ohio State University Extension    

Gary Gao
Horticulture Extension Agent
Clermont County

Joe Boggs
Horticulture Extension Agent
Hamilton County

Jim Chatfield
District Specialist and Assistant State Specialist
Northeast District, and Horticulture and Crop Science

The soil test is an excellent measure of soil fertility. It is a very inexpensive way of maintaining good plant health and maximum crop productivity. The standard soil test provides the status of phosphorous (P), potassium (K), calcium (Ca), magnesium (Mg), pH, cation exchange capacity, lime requirement index, and base saturation. Additional tests are also available for iron (Fe), zinc (Zn), manganese (Mn), soluble salts, and nitrates.

The Research and Extension Analytical Laboratory (REAL-LAB) run by The Ohio State University was closed in December 1998. However, there are several commercial labs that offer the same or similar tests as were used by the REAL-LAB. There are also soil and tissue testing labs administered by land grant universities such as Michigan State University, Penn State University, and the University of Kentucky. For a fee, most of these labs will test soil, soilless media, plant tissue, compost, and water. The amount of fertilizers, timing of fertilization, and amount of soil pH modifying agents such as lime, will be given according to the type of crop, based on soil test results. Refer to Table 1. for a list of these soil and tissue testing labs in Ohio and neighboring states.

Why Do I Need to Soil Test?

Soil fertility fluctuates throughout the growing season each year. The quantity and availability of mineral nutrients are altered by the addition of fertilizers, manure, compost, mulch, and lime or sulfur, in addition to leaching. Furthermore, a large quantity of mineral nutrients are removed from soils as a result of plant growth and development, and the harvesting of crops. The soil test will determine the current fertility status. It also provides the necessary information needed to maintain the optimum fertility year after year.

Some plants grow well over a wide range of soil pH, while others grow best within a narrow range of pH. Most turfgrasses, flowers, ornamental shrubs, vegetables, and fruits grow best in slightly acid soils which represent a pH of 6.1 to 6.9. Plants such as rhododendron, azalea, pieris, mountain laurel, 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.

Most soil nutrients are readily available when soil pH is at 6.5. When pH rises above this value, nutrient elements such as phosphorus, iron, manganese, copper, and zinc will become less available. When soil pH drops below 6.5, manganese can reach a toxicity level for some sensitive plants.

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 samples can be taken in the spring or fall for established sites. 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.

How Frequently Should I Soil Test?

A soil test every two to three years is usually adequate. Sample more frequently if you desire a closer monitoring of the fertility levels, or if you grow plants that are known to be heavy feeders.

What Soil Sampling Tools Do I Need?

A soil sample is best taken with a soil probe or an auger. Soils should be collected in a clean plastic pail or box. These tools help ensure an equal amount of soil to a definite depth at the sampling site. However, a spade, knife, or trowel can also be used to take thin slices or sections of soil.

How Do I Take Soil Samples?

1. Remove the top debris, residue, or turf thatch from the soil surface before taking the sample.
2. Sample gardens, trees, shrubs, flower beds, and orchards 6-8 inches deep.
3. For the lawn, lift the sod and sample 3 inches deep.
4. Sample a row crop field or garden between rows to avoid fertilizer bands.
5. Sample when soils are suitable for spading or plowing.
6. Take separate samples from fields that have received different fertility programs.
7. Take separate samples from soils that are distinguishable by color (i.e. light vs. dark), drainage, or other factors.

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 should contain 10-15 cores or slices. Each core or slice should be taken at the same depth 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.

How to Prepare Soil Samples for Submission

1. Contact a soil testing lab for instructions, soil test kits, and appropriate forms.
2. Break up lumps and air dry the soil at room temperature with no artificial heat.
3. When the soil is dry, mix well and crush soil lumps to the size of wheat grains or smaller.
4. Take about one pint of the composite sample and place it in the sample bag associated with the kit.

Where Do I Send My Soil Sample?

You need to mail soil sample(s), completed sample form(s), and appropriate payment to the soil testing lab you selected. Generally, soil testing labs will provide a complete set of instructions, either with sample kits or upon request. Follow the instructions carefully.

How Long Does the Soil Test Take?

Soil test results and fertilizer recommendations are usually mailed in two weeks, depending on the labs you work with. Make sure you fill out the form for the types of plants you grow or will be growing. Otherwise, no recommendations will be given. Indicate whether the crop is planted, or is to be planted. Incomplete forms may cause delays in receiving results and recommendations.

What Kinds of Soil Tests Are Available?

The authors would like to thank Jane Martin and Maurice Watson for reviewing this fact sheet.