Garfield County Cooperative Extension

Soil Testing

Rick Nelson Extension Educator, Ag/4-H

Most gardeners think that soil tests are done only to find out what nutrients are deficient. However, it is just as important to know if you have adequate levels of nutrients so you don't add unneeded fertilizer. The most basic soil test checks pH and the levels of phosphorus and potassium. Most of the lawn and garden soil tests that come out of our soil-testing lab show more than adequate levels of both phosphorus and potassium. If those nutrients are not needed, applying them is a waste of money and can be a source of pollution. In extreme cases, excess phosphorus can interfere with the uptake of micronutrients. So, if you haven't taken a soil test in several years, take one this spring.

Begin by taking a representative sample from a number of locations in the garden or lawn that goes from the surface to 6 inches deep. Mix the samples together in a clean container and select about 1 pint of soil. For more detail on taking a soil test, ask for L-249 soil Testing- the Right First Step at your local Extension Office. Take the soil to your County Extension Office to have tests done at the OSU Soil, Water, & Forage Analytical Laboratory for a small fee. A soil test determines fertility problems, not other conditions that may exist such as poor drainage, poor soil structure, soil borne diseases or insects, chemical contaminants or damage, or shade with root competition from other plants. All of these conditions may reduce plant performance but cannot be evaluated by a soil test.

What a Soil Test Does Not Tell You

Though soil tests are useful for identifying nutrient deficiencies as well as soil pH, they do not tell the whole story. We often receive soils from gardeners that are having a difficult time growing crops even though the soil test shows the pH is fine and nutrients are not deficient.

Here are some factors that can affect plant growth that are not due to nutrient deficiencies or pH.

Not enough sun: Plants need a certain minimum amount of sun before they will grow well. As a general rule, flowering (and fruiting) plants need at least 6 to 8 hours of full sun per day. There are, of course, exceptions such as impatiens that bloom well in shade. Move sun-loving plants out from the shade or use plants that are better adapted to shady conditions.

Poor soil physical characteristics: Roots need oxygen as much as they need water. A tight clay soil or excessive water can restrict soil oxygen levels as well as make root penetration of the soil difficult. Increasing the organic matter content of clay soils can help break them up. Add a 2-inch layer of organic matter and till it in.

Walnut trees: Walnuts give off a natural herbicide that interferes with the growth of some plants such as tomatoes. Vegetable gardens should be at least 50 feet away from walnut trees if possible.

Tree roots: Trees not only compete with other plants for sun but also for water and nutrients.

Extra water and nutrients may be needed.

Shallow soils: When new homes are built, the topsoil is often stripped off before the soils are brought to grade. Though the topsoil should be replaced, it sometimes is not or is not replaced to the same depth as it was originally. You are left with subsoil that usually does not allow plants to grow well due to a lack of soil structure. Adding topsoil to a depth of 8 to 12 inches would be best but this often is not practical. In such cases, try to rebuild structure by adding organic matter and working it into the soil.

Too much phosphorus: Most soils of the Southern High Plains are naturally low in phosphorus. However, soils that have been fertilized for a number of years may have phosphorus levels that are quite high. As a matter of fact, many of soil tests received show phosphorus levels in the "high" category. Too much phosphorus can interfere with the uptake of some micronutrients such as iron, manganese and zinc. High phosphorus soils should only be fertilized with fertilizers that have relatively low amounts of phosphorus.

Improper watering: Roots develop where conditions are best for growth. Shallow, frequent watering leads to roots developing primarily near the surface of the soil where the soil is moist.

Such shallow root systems are easily damaged by heat and any interruption in the watering schedule. It is better to water less frequently and to a greater depth to encourage a deeper root system that is less sensitive to heat and water stress.

Watering during the evening can also be detrimental to plants if the irrigation wets the foliage.

Many diseases are encouraged by free water on the leaves. Watering late in the day often will   keep the foliage wet until dew forms. Dew will keep the foliage wet until it evaporates the next morning. It is better to water early in the morning so leaves do not stay wet as long. If you must water late in the day, consider drip irrigation if practical (such as in a vegetable garden).

Overwatering: Roots need to breathe. In other words, they must have oxygen in order to survive. Be careful to not water so much that the soil remains saturated. Water deeply but allow soil to dry somewhat between irrigation events.

Gardening 101

The Garfield County Master Gardeners have scheduled a workshop Saturday, February 28 held at Autry Technology Center. Six sessions are planned to ready one for the upcoming gardening season. Details are available from the OSU Extension Center 580-237-1228 or any Master Gardener.

Reference to commercial products or trade names is made with the understanding that no discrimination is intended and no endorsement by the Oklahoma Cooperative Extension Service is implied. 

Oklahoma State University, U.S. Department of Agriculture, State and local governments cooperating. Oklahoma Cooperative Extension Services offers its programs to all eligible persons regardless of race, color, national origin, gender, age, religion, disability, or status as a veteran and is an equal opportunity employer.



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