Deficiencies not always detected by soil testing

In mid-October, one of my articles was on fall garden work and soil testing. 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 who are having a difficult time growing crops even though the soil test shows that nothing is 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 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 to12 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.

* Added soil: Sometimes soil is added to smooth out an area or topsoil is added but only a shallow layer is used. Roots will not go from one soil type to another. Always blend in added soil to existing soil so there is a gradient from one to the other. For example, let's say you want to add 6 inches of topsoil. Add 3 inches and till in to a depth of 6 inches and then add the remaining 3 inches of topsoil. You now have a gradient that goes from the topsoil to the existing soil.

* 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, use drip irrigation if practical (such as in a vegetable garden).

* Gas Leaks: Make sure that a natural gas line does not run under the affected area. Leaks from that line can kill existing vegetation. If this is the cause, everything in that area will be killed.

These are a few reasons why you maybe disappointed in plant growth and performance. If your soil tests do not show an inadequacy of nutrients, check for the above conditions. If you need to amend your soil or transplant flowers start making a plan now instead of waiting until its time to begin the work!

Editor's Note: Delta George is a K-State Research and Extension agriculture and 4-H extension agent assigned to Bourbon County. She may be reached at (620) 223-3720.