By Darrell Blackwelder
Cooperative Extension
Spring is the time of year when gardeners ask questions about their soil, especially those trying to prepare both floral and vegetable gardens in red clay.
One woman called complaining that her sick red clay soil wouldn’t grow anything.
The biggest misconception about clay soils is they tend to be poor and non-productive. All one has to do is observe the massive growth of a mature oak tree or literally tons of tomatoes per acre on these soils and the myth is easily quelled by their productivity. Tight clay soils are indeed a challenge, but with proper modification, these soils can easily produce beautiful ornamentals and bountiful garden vegetables.
The workability of clay soils must be improved with amendments. Amendments are necessary to improve drainage and pore space or oxygen. Oxygen is a very important element necessary for root growth and expansion.
The best amendments for clay soils are pine bark humus that is less than half an inch in diameter, composted leaf mold or Permatil. Permatil, locally produced in Gold Hill, is slate rock that is heated and expands, providing pore space in clay soils.
Select leaf mold and other composted materials that are completely composted and not merely aged. Green or uncomposted organic materials compete with plants for nutrients, especially nitrogen and sulfur, resulting in nutrient deficiencies and poor plant growth.
Peat moss, sand, hardwood bark, green sawdust, wood chips and pine straw are not recommended as amendments for clay soils. Adding these materials will not improve the physical properties of a clay soil. In fact, sand added to clay makes the soil more like a brick.
Amendments to clay soils must be incorporated to at least 25 percent by volume to be effective. For example, to result in approximately 8 inches of amended soil, a minimum of 2 inches of the amendment should be incorporated into the top 6 inches of soil. This also helps raise the bed, which will not only improve drainage but will also make bedding plants look more attractive.
Incorporating up to 50 percent by volume will probably improve plant growth. Incorporating over 50 percent may have a negative effect on plant growth, while incorporating less than 25 percent by volume is a waste of time and material.