Darrell Blackwelder

March 25, 2004

     Often I receive calls from gardeners complaining about our soils. The biggest misconception about clay soils is they are poor and non-productive. Observing large growth of trees and tons of vegetables per acre easily quell this myth. Tight clay soils are difficult to manage; however, with proper modification these soils can easily produce both ornamentals and garden vegetables.

     Clay soils can be improved by amending the soil to improve drainage and oxygen The best amendments for clay soils are pine bark humus that are less than 1/2" in diameter, composted leaf mold, or Permatil.  Permatil is slate rock, heated to provide pore space in the soil.

      Be careful when selecting leaf mold, and make certain that the material is fully composted and not merely "aged." Green materials compete with plants for nutrients, especially nitrogen and sulfur, resulting in nutrient deficiencies and poor plant growth.   

     Peat moss, sand, hardwood bark, sawdust, wood chips and pine straw are not recommended as an amendment for clay soils. Addition of these materials will not adequately 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% 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% by volume will probably improve plant growth. Incorporating over 50% may have a negative effect on plant growth, while incorporating less than 25% by volume is a waste of time and material.