How to Fertilize Clay Soil


Growing lawns and gardens in clay soil can be frustrating because clay can form a barrier against roots, nutrients and water. Roots need to spread. Soil needs to drain. How to fertilize clay soils that can be nearly impenetrable is complicated. The short answer is that clay soils first have to be amended; a gardener adds material including fertilizer to the top layer of clay, loosening it to encourage plant growth and so that it can accept nutrients and water.

Amending clay

Step 1

Test your soil to find out what kind of fertilizer you need to add. Different types of clay need different types of fertilizers and so do the plants you propose to grow. Most states have agricultural extension services that can test your soil or tell you how to get it tested.

Step 2

Mix your amendment ingredients. There is no accepted formula for amendments to clay soil. Your mix should ideally be equal amounts of compost, pine bark, manure and gravel. All three organic materials are good, but if you only have one or two, use those. You should include gravel or shale to promote drainage. You can add non-clay, weed-free commercial topsoil.

Step 3

Use your spade and shovel to break up the clay about 8 inches deep, about the length of a shovel blade.

Step 4

Spread a 2-inch layer of your mix over the area. Break up clods of clay into ever smaller chunks. Work the amendment into the soil.

Step 5

Spread a second 2-inch layer, adding the fertilizer or lime that your soil test said was necessary. Work this layer in the full 8 inches.

Step 6

(Optional) As you amend your clay, it will become higher than the surrounding soil, helping to improve drainage. It will eventually settle some. To keep it higher, build a barrier around it.

Tips and Warnings

  • Sand is useful in potting soils, but if you add it to clay, the soil can pack even harder. Decomposing wood chips or sawdust take nitrogen from the amended clay. The manure in wood chips or sawdust used to bed farm animals will provide nitrogen for decomposition, but you shouldn't use these chips or sawdust to amend your clay unless they are well rotted.

Things You'll Need

  • Shovel
  • Compost, or humus (You can buy compost or make it yourself using yard waste, grass clipping, leaves, or vegetable peelings and other vegetable household waste.)
  • Pine bark soil conditioner, sometimes sold as pine bark mulch. (A pea-sized grind from one-fourth to one-half inch is ideal. Straight pine bark is good, but larger nuggets sold as "pine bark mulch" can sometimes be too large.)
  • Aged manure. (If it has been well composted, manure will lose its smell. Fresh manure can add too much salt to the amended clay. It can also cause roots to dry out and burn.)
  • You can add commercial topsoil to your amendment mix if you want a raised bed, but topsoil available locally is sometimes the same as your clay, only it hasn't been packed down. It also might contain too much sand and weed seeds.
  • Pea gravel. (This helps the clay drain and not pack down. It is sometimes called 78, meaning it is about a quarter of an inch wide. Expanded shale is good but it is more expensive.)


  • Amending clay soils
  • Improving Soil
  • Improving Clay Soil In Your Yard

Who Can Help

  • Soil testing
Keywords: fertilizing clay, amending clay, growing in clay

About this Author

Richard Hoyt, an internationally published author of 26 mysteries, thrillers and other novels, is a former reporter for Honolulu dailies and writer for "Newsweek" magazine. He taught nonfiction writing and journalism at the university level for 10 years. He holds a Ph.D. in American studies.