How to Amend Your Clay Soil

Have a soil test done before you add amendments (see CSU Fact Sheets 0.501 and 0.502). A soil test will tell you exactly what nutrients your soil needs. Adding fertilizer at the same time as other amendments can help save work. Private companies and Colorado State University soil testing laboratory can test soil samples. Home test kits are not recommended, as they are not very accurate on Colorado’s alkaline soils.

Digging In Amendments
The ideal amount of organic soil amendment is 25-50% by volume. Less than 25% will not provide enough aeration, and more than 50% is actually detrimental to plant growth. This is equivalent to spreading a 3 to 4 inch layer of amendment over your soil and digging it in to a depth of 8 to 24 inches. Break up any clods of clay in the process.

Make sure that the organic matter is incorporated to the full depth of the bed. If the soil is difficult to break up, try irrigating the area, waiting a day or two, then spading again. The wetting and drying process helps break up clods. Clay soil is easiest to till when it is slightly moist. Don’t dig if the clay is wet and sticky.

Raised Beds
Building completely new beds with new soil raised about ground-level is another way to get around clay soil. However, you can also create raised planting areas when adding soil amendments. Simply rake the amended soil into a smooth berm, or build an edge. The amended soil will settle somewhat as the organic material decomposes. If a level surface is desired, take away some of the amended soil for use elsewhere on your property.

Recommended Soil Amendment Materials

Homemade Compost
Compost, or humus, is decomposed plant material. It makes an excellent organic amendment for clay soils.

  1. From your lawn and garden: save yard waste, grass clippings, spent annuals, etc.
  2. From your trees: save raked-up leaves in fall, and grind them up by making several passes over them with the lawn mower. Leaves will decompose much more rapidly when ground up.
  3. From your household: save vegetable peelings, canning wastes, coffee grounds, etc. (but not meat or animal by-products).

“Cold” composting simply means piling your organic debris somewhere and letting nature take its time decomposing it. This may take a year or two, depending on the materials in the pile. Putting mulch on your beds and letting it decompose there is a simple form of cold composting.

“Hot” composting means building a pile that contains both nitrogen-rich (usually green) and carbon-rich (usually brown) materials, keeping it moist, and turning it regularly to encourage the microbes that carry out decomposition. The pile heats up from the microbes’ activity. In our area, the elevated temperature rarely becomes hot enough to kill weed seeds and disease pathogens that may be in the pile. Avoid composting weeds with flowers or seeds, or diseased plant materials.

Detailed information on composting is available from CSU Fact Sheet 7.212 and other CSU publications.

Purchased Compost (Humus)
Few homeowners have sufficient quantities of compost to amend heavy clay soil. Compost can be bought in bags at gardens centers, or by the truckload from companies that sell mulch or topsoil.

Pine Bark Soil Conditioner
Finely- ground nuggets sold as “pine bark mulch” are an excellent soil amendment, and have the advantage of being a local and renewable resource. A pea size grind (1/4 to 1⁄2 inch is ideal. The larger nuggets also sold as pine bark mulch are too big. Brands and names of products vary, so look or ask before you buy. Straight pine bark is a better deal.
There are other products on the market called “soil conditioners” but many are too fine to provide the needed pore space.

Composted Manure
Well-aged manure is an excellent soil amendment material that also provides some minor nutrients, though its fertilizing capacity is often over-estimated—most analyze at about 1-1-1 . For example (in order of nitrogen,  phosphorus, potassium): cow manure, dried: 1.3/.9/.8, hen manure, fresh: 1.1/.9/.5, horse manure, fresh: .6/.3/.3.
Once it is sufficiently composted, manure has no ‘barnyard smell.’

Peat Moss
Peat moss is a long-lasting, although expensive, source of organic matter. While best used in sandy soils, due to its substantial water retention capabilities, it is also worth considering as an occasional amendment in clay. Peat is hydrophobic, meaning it is very difficult to rewet once it dries out. Make sure it is thoroughly incorporated into your soil. It’s best to use peat in combination with other organic amendments.

While Canadian peat is a sustainably harvested, renewable resource, Colorado peat is not. It also has an unacceptably high pH. Most commercial sources no longer sell Colorado peat, but if you come across it, don’t use it.

Not Recommended Soil Amendment Materials

Sand
Sand is not a good amendment for clay soils. Any mixture less than 70% sand in 30% clay actually packs more densely that straight clay. This makes a readily compactable soil that isn’t fun to garden in. Add a bit of water and make your own adobe bricks.

Gypsum
Gypsum “clay buster” sold in garden centers is not useful in alkaline clay soils. It actually will worsen your soil structure.

Fresh Manure
Fresh animal manure (in addition to being fragrant) has too much ammonia to use near plants. It will dry out roots and cause burned edges on the leaves. It may also contain the bacteria E. coli, and should never be used in a vegetable garden. Compost it until it no longer smells like the barnyard; once decomposed it makes an excellent soil amendment for ornamental garden beds. Manure is also a good nitrogen source in the compost pile, to offset higher-carbon materials like dried leaves and plant stems. Don’t use human waste or pet wastes, as these can transmit diseases to humans. Cow, horse, rabbit, llama, and chicken manures are fine.

Fresh Wood Chips or Sawdust
Wood chips can take years to decompose, and wood needs a lot of additional nitrogen to balance its high carbon content. If wood chips or sawdust are decomposing in your garden soil, they will take nitrogen from the soil to the detriment of your plants. Of course, undecomposed materials can be used on top of the soil as mulch.

Whatever you use to amend your soil, be sure to mix in the new materials thoroughly. If amended soil is just spread over the existing soil, plants will not root into the clay underneath, and the plants will dry out in hot weather.
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Contributed by Nadine Salmons, Certified Colorado Gardener

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