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Improve Your Garden Soil this Fall

By Robert Cox, Colorado State University Cooperative Extension Agent, Horticulture

All things are not created equal.

Colorado, for instance, is no Iowa when it comes to topsoil. That's one reason gardening here is filled with challenges that result in a variety of plant problems. But, what nature didn't provide, mankind can, at least, improve upon. This fall is a good time to begin. Start improving your soil now by adding organic matter. You can till or spade in leaves, cornstalks and other plant residues.

Consider the curbside garbage that goes out each week. Does it contain recyclable materials such as pine needles and grass clippings? If so, incorporate them at a depth of 6 to 12 inches into your soil.

What will you get in return? A soil with more nutrients that also favors root growth. In addition, you'll be relieving already-clogged landfills.

An ideal soil for plant growth contains about five percent organic matter, but most existing Front Range soils contain two percent or less. A three-inch layer of material, however, is about all that a rototiller can mix into the soil at one time. For best results, shred the organic matter and wet the soil before tilling.

Here are some additional considerations for improving your soil:

  • It is essential to improve soils before seeding or sodding a lawn. Unimproved soil is a major cause of poor lawn performance or failure. Annual flower beds and vegetable gardens also need continued amendment and will become fertile and productive over the years.

  • Earthworms can help improve soils, but they prefer soils with plenty of organic matter. You'll notice earthworms becoming more prevalent in organically rich soils. When introduced into poor soil, they may die out.

  • Adding sand to a clay soil might seem a simple method of breaking up the clay. Clay and sand, however, make a nice adobe brick! Organic matter, rather than sand, is much more beneficial to clay soil.

What types of organic matter can you add to your soil?

  • Most fall leaves are good additions, although some argue that cottonwood leaves are too alkaline to use in our alkaline soils. Cottonwood leaves are organic, however, and when broken down, humic acids result. The advantages of cottonwood leaves, therefore, outweigh any disadvantages.

  • Hay and straw should be free of weed seeds.

  • Weeds can be turned into the soil, if they don't have seeds on them.

  • Use crop residues such as cornstalks, tomato vines, and squash plants. These can be chopped up and turned under the soil or composted. Don't, however, use diseased plants. Dispose of them.

  • Pine needles make a good addition to the soil, despite their acidity. Front Range alkaline soils resist change but several years of heavy pine needle applications would create a somewhat more acid soil.

  • Grass clippings are best left on the lawn, but turning them into the soil or using them as mulch is strongly preferred over putting them out as trash.

  • Kitchen wastes, such as potato and banana peels, apple cores, lettuce leaves and grapefruit rinds benefit the soil or the compost pile. Coffee grounds are acceptable. Egg shells can be turned into the soil, although they aren't needed to add calcium to our already-calcium rich soils, as some suggest. Don't use meat scraps in compost.

  • Manures are a good addition if they are well-aged and relatively free of weed seeds. Feedlot manures may contain excessive salt levels. Manures contribute some plant nutrients, but their primary benefit is improving soil texture. Stable litter, often a mix of manure and sawdust or straw, also works well.

  • Sawdust and wood shavings can be slow to break down in the soil, but both are good amendments. To speed decomposition, just add some nitrogen fertilizer. Never use walnut sawdust or shavings; walnut contains a compound that is toxic to other plants.

  • Wood ashes have been promoted as a good source of potassium. They aren't recommended, however, as Colorado soils are potassium-rich. Wood ash also is quite alkaline.

  • Sphagnum peat is recommended, because it is acidic and therefore helps alkaline soils. It, however, is expensive. Hypnum and sedge peats are of less value as soil amendments.

  • Sewage sludge is a good soil amendment, but may contain high levels of heavy metals such as lead and cadmium. Industrial flow into municipal sewage systems often contains these heavy metals. Sewage sludge often is used on golf courses. At home, it's safest to use it on lawns, flower gardens and other areas not used for growing edible produce.

Photo: Judy Sedbrook

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Date last revised: 01/05/2010