How to improve clay soil

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Last Updated: 
February 19, 2003

CORVALLIS - When you walk about your yard on a wet day, do your shoes stick in the mud? Could you make ceramic pots out of the soil in your garden? Odds are you have clay soil, one of the biggest challenges to the home gardener.

Finely-textured clay soils are difficult to work up and develop into a good seed bed. If the clay is dry, it tends to be very hard and lumpy. If it is wet, it tends to be very sticky and difficult to manipulate.

The best way to improve clay soils is to mix organic materials thoroughly with existing soil, explained Ann Marie VanderZanden, Master Gardener coordinator for the Oregon State University Extension Service.

Bark, sawdust, manure, leaf mold, compost and peat moss are among the organic amendments commonly used to improve clay soil. Two or three inches of organic materials should be spread and rototilled, forked or dug into the top six or seven inches of your garden beds.

Sand can help make clay soils more workable, but it should be added only after the addition of organic matter, or a cement-like mixture may result. Two inches of sand along with two inches of organic matter will further improve the texture of clay soils.

When a large amount of organic material is added to the soil, microorganisms multiply rapidly. Since they construct their bodies from the same nutrients that plants use, soil nutrients can be relatively unavailable for a time after an addition of manure or compost. This condition may persist until the organic material is broken down and nutrients are released.

To overcome the temporary lack of nutrients, VanderZanden suggests adding organic material to the soil in the autumn, or smaller amounts at a time in the spring, when warm weather will hasten break down. You can also sprinkle in some fertilizer, such as ammonium sulfate at the time of tilling to give an immediate source of nutrients.

The regular addition of organic material can raise the soil nutrient level to a point where the need for synthetic fertilizers may be greatly reduced or eliminated, explained VanderZanden.

"Fertile soil with good tilth does not come about with a single or even several additions of organic material, but from a consistent soil-building program," she said.

Organic matter in soil serves as food for earthworms, insects, bacteria and fungi - they transform it to soil nutrients and humus. Through this decomposition process, materials are made available as foods to growing plants. In finely-textured clay soils, organic material creates aggregates of the soil particles, improving drainage and making it easier to work.

Author: Carol Savonen
Source: Ann Marie VanderZanden