Don't till soil until it's ready

By ED PERRY

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Over the past few days, I've spoken to several people who want to plant bare-root trees or begin preparing soil for spring planting. However, in most cases, soils throughout the Stanislaus County area are still very wet from our heavy rains. It's really best for your garden's long-term health to resist the urge to work the soil when it is still wet. Whether you use a tiller or just a garden spade, working wet soil can badly compact it, and the negative effects can last for many years.

One of the most important things you do in your garden is maintaining or improving soil structure, which is necessary for ensuring drainage. You can easily turn a well-draining soil into a poorly draining soil by cultivating or tilling the soil when it is wet. Working in wet soil causes compaction, which occurs when the large air spaces or pores between soil particles are collapsed. Without large pore space, water penetration becomes very slow. The remaining smaller pores may fill slowly with water after an irrigation and drain even more slowly because water is held strongly by soil particle surfaces. Therefore, water movement to lower depths slows down, and little or no air space is left in the compacted soil. If deprived of air for only a few hours, the fine, absorbing roots of most plants will die. Plants will not be able to root deeply in soil that is dry beneath the compacted layer. Also, soil tilled too wet usually dries into hard clods, making preparation of a good seed bed difficult.

Sandy soils tend to dry fairly rapidly, allowing you to begin preparing your garden fairly early in the season. Even so, you should not work sandy soils when they are wet. If your soil contains even moderate amounts of clay, it is even more critical to wait for the right moisture content before tilling. Clay soils, because they are composed of smaller-sized particles, have a greater tendency to compact than sandy soils.

It's best to prevent compaction in the first place. To determine whether your garden's soil is dry enough to work, dig a trowel full of soil and squeeze it in your hand. Soil that crumbles through your fingers when squeezed is ready to garden. However, if the soil forms a muddy ball, give the soil a few days to dry, then sample again later.

Once you have determined that the soil is dry enough for working, be sure not to over till. Rototilling until the soil is pulverized into fine dust also damages soil structure. Till the soil just enough to break any surface compaction and to incorporate compost. Stop tilling when the soil is in a good seed bed condition, with the soil particles aggregated into small clusters. This condition will allow good air and water penetration.

Adding a good soil amendment, such as compost, will help improve soil structure. Wait for a dry, warm spell during winter to work compost into the soil. If the compost has been sitting on the soil surface as a mulch, it has been insulating the soil. As spring comes, the insulating effect will prevent the soil from warming up. As soon as the soil is dry enough, mix the compost into the soil to speed the warming process, which will make it possible to plant earlier.

Ed Perry, a farm adviser with the UC Cooperative Extension, can be contacted at 525-6800 or ejperry@ucdavis.edu.