Soil layers can restrict drainage

by Ed Perry

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Though it's midwinter, it's not too early to start thinking about your spring garden. This is especially true if you need to improve your soil by correcting drainage problems or by adding amendments to improve soil texture. Most soil problems are easier to correct before than after you've planted your garden. It may be impossible to correct some soil problems after planting trees and other perennial plants.

While many physical and chemical soil problems can affect plant growth, a common one in the Northern San Joaquin valley is soil layering. Soil layers may exist naturally or result from landscape maintenance activities. Either way, soil layers can make it difficult to grow plants, whether shallow-rooted annuals or deep-rooted perennials. Layering can result in poor growth caused by too little or too much water, root diseases, poor root development or a combination of these.

You may be able to correct problems relating to soil layering before you plant, depending on the type of layer. Hardpan, which is a term often mistakenly used for any soil drainage problem, is an impervious layer that occurs naturally in some very old soils. Hardpans exist at various depths and have a different texture than the surface soils. Claypan is another type of natural, dense layer. Pan layers may be from a few inches to several feet thick, and just about as hard as concrete; they are an almost solid barrier to root growth and water movement.

Another type of layering may exist in soils that were deposited by moving water, called alluvial soils. Alluvial soils often have layers of different textures, like sand, silt or clay, with sharp boundaries between layers. Water saturates the soil at the bottom of each of the layers, forming a temporary "perched" water table. This saturated condition may last a week or more after irrigation, causing serious root damage.

Some "unnatural" soil layering may result from landscape maintenance activities. Land grading moves soil of one texture on top of another soil, creating drainage problems. This often happens when landscapes are changed, and becomes a serious problem for existing trees and shrubs. Or, mechanical compaction can be caused by running heavy equipment over the soil, especially when the soil is wet. Over time, even foot traffic or light equipment can cause soil compaction in landscape soils, especially in turf-grass areas. Turf-grass areas may need periodic aerification to break surface compaction and allow water and air penetration.

It's important to know that chemicals or soil amendments will not correct problems related to hardpans, claypans and other layered soils. You must mechanically modify or eliminate soil layers if you expect healthy, long-lived landscape. You need to physically break the hardpan, claypan and other layers before planting, as it's almost impossible to modify the soil around established trees and shrubs. If the hardpan or other layer is well below the potential root depth of the plants you plan to grow, you may not need to physically break the layer. For example, you should be able to grow turf grass and many annual plants if the layers are below 24 to 36 inches. However, you will need to water very carefully to prevent a perched water table.

If you need to physically break a hardpan layer, or modify your soil in any way, be sure to wait until the soil is relatively dry. By working your soil when it's wet, you can cause physical compaction and make drainage even worse.

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