The word “hardpan” is often loosely used to mean any dense layer in the soil. Because different types of dense layers have different characteristics, this work uses separate words for dense layers formed by different causes. They all have in common the fact that they are distinct layers in the soil, and permeable soil lies under them.
Hardpan is a cemented layer in the soil, usually from 2 to 4 feet under the surface, although erosion can wash away some of the soil that covers it, or even expose it on the surface. It is formed chemically, by lime, iron or silicate materials depositing at the same level and cementing the soil particles together. This cementing takes place very slowly. If you break up hardpan, it won’t form again during your lifetime.
Caliche is a lime-cemented hardpan found in the arid Southwest. Like other hardpans, it is impervious to water. It is white or light in color, and the soil over it is usually alkaline.
Claypan is a layer of clay soil. Claypan sometimes forms on top of hardpan as a separate layer. The clay may be hard when dry and softer when wet, but always impedes the flow of water, causing drainage problems.
Fragipan is a layer of dense, compact cemented silt and fine sand. Like other pans, it impedes the movement of water. Fragipan is hard when dry, but brittle and fragile when wet.
Plowpan is formed by the action of moldboard plows compressing the soil about 7 inches under the surface. The sole of the plow, which bears its weight, slides over and compresses the soil under it. Plowpan is found in areas that used to be farms. It is a dense, compacted layer a couple of inches thick.
Traffic pan is a term occasionally used for soil that has been compacted by foot traffic. For more information, see Compacted Soil .
There are two general approaches to gardening on hardpan. One is to break through the hardpan layer, allowing water and roots to penetrate it. The other is to leave it in place and garden on top of it.
Hardpan and caliche, the most difficult pans to break through, might require heavy equipment. Test its thickness and hardness by digging a hole with a shovel to the hardpan, then chipping through it with a crowbar, pick, or mattock. If it isn’t too hard or too thick, this method might be feasible. Break a hole through the hardpan near each tree or shrub to allow drainage. Put the soil back in the hole. A posthole digger or rented soil auger makes digging individual holes easier. For large jobs, hire a contractor with a posthole digger mounted on a tractor.
Farmers use heavy equipment to break up hardpan. If possible, hire a contractor to use a chisel plow to rip the hardpan. Chisel plows penetrate through the hard layer and pull it upwards, shattering it into pieces.
In areas where a chisel plow won’t work, consider hiring a backhoe. Even a small backhoe can break through hardpan quickly, digging several holes to allow drainage.
Softer pans, such as plowpan or claypan, are easier to break through. Either can be dug through with a shovel, perhaps with the help of a pick. Break up claypan or fragipan when they are wet.