Hardpan

How can digging both cause and correct hardpan?

Before we can answer that question, we need to know what hardpan is.

Hardpan stops roots in their tracks (steevo2005)

Hardpan is a layer of soil so dense that air, water, and roots can barely move through it, if at all. Think of it as a saucer under the teacup of life. Unlike compacted soil, which is a general condition of not enough macropores and micropores, hardpan is a distinct layer, usually found 4 to 40 inches below the surface, that reduces or halts drainage, roots, and gas exchanges altogether.​

Causes of hardpan

Plowing, digging, or rototilling to the same depth every year can compact the underlying soil so much that it becomes hardpan. In addition to digging and plowing, there are several other things that can cause hardpan to develop:

• heavy traffic, such as the construction equipment used to build your home
• heavy chemical use, which kills beneficial soil microorganisms
• extended drought and improper watering can also cause hardpan

Hardpan can also occur naturally when layers of silica, iron oxides, salt, or calcium carbonate fuse and bind soil particles together. Glaciers can cause hardpan, by compressing layers of those minerals.​ ​Soil pH and soil structure are major factors in the development of hardpan.

Acidic soils are far more likely to cause calcium and iron to form hardpan layers, than alkaline soil. Soil structure is another important factor. Clay particles are very small and already tend to become compacted. In clay soil, rain or irrigation followed by high temperatures can also create hardpan. However it occurs, plant roots don’t like it.

Do you have hardpan?

Poor drainage is the first sign of hardpan, followed by a general failure to thrive. If you have a soil tester, you can take a few 2 or 3 foot deep samples of your soil, to see if there is a hardened layer. If you have a post hole digger, you can basically do the same thing, with more soil and more effort. The important thing is to look at the layers of soil as you bring them up. If you reach a layer where all the plant roots start growing horizontally, you have hit hardpan.

Managing hardpan

Once a layer of hardpan has formed, it takes brute strength and proper soil amendments to correct the problem. Basically, you can either dig down into the hardpan layer, breaking it up, or add significant amounts of organic matter and let nature takes its course.

If you opt for the brute strength method, you will have to wait for the soil to be dry. [Digging wet soil is never a good idea.] You can use a digging fork, spade, or broadfork to break through the hardpan layer. As you do this, you will want to make sure that the subsoil is not brought up and mixed with the topsoil.

To prevent compounding the problem by even more digging, it is important to add significant amounts of organic matter, in the form of aged compost or manure, or peat, to improve the soil structure, as you dig. Earthworms and other soil dwelling creatures will, over time, break through a thin layer of hardpan, but only if enough organic material is added to the soil.

Treating acidic soil with lime can help break up the chemical bonds that hold a hardpan layer in place. This is not an option in regions with alkaline soil. You can also top dress an area repeatedly with organic matter to treat hardpan, but it may take years before you see results.