Dealing with Compacted Soils

20160309_134005Last week I shared how to determine the make-up of your soil. This knowledge is helpful, but it doesn’t solve the problem of soil so hard, you can’t get a shovel into it. That’s what we’ll cover today.

Most often, soil that is rock-hard is mostly clay. Sand can get hard, too, but it’s much more forgiving. They don’t make pottery out of sand. So what do you do with your compacted clay? Here are some do’s and don’ts.

Don’t replace it. Our landscaper initially wanted to haul away all the nasty soil left by the builder and replace it with topsoil. I said no. There was no way he was going to remove and replace an 18 inch deep layer of soil, and that’s how much most plants need. Besides, replacing soil leaves a sharp disconnect between the layers, which acts as an impediment to both water and roots. It’s much better to improve what you already have.

Don’t add gypsum. There’s a widespread belief that adding gypsum to your clay soils will “break up the clay.” Unfortunately, this is simply not true. (I wish it was that easy! ) For one thing, gypsum is calcium sulfate. Colorado soils are already mostly calcium. Adding more won’t accomplish anything. The sulfur isn’t sufficient to lower your pH either.

This issue came up years ago during my master gardener training. Our instructor’s simple reply convinced everyone. He explained that calcium compounds (our soil) combined with gypsum yields Portland cement.

Don’t add sand. Again, this isn’t the answer. Adding sand will actually decrease the drainage of clay soils. Think grout. Adding sand only works when you get to the point where your soil is mostly sand. By this time, you would have done better to simply haul your soil away and replace it!

Add humus! The optimal humus content for garden soils is 5%. If your soil is rock hard, it’s highly likely that you don’t have that much. Spreading a 2 to 3 inch layer of compost (or peat moss, although that’s more expensive) over the ground, and tilling it in, is a good beginning. Add more every year until you reach 5%, and then add just enough to maintain that percentage. This is the goal for sandy soils as well.

Avoid walking on the soil. Foot traffic is a significant cause of compaction. Instead, create permanent paths or place stepping stones. If you must step where the plants are growing, place a board down first to distribute your weight over a larger area.

Use mulch. Mulch acts in the same way as that board, spreading your weight over the soil so it doesn’t cause as much of a footprint. It also keeps pounding rain from adding to the compaction.

Aerate lawns and around trees where foot traffic can’t be avoided. Be sure to use a large machine that pulls the plugs, and go over the area at least twice. Colorado State University extension recommends two aerations per year, spring and fall.

Avoid over-tilling. With rototillers, it’s tempting to till until you have an evenly fluffy dirt pile. But excessive tilling destroys soil structure. All those little pieces will just pack down denser than they were to begin with. Consider using a garden fork to incorporate your compost, and then avoid further tilling with mulches and permanent paths.

Never work clay while it’s wet. There’s a reason that it’s used to make dishes and bricks! If squeezing a lump of clay causes it to crumble, it’s time to dig.

And finally, here’s a bit of inspiration. Clay soil won’t succumb to even the best treatment during the first season. It takes time to achieve results. When we lived in California, my yard was comprised of adobe clay. Yes, the stuff they make bricks out of. It was unbelievable difficult to work with. Too wet, and it was slimy. Too dry and we had to use a steel pike to break it up. And that golden spot inbetween lasted less than a day as the soil dried rapidly.

We added whatever organic matter we could get. Mushroom compost (sold relatively cheaply in our area). Chicken manure. Rabbit droppings. Grass clippings. Shredded leaves. We built raised beds that never saw a footprint. I pulled any weeds by hand. I mulched. And amazingly enough, it worked. Long before the time we moved, nine years later, the only gardening tool I needed was a small trowel. The soil was deep, rich, and loose, and my plants thrived.

Don’t give up. There’s hope.


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