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-Pat Welsh Gardening

The important thing to impress on the kids is that in a garden it would be impossible to add an equal amount of sand to clay. In a practical example, years ago I knew of a garden in Fallbrook California where the soil was heavy clay. Instead of adding organics to improve his soil, the gardener added wheelbarrows full of sand to the clay because an English garden book gave that advice. He kept adding more sand every year to a long flower bed filled with perennials. Eventually the soil was so ruined that drainage was nil and many plants died from root rot.

To answer your question: There are a couple of ways that organic materials can be added to existing lawns. One way is to rent an aereating machine that goes all over the lawn and makes round holes in it. The next step is to rake up and throw away or compost the plugs that the machine pulls out of the lawn. After that spread ground bark onto the lawn and rake it into the holes. Follow up by fertilizing and watering the lawn. The best time to do this job is fall. September or October would be ideal. This will help increase drainage.

Please note that while answering the first gardener who wrote to me on this subject I recommended adding soluble gypsum to help break up clay soil. When clay soil fails to drain due to its alkalinity, applying gypsum according to package directions can aid drainage. One can mix it with the ground bark recommended above and rake it into the holes along with the bark or apply it separately once or twice a year on top of the ground and water it in.

Another idea for adding organics to lawns is to top the lawn with dry cow manure in fall or early spring. Back in the years when I had a lawn I did this job every fall but my lawn was growing on sandy soil, so it was okay. First I would cut the lawn short, then top with manure and follow up with water and the grass soon bounced back greener than before. The problem with using this technique on clay soil is that bagged cow manure is often salty and clay soil can retain salts. A better way is to use any fine-textured mulch recommended for topping lawns. By topping the lawn annually or bi-annually in spring and fall you can gradually add to the organic structure of the soil under a lawn, but not as much as if you aereated first and then raked the organics into the holes.

However, I would like to give them something to think about as positive alternatives for soil enrichment that can be done in their yards. Many of us have yards that are likely placed over thick, compacted layers of clay. (I can tell by the runoff when the sprinklers run in our neighborhood.) How do we add organics to the yard without tearing up the grass that is there?

Clay soil is nutritious. Plants will grow in it, but you are right a raised bed can be a great help when first planting. Luckily it does not need to be very high. For a deciduous fruit tree you could build a raised bed that measures 5×5 and is at least 4 inches high and fill it with a good grade of top soil.
What is important here however is that you must dig up the clay beneath the raised bed to about a foot deep and mix gypsum into that and also mix some of your top soil into the top layer of the native soil below the raised bed. This is so you do not create a hard horizon between two soils which will stop roots from penetrating.

If you create a hard horizon between friable soil above and hard clay below you are making hardpan, which happens when builders in housing schemes throw a layer of top soil on top of bulldozed ground. Plant roots tend to stay up in the top layer and never penetrate the lower layer, and then the hard lower layer stops drainage. This is a genuine problem for many gardeners in new homes.

Despite having explained the system above which will help prevent rotting roots in clay soil by use of a raised bed, my belief is that you can garden in clay soil and plant straight into it, just as it is since I have seen it done many times. The surprising thing is once you begin digging into clay soil and mulching the top of it, eventually the very act of gardening in it and the roots getting into it will help break it up and plants will grow. Clay is minerally rich and plants will often thrive in it better than poor sandy soils that drain well but do not retain nutrients.
The wrong thing to do is to dig planting holes and fill them with organic soil amendment since that creates pockets of soggy ground that fill up with water and rot roots. Also roots will think they are in a container and go around and around inside the amended plant hole and never get out into the surrounding soil.

Many scientific experiments have been done to show that the best way to deal with clay soil is to plant directly into the native soil, though plant a little high, and then continually mulch the top of it. The roots of plants have to get out there eventually so the sooner the better. Rough up the sides of planting holes, instead of making a smooth hard edge. Even clean unsalty horse manure can help break up clay when laid on top like mulch before the rains, but I strongly emphasize must not be salty. Manure brings earthworms and they also help break up the ground and make it drain. (When using horse manure, be sure your tetanus booster is up to date.)

One important factor is that if the clay is compacted due to alkalinity, adding gypsum can aid in breaking it up and making it drain, but you should add the gypsum every two years and a half-coffee can full of it dug into the bottom of every planting hole. Gypsum will not correct drainage problems caused by compaction from walking on the ground or heavy equipment but it will greatly help soil to drain when the compaction is caused by alkalinity.

I have known a few gardeners who have hauled away truckloads of pure sandy or clay soil in gardens built on sand or clay and replaced with top soil but this is not necessary and is very expensive. (Again, one must be sure not to create a hardpan layer beneath the top soil. More details on all this are in my book.)