Too Much Clay
One of the most common problems with the structure of soil in Tucson is excessive clay which locks in water and nutrients. Native plants are adapted to this soil but most other plants we grow in our gardens are not.
One possible solution is to mix sand in with the clay but because a little clay goes a long way, it would take an awful lot of sand to achieve a mixture suitable for non-native plants. This method runs the risk of not adding enough sand to prevent the clay from acting as a mortar causing the mix to form a sort of concrete.
For small areas, it could be practical to remove all of the soil and replace it with a better mixture of sand, silt, and clay. I wanted to plant a small square of lawn in my backyard so that the dogs would have a cool soft place to lay during warm weather. Unfortunately, the soil in my backyard consists of about 90% clay and because I need low maintenance, I decided to remove about 2 feet of the clay soil and replace it with sandy soil which I amended with organic materials. The lawn is growing quite well right up to the edge of the sandy soil but the job was very labor intensive.
Care should be taken when acquiring replacement soil to avoid soil that may be contaminated with toxic pollutants. One option if you have good soil in part of your yard is to move some of that soil to another area.
Probably the most common treatment for clay soil is to amend it with organic materials. The organic particles help gather the clay into clunks or aggragates and make space for air, water, and plant roots. This technique is less expensive than those previously mentioned but unfortunately the organic material decomposes over time and must be replaced which may be practical for a bed of annuals but less practical for large bushes and trees where the selection of native varieties may be a more practical alternative.
Some organic materials that are commonly used are:
- peat moss
- bark
- compost
- manure (provides a small amount of nutrients but steer manure may be very salty)
- materials from chipping and shredding trees
- sawdust
- straw (beware of viable seeds)
Microbes will decompose organic matter providing nutrients to the plants. This healthy aerobic decomposition requires water, air, heat, and of course the microbes. Between plantings of annuals, fresh organic matter should be spread on top of the soil then the soil turned under to mix in the organic matter and loosen the soil. If salts have wicked up to the surface, that thin layer can be removed first.
Decaying organic matter will take nitrogen from the soil, so a high nitrogen fertilizer may be needed when adding organic materials to improve the soil structure.