Improving Poor Soils

by Don Janssen, UNL Extension Educator

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No amount of money invested in plant material and support systems can make up for poor soil. No matter how you cut it, expensive plants in poor soil is a bad investment. This seems to be a fairly common problem, particularly in some of the newer developments. When construction is complete, often there is not much good soil left.

Most plants grow best on well drained, well aerated, fertile topsoil. Plants do best with a nice, even moisture supply, but also require an open, well aerated soil for good root penetration and to allow the roots to breathe. A tight clay soil limits root growth because of the lack of air available in the soil, and simply by the resistance of the soil to root penetration. Resilience in plants, and their ability to withstand stress, is directly related to the size and health of their root system. It would be better if trees and shrubs just rolled over and died immediately in this situation – that would force us to investigate the problem. But, they struggle to survive for years, performing poorly, and disappointing the homeowner because they fail to live up to expectations.

The same types of problems occur with lawns. On heavy clay soils grass is just not as vigorous and thick as it should be. The poorer the soil the shallower the root system and consequently the less able to deal with stress – whether from drought or pests. When we get into a hot dry growing season, lawns on poor soil are the first to burn up or the first to turn brown due to feeding by chinch bugs or white grubs.

So what do you do if your property has very poor soil? When considering the cost of landscaping, keep in mind that you often have to spend as much or more on soil preparation as you spend on plant material. The way to improve clay soils is to add large volumes of organic matter such as compost, aged manure, peat moss, or similar materials. The organic matter is the glue that binds fine clay particles into irregular granules that have air and water holding spaces.

However, providing good soil involves much more than just digging a planting hole and filling it with a nice porous mix of good soil and organic matter. If you dig a hole in tight clay soil, amend it with the finest topsoil and organic matter available and put the plant in, you now have a plant in a bathtub with no drain. Some believe that to improve drainage you just add gravel to the bottom of the planting hole. This gives you a bathtub with gravel in the bottom – the water still has nowhere to go.

The key to good site preparation is to prepare the area where the roots can grow. This means width not depth – most roots will be in the top foot or less of soil. If you are lucky enough to have good well drained garden topsoil, just plant into it. All too often though, you have to deal with that tight clay subsoil and very little topsoil – in which case you need to provide a wide area where roots can grow and thrive. The best solution may be to incorporate organic matter into the top few inches of existing soil and also build a layer of good topsoil and humus mix on top of that, so that you have eight to twelve inches of good soil. Make the prepared area as wide as possible to allow plenty of room for root growth.

Improvements in soil physical structure, and also major improvements in pH and fertility, need to be made before trees, shrubs, or lawns are planted. If plants are healthy and vigorous to begin with, they are much more able to tolerate attacks by pests and those problems are much easier to deal with. On the other hand, plants that are weak to begin with due to poor soil conditions are much less able to fight off pests.