A Layman's Approach to Soils

Everything we do as horticulturists starts with our soils.
 
I want to emphasize I am not an expert in soils. Far from it. I?m a general horticulturist, a horticultural jack of all trades, so to speak. But I do want to share some of the information I?ve gathered and observations I?ve made about soils, water and roots.
 
To have horticultural success in a landscape, the best of scenarios is to have maximum effect with minimum effort. In other words, we want our plants and landscapes to thrive, without having to work our tushes off. Plants, through natural selection and evolution, have adapted to thrive most prolifically in their native habitat and climate. They like the soils from whence they come. It makes sense then that a plant will perform better the closer we come to recreating its native environment.
 
As horticulturists, we should look at landscapes as small but very complex eco-systems, composed of many elements affecting one another. Soils affect water retention and wetting patterns, root penetration and density, aerification levels, nutrient availability, and the vitality of micro-organism communities. Likewise, the vitality of micro-organism communities affect all of the other afore-mentioned. In fact, each of these broad categories affect one another in many ways. Sometimes, I feel a bit overwhelmed. Fortunately, nature is often forgiving. All we have to do is get the big picture items more or less straight, and we?ll achieve success in our horticultural endeavors. As we look at our landscapes in a ?big picture? mode, and view our individual plants as complete and complicated organisms, we begin to see those elements which contribute to our successes and failures. I do want to emphasize the word ?contribute?. Unless it is some radically detrimental force such as unexpectedly severe cold or drought, most factors merely influence plant health. If we can build and maintain our landscapes in such a way as to have most of those factors working for us, we are likely to achieve success.
 
Water moves through soil both vertically and horizontally. So do roots. So does air. The vitality of a plant?s root system and, of course, the whole plant, is dependant on the other two.
 
In natural settings, a tree?s root system will extend out to a distance of 1 1/2 to 4 times the diameter of it?s crown. I?m certain this is generally true of our other landscape plants. Therefore, it makes sense that we provide our plants with wide wetting patterns. Here in the desert southwest there has been a large scale conversion to drip irrigation. Unfortunately, as we learn how to best manage this new technology, we?ve made some mistakes. The most common one I see, (other than people running drip systems every day for ten minutes), is the use of inadequate numbers of emitters for larger landscape plants, especially trees. Large canopy trees are often introduced into landscapes with just 2 or 3 emitters placed close to the trunk. While such a system is adequate to get the tree started, the requirements of the tree quickly exceed the availability of water and root spread. The consequences for this are profound. As the canopy becomes more massive, the wind pressures exerted increase dramatically. For a tree to remain standing, it requires a strong root system physically anchored in the soil. The further the root system spreads, the greater the stability. If the emitters are placed only at the base of a tree, the roots have no reason to spread. They remain in a little pocket at the base of the tree, the winds come, and down goes our tree. Unfortunately, as the tree becomes larger and more valuable, the more likely this is to happen.
 
To prevent this, provide enough emitters to extend out at least to the trees mature canopy dimensions, in a star pattern of at least four rays, with the emitters set closely enough to have overlapping wetting patterns. The roots will grow through that continuous wet space, well away from the trunk, and provide stability. Remember though, any nearby plants with their own emitters contribute to the wetting pattern as long as they overlap into the trees wetting pattern. Therefore, take underplantings into account when determining emitter placement.
 
Another advantage when considering wide and deep wetting patterns is nutrient availability. It makes sense that the greater the amount of soil space a plants? root system occupies, the greater will be the bulk quantities of nutrients available. So, not only should we be watering wide, but deep as well. I encourage people to think of the run times of most drip irrigation systems in terms of hours. Soak deep and soak wide. But there?s another important part to this story. Soak infrequently. The root systems of our plants need air as much as they need water and we need to irrigate infrequently enough to allow air to repenetrate the soil. How infrequently depends on the soil?s air and water holding capacities, and the water requirements of the plant. Many plants in the wild often go for long periods without rain. Remembering the principle of wanting to recreate our plants? natural environments, we should attempt to wean our plants from frequent irrigation. Even plants which we think of as moderate or high water users often benefit greatly by a reduction in frequency, so long as we irrigate wide and deep. Roses come to mind. I know that most of the roses in our valley get watered every day, or every other, in the heat of our summers. In our rose garden, last summer we irrigated our roses every seventh day, down from every sixth the year before, and they performed wonderfully. They have enough emitters that to soak the entire bed, and we also use an organic mulch to help reduce moisture loss. This coming summer, I plan on watering them every eighth day, and I?m certain they?ll be fine. However, I?ll be keeping an eye on them. That is another key to successful irrigation. Slowly wean your plants off of frequent irrigation, but watch for drought stress. That?s why keeping good records of your irrigation schedules is important. You can revisit last years? schedule, and read any notes you made about plant performance. If you?re too ambitious in backing off an irrigation schedule, note the stress you?ve caused, and bump up the frequency. The following year, you see the note and schedule accordingly. Remember, as you reduce the frequency, do so gradually.
 
One final note on frequency of irrigation. Plants are usually lumped together in irrigation zones based on location, not on water requirements. If multiple species are being watered by the same valve, somewhere in that zone is an ?indicator plant?. The indicator plant requires the most frequent irrigation at a given time of year. If you?re backing off on frequency from the year before and you notice the Lantana?s have wilted a bit, you can safely determine that Lantana is your indicator plant for that time of year. In years following, to make sure you?re watering frequently enough, check your Lantanas. If they?re doing ok, the other plants are likely to be doing fine with that schedule. Remember, your indicator plant may change with the time of year, and to minimize risk, make changes at times when you?ll be able to observe results. If you go to far, it?s important that you observe the results and make corrections before any permanent damage is done.