The foundation of any happy garden is the soil. In fact, some might argue that a gardener’s job isn’t to grow plants but to cultivate healthy soil. In our desert, we are lucky to have plenty of sunshine, but there are some specific challenges to making our soils support the plants we want to grow.
The structure of soil is made up of minerals, water, air and organic matter. On average, the minerals are about 45% of the soil composition, the organic matter about 5%, and air and water about 25% each. In desert soils, the organic matter portion may be 1% or even less. This is the first of three factors that makes desert soils difficult for non-native plants.
Another property of desert soils is their high pH, which refers to the concentration of hydrogen ions (H+). Low pH indicates a high number of hydrogen ions (and thus acidic conditions) while high pH numbers indicate low numbers of H+ ions and alkaline conditions. Soil pH has an affect on the types of mineral nutrients that form in the soil and whether those nutrients are in a form that can be processed by plants and by the tiny organisms present in the soil — soil microbes.
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Our desert soils are alkaline, chiefly due to the fact that we do not have much rain (rain makes soils more acidic). This low rainfall also leads to our soils building up calcium, magnesium, potassium and nitrogen. The term alkaline just means that the soil pH is above 7. However, some gardening references mistakenly confound this with soils which contain an unhealthy amount of salts, which are called salt-affected soils. Native desert soils may have both issues, as can our garden soils.
The third challenging characteristic of desert soils is the caliche hardpan. This is a layer of calcium carbonate that forms in some soils. It can be as thick as several inches and concrete-like in its hardness. Whether you have it in your own garden, and how deep, depends on a number of factors, such as soil type, previous irrigation, prior plants, and the like. The caliche makes our lives difficult as gardeners because it can act as a barrier to plant roots and to water drainage. It can also make the surrounding soil more alkaline than it is already. Thus, for larger plants, ones that need well-draining soils, or ones that prefer more neutral soil pH, you will need to remove the caliche before planting.
Caliche is found as a hard layer in our desert soils. It can be a few inches to over a foot thick and can be found at different depths.
Testing your garden soil
Your garden soil can vary greatly, depending on how it’s been treated in the past, where it came from, what types of plants have grown in it before, and various other factors. The best way to determine the type of soil you have and its properties is to do soil tests. Soils are primarily classified according to their textures, which depends on the sizes of the particles, so this is the first thing to determine. Sandy soils have large particle sizes, clay soils have very tiny particles, and silty soils are in between. The soil’s texture can be graphed out on a soil texture triangle, with the ideal garden soil being somewhere in the middle of the triangle with roughly equal components of all three soil particles, along with at least 5% organic matter.
You can do a quick-and-dirty soil texture test just by putting some soil into a jar of water, shaking it up, and letting the soil settle out over 24-48 hours. Sand, with the coarsest texture, will settle out at the bottom, followed by silt and clay, with organic matter floating at the top. By measuring the thickness of each component you can figure out the type of soil you have using the soil texture triangle. You can also figure out the texture of your soil using your hands. This helpful website describes how to do both the jar test and the ball and ribbon tests to figure out your soil texture.
You may also want to do a more formal soil test, where you send samples of your garden soil to a lab. The soil tests check your soil’s pH, levels of nutrients such as nitrogen, potassium, and phosphorus, and can even test for heavy metals such as lead and arsenic. The latter is particularly important if you’re planning to grow edibles in your garden soil, as some plants can take up heavy metals into their structures. In an urban environment, it’s better to be safe than sorry, since you don’t know what your garden’s soil may have been exposed to. As an example, if you live in an older house, there may be lead paint fragments around the structure; this lead could then find its way into your garden veggies.
To determine whether you have caliche, the best thing to do is dig a test hole (or several). If you come across a hard white layer, you will need to get out your pickaxe (or jackhammer). You may not need to remove the whole layer — it depends in large part on whether you’re planting a native plant and how quickly the water drains out of the hole you’ve dug (see my article on how to plant a tree). I’ve come across very thick (8-9 inch) layers of caliche in my yard, but I remove as much as I can and test the drainage. If it’s OK, then I go ahead and plant, since I am putting in native or desert-adapted trees.
Our native soils tend to have variations in pH, organic matter, and other characteristics, depending on where you look. In a natural desert landscape, plants grow far apart, and soil beneath a mesquite tree, for example, will have more organic matter than bare soil. It will also have more nitrogen and the soil will be more fertile and less alkaline than the bare spots. In our gardens, we tend to grow plants a lot closer together than they grow in the wild. The mineral content of our soils is generous, but the minerals are not necessarily in forms that plant roots can absorb. To improve the fertility of our soils and to grow plants in a garden setting, you will probably need to add organic matter. You can use compost, or check out this article about organic fertilizers.
Helpful resources
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Do you have any gardening topics you'd like to see covered in the Tucson Garden Guide? Email me at dheusinkveld@tucson.com with your suggestions and questions. Thanks for reading!
