Arizona Soil

 

Soil is formed when rock is broken down by climate, organisms, and vegetation over a period of time. It is made up of weathered rock fragments and decaying remains of plants and animals (organic matter). It also contains varying amounts of air, water, and micro-organisms. It furnishes mechanical support and nutrients for growing plants. Most Arizona soils developed under desert or scrub vegetation. However, because of the wide variation in elevation and climatic conditions found throughout the state there are wide differences in the types of soil profiles found. Interesting, only tropical soils are not found within Arizona. Important physical properties of a soil is color, texture, structure, drainage, depth, and surface features. These properties can vary greatly in one's own backyard. The color of the soil can be a good indicator of drainage. Red/brown soils usually indicate good drainage while yellow soils have medium drainage and gray soils usually have poor drainage.

Here in the High Desert, we have alkaline soils. The relative acidity or alkalinity of soils is expressed as pH, which is a measure of the relative number of free hydrogen ions. The pH scale goes from 1 through 14, with 1 being the most acid and 14 being most alkaline, and 7 being neutral. Each number represents a tenfold change in acidity or alkalinity. For example, a soil with the pH of 9 is ten times more alkaline than one with a pH of 8. Generally, soils are acidic where it receives more than 20 inches of rainfall in a year. Areas with less than 20 inches of rainfall in a year, such as the High Desert where average rainfall is between 12-16 inches, the soils are alkaline. This is due to the calcium carbonate content of the soil which is not leached through the soil because of our limited rainfall.

Why is this important to the gardener?..... Because we need to learn to work with the soil on our property. It is much easier and cost effective to work with Mother Nature than against her. Gardeners in Cochise County can pursue two possible strategies. The first is to attempt to alter the pH of the soil by the use of amendments so they can grow acid-loving plants. To illustrate what amending the soil would take lets assume that a cubic foot of soil weighs 100 pounds and has a 1 percent calcium carbonate content. This means that there would be one pound of calcium carbonate present. It takes one pound of sulfuric acid or its equivalent to neutralize one pound of calcium carbonate. Therefore, to decrease soil pH from alkaline to neutral or acid it would require equal mixing of one pound or equivalent of sulfuric acid throughout the cubic foot of soil. If the soil had 10 percent of calcium carbonate then 10 pounds of sulfuric acid or its equivalent would be needed to neutralize the soil! The second choice is to go native and grow plants that are adapted to our soils. Not only do these plants grow well in our High Desert soils but they are adept to cope with harsh climate conditions of heat, wind, and drought. There are also many non-native adapted plants which do well in Maricopa county.

Although attempts to lower the pH of soils can be successful in the short term, they require repeated additions of amendments to maintain the change in pH. Ultimately the gardener will lose to Nature. The best strategy is to leave the soil pH alone and plant those plants that are adapted to the soils of our area.

What's Gardening Like In The low desert?

"Different" is the first word that comes to mind-followed by "frustrating" and "rewarding" in about equal measure. Almost everything about gardening in the low desert is different, from the soil to the sunlight to the planting dates to what you plant. Some differences are good; some aren't.

Elsewhere a rubber tree is an indoor plant; the one outside my window is house-high and trying to take over my front walk. Bougainvillea, hibiscus and jacaranda all decorate front yards on my block. I have several thriving pepper plants that are approaching the five-year mark; I harvest my first peppers in March, about the time I set out my last tomato plants. My grape vines and peach trees set fruit in mid-April. If I'm lucky I may harvest my own bananas in a couple of years.

On the other hand, seed catalogs are terminally disappointing. I've learned to skip from the colorful pictures to the U.S.D.A. hardiness zones and check those first. Then I scan the copy for clues as to why a wonderful new find won't grow here. Many beautiful and useful plants -- among them cherries, blueberries, tulips, daffodils and peonies -- simply cannot take the heat and dryness. Even many of the subtropicals that enliven gardens in other Zone 9 areas often won't survive here.

When it comes to plants I have learned never to say "never." Someone somewhere always manages to grow the most unlikely things in the desert. Even acid-loving, cool-temperature plants have been successfully nurtured by gardeners of my acquaintance. While I admire their fortitude, I think they're working way too hard. It is easier and more satisfactory to admit we're different and go from there.

Most of our notions about gardening arose in England or the eastern United States. The farther you move away from those conditions, the more you have to adapt. Scheduling vegetable crops, for example, can be just plain backwards in the desert. Winter is the only time to grow lettuce and other cold-weather crops. So-called warm-weather crops -- tomatoes, eggplant, peppers -- stop setting fruit when the daytime temperature climbs over 100 degrees, as it does for five months out of the year here in Phoenix, Arizona. Peppers, tomatoes and eggplants are set out in February or March in semi-shaded locations and nursed through the summer. Spring and fall are the big fruits hanging on through the summer. Even melons, the ultimate heat lovers, must be handled delicately. If you don't shade the developing fruit it will scald under our blazing summer sun.

Pests are a good-news, bad-news proposition. Many of the diseases and insects that plants face in other areas simply don't exist here. However, like everything else in the desert, the pests we do have are tough and adaptable.

Currently my nemesis is the whitefly, primarily a houseplant or greenhouse pest elsewhere. Here it is a common summer pest of just about everything outdoors. In Phoenix we've recently encountered a race of whiteflies that's immune to most pesticides. Soap sprays help, but only somewhat.

Various caterpillars also flourish in the long-season climate. In addition to cabbage loopers and tomato hornworms, we have some indigenous types. For example, the aptly named grape leaf skeletonizer hatches out of an innocuous-looking egg cluster on the bottom of a grape leaf. The caterpillars march across the leaf, eating it down to a skeleton, then divide and conquer the rest of the leaves. They can strip a grapevine bare of leaves in just a few days. Fortunately Bacillus thuringiensis works as well in the desert as anywhere else.

Then there are the exotics, such as the 4-inch Palo Verde beetle, which bores holes into trees to lay eggs that hatch into grubs. Those grubs can grow up to 6 inches long and an inch thick, equipped with jaws that can chew through solid wood. You don't even want to know what they can do to tree roots. Wise gardeners check the base of trees for the characteristic holes.

Low desert soils may have a high clay content, low organic material content (less than 1%) and a high pH (meaning the soil is very alkaline). The high clay content helps the soil to retain water and nutrients. This sounds great, but that same clay also compacts easily, can be void of vital oxygen, and can make it difficult to dig planting holes. At the other end of the spectrum, you may have sandy soils, which like clay soils have very contain little organic matter and do not retain moisture. Soils are nearly impossible to change. The likelihood you will be able change your soil texture is pretty slim. The best plan when designing your landscape is to choose plants that have evolved in the desert and are adapted to our soils. Trying to grow a plant that prefers the loose, acidic soils found back east will only end in frustration for you and death for the plant.   
 
Organic soil amendments can improve the nutrient content, the water holding capacity and improve water penetration. Adding organic material to the backfill of planting holes was long been believed to help establish new plants. This is no longer recommended. Incorporating organic material into our soils can actually have a detrimental effect on many desert plants. Organic material is best used on the soil surface as a mulch both as a temperature and moisture regulator, and to reduce weed growth.  The mulch material should be spread around plants, about two-inches deep, out to the drip line.  Be careful not to put the organic material directly against the trunk or stems of the plant.
 
Organic material comes in many forms:

Compost – can be made at home from vegetable waste or leaves or purchased at  a nursery and has a low nutrient content but improves water holding capacity.
 
Manure – use manure only from plant eating animals such as cows, sheep, horses, rabbits and chickens.  It needs to be well aged (six months to a year) or it could damage your plants’ roots with its high salt content.
 
Leaves – best if composted first, but if you have leave drop in your yard, consider leaving it under your plants rather than raking it up. Leaves contain nutrients that are important for a plant’s health.
 
Bark Products – you can obtain this material from your local nursery or garden center.  The ground bark pieces eventually interlock and will not be disturbed by the wind.  Bark could be used as a total landscape cover in place of crushed granite.
 
A hardpan soil layer causes gardeners much grief. Hardpan is created when builders spread excavated subsoil over the soil surface and repeatedly drive heavy equipment over it. A hardpan layer close to the surface may be broken up with these steps:
 
1. Till the soil to a depth of 1 foot or more.
 
2. If tilling is not possible, drill through the hardpan with a soil auger to the porous soil below.
 
3. A hardpan layer may require the installation of a French drain and/or water chimneys.
 
4. Use raised planting beds and in fill with topsoil.
 
You may hear the term ‘caliche’ used when discussing soils.  Caliche (Ca – leach –ee) is layer of soil in which the soil particles are cemented together by calcium carbonate (CaCO3). These concrete-like pieces range in size from one inch to several feet across. Layers of caliche can be loose or may be found in solid formations.  The only time caliche is a problem for plants is when the layers of material doesn’t allow water to drain through it.  Aside from removing the pieces of caliche, there is no practical way to eliminate it from our soils.

Soil Problem-solving Guide

The table below lists common soil problems. Each problem includes indicators to test for the problem, and practices that help or hinder. Use the links in the table to go to the specific information about practices and indicators.