Every Landscape Architect Should Know This about Desert Soils

Desert soils have characteristics which make them unique among all the other types of soils. These characteristics encompass its chemistry, structure, biological activity and consequently how plants respond after planting. Desert soils are stable under a desert climate. Once manipulated they are no longer the same and impact management decisions long after landscapes have been installed.

Surface of typical sandy loam soil in the eastern Mojave Desert of Las Vegas, Nevada, with no organics in it

Rainfall and Desert Soils

Desert soils are characterized by the desert climate where they were formed and the rocks or geology that decomposed to form them. The dominant climate feature of deserts that impact soils the most is rainfall. Not total rainfall, but effective rainfall.

What’s the difference? Total rainfall is the amount of water that falls from the sky. This is measured using rain gauges. We hear it on the news. “Total rainfall yesterday was one quarter of an inch.” But that is not effective rainfall.

Effective rainfall is the amount of rain that actually enters the soil, stays in storage and supports plant growth. This can be far less than total rainfall. If total rainfall comes down slowly then a higher percentage is “effective”. If rain falls in a torrent, much of it runs off the surface and therefore a much smaller percentage is “effective”.

For deserts in general I have heard the magic number “40%” tossed around to convert total to effective rainfall. Personally, that sounds high to me for the Mojave Desert because the soils are much different than other deserts in North America. The primary reason they are different? The amount of plant material present. Why is there a different amount of plant material? Rainfall. Give the Mojave Desert more rainfall and the soils change.

Amount of Rainfall Dictates Number, Size and Type of Plants

The amount of rainfall dictates the size, number and the type of plants found in deserts. For example, compare the size, quantity and type of plants growing in the “lush” Sonoran Desert (with an average of ten plus inches of rainfall) versus the more barren Mojave Desert (four plus inches of rainfall). Most parts of the Sonoran Desert receives 150% or more water than the Mojave. More rainfall supports a greater number of plants and these plants tend to be larger.

Eastern Mojave Desert near Las Vegas, Nevada

By adding more water to desert soils through irrigation, the size, number and type of plants will or can be increased… artificially. Little else needs to be done to the soil than add water IF plants which originate from deserts are used. Let’s call these “desert plants”.

Desert Plants Better "Tolerate" Desert Soils

This doesn’t mean that desert plants don’t appreciate soil improvement but desert plants are better suited in “tolerating” the unimproved soil conditions of a raw desert soil. When propagating prickly pear cactus (nopal cactus) from pads in the Sonoran Desert of Mexico it is a common practice to amend the desert soil with manure at the time of planting. In a demonstration of growing these cacti from pads in the Mojave Desert with and without soil amendments I have witnessed a substantial improvement in growth when compost is added to the soil rather than applied to the surface after planting.

Desert Soils Change When Irrigated

When “effective rainfall” is increased artificially, desert soils change chemically, biologically and physically. Soils that were relatively unchanged for thousands of years, stabilized at around 2 inches of effective rainfall, now receive 50, 60 even 90 inches of effective rainfall. It’s silly to think nothing will change. As the saying goes, “Somethin’s gotta give.”

Unless a footer is constructed well, desert soils "settle" when irrigated as they change

Changes to the soil occur rapidly as a result of this much “effective rainfall”. Minerals in the soil that were stable, like gypsum or limestone, begin to solubilize quickly. Chemical changes occur. The pH of the soil begins to drop as plants grow and plant roots pump out carbon dioxide into the soil which dissolves in water to produce carbonic acid.

Roots of plants constantly die and regenerate providing food for microorganisms. As these microorganisms begin to colonize these “enriched” soils, they pump more acids into the soil. Changes in soil pH might be faster if a unit change was linear. Changes in pH are not linear. They are logarithmic. A change of one pH unit is similar to a one unit change in seismic activity on the Richter scale. A one-unit change equals a factor of 10.

Little Rainfall = High Salts

Desert soils commonly have an abundance of salts because of low rainfall. Salts are removed from soils by flushing them with water. Salinity of a soil drops as elevated amounts of “effective rainfall” wash these salts past the root zone of plants.

Salts move from the soil in the irrigation water as this water dissolves salts, wicks to new locations and evaporates

Artificial Desert Rainforest

Just as the “effective rainfall” of an irrigated urban landscape is not natural, the number and size of landscape plants installed in them is not “natural” or sustainable without this irrigation. Other “inputs” are needed to sustain this “artificial desert rainforest”. The type of inputs needed are dictated by the type of plants selected for a desert landscape. One of my favorite sayings is, “The further our landscapes drift from a true desert environment, the more time, energy and money is needed to sustain them.”

Mass Planting and Irrigation Create Fertilizer Demands

We know there are 16 or 17 nutrients that plants absolutely need to stay alive. Some of these like nitrogen, phosphorus, potassium, calcium, magnesium and sulfur are needed in large amounts. Others, considered minor elements, are needed in much smaller amounts. If the solubilized minerals in the soil don’t release nutrients fast enough to keep plants healthy, then additional fertilizers must be supplied to make up the difference.

Even though a soil can be rich in nutrients, plant demand for some nutrients may exceed the soils ability to supply them or supply them in the proper form. Some examples are the extra calcium needed by intensively grown turfgrass on sand based greens, calcium deficiency of apples and pears grown on soils full of lime (calcium) or iron deficiency induced by a high pH in soils with an abundance of iron.

Add Organics to Desert Soils

Desert soils typically have low levels of organic matter. Organic matter in soils result from the death and decomposition of life (plants, animals and other organisms) that lives in it or above it. Climates that have more rainfall support more life. Soils influenced by these climates consequently have more organic matter in them than soils which have evolved under a desert climate.

Climates that have little rainfall support less life. Since our hot deserts have little rainfall, native or raw desert soils are low in organic matter. The raw, untouched soil of the Mojave Desert contains less than 1% organic matter.

More Water = More Organics

Desert soils that evolved with irrigated agriculture contain an abundance of organic matter compared to the raw soils of the Mojave Desert. Levels of organic matter in “farmed” desert soils can reach 4 to 5%, a level approaching the rich grassland prairies of Nebraska and Kansas. Why? Because of the artificially applied “effective rainfall” used to grow crops. Water is life. Water applied to desert soils evoke change.

Building organic matter in soils takes time. Organic matter in soils builds rapidly in the hot, wet tropics. In the grassland prairies, at 20 to 30 inches per year, it can take decades. Since organic matter never increases in desert soils without irrigation it must be added “artificially” when it is needed.

Desert Landscapes: Are they Really?

So-called “desert landscapes” no longer consist of “desert plants”. Nurseries and landscapers sell landscapes that look good with no regard for a plant’s origin. Nondesert photinia and mockorange are planted on the same valve and grown alongside Baja Fairy Duster or Arizona Rosewood and mulched with rock. In three to five years, the organic matter used at the time of planting decomposes and the soil “mineralizes” (loses its organic matter). The soils collapse and lose their porosity and ability to drain. Soil pH begins to drift upward. Biological activity declines. The non-desert Photinia and Mockorange begin yellowing and scorch while the more tolerant desert plants continue growing and appear healthy.

More Plants = More Landscape Inputs

The primary reason for these types of landscape failures is the combination non-desert plants with desert plants and managing this landscape as if it were a “desert landscape”.

I prefer the concept promoted by Dr. Warren Jones when he was with the University of Arizona – Mini Oasis. This type of desert landscape design grouped plants with similar needs together. Plants that grew well under rock mulch were grouped together and watered and managed similarly. Higher water use plants, many times non-desert plants, that benefited from higher levels of soil organic matter and wood chip mulch were grouped together. This concentrated plants which required more time, energy and money in one location, usually close to where people congregated.

 “The further our landscapes drift from a true desert environment, the more time, energy and money is needed to sustain them.”