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.
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| 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.
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| 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.”
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| 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.
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| 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.”
