Monday, July 15, 2013

Six Simple Soil Tests That You Can Do Yourself

You planted a tree, shrub, rosebush, or groundcover in your yard, and it died. 

It was the healthiest plant you could find.  You prepared the planting hole according to the best information available.  You watered and fertilized it diligently.  You were excited when it put out its first leaf of new growth; and, now you are devastated. 

Never fun, and often expensive, dead and dying plants are no joking matter.  There are many possible reasons, of course.  A lack of water in the hot summer is deadly to plants.  So also is too  much water.  It can lead to rotting roots.     

In many cases, however, the causes of dead and dying plants can often be traced back to the soil itself.  To be specific, there are five common soil problems that can, and often do lead to serious plant health issues.

Caliche is an accumulation of lime or as it is chemically known, calcium carbonate.  It is a natural and common mineral in the desert and, in a refined form, is a basic component of concrete.  Caliche can be found in the soil as small crumbs, thick or thin lenses, or solid sheets.  It is grey in color and it is hard, very hard.  If you have ever tried to dig through it, you will know what I mean.

Salinity, a buildup of naturally occurring salts in the soil, is also a common problem.  Some salts, in beneficial amounts, are considered to be plant nutrients.  Magnesium, zinc, and boron are examples.  When these and other salt forming chemicals reach toxic levels in the soil, however, they can seriously injure or kill sensitive plants.  We solve high salt concentration problems by leaching with extra water during an irrigation event to wash the salts down and out of the root zone.

The only exception to this rule is sodium.  Sodium salt is a special case because it ties itself chemically to the individual soil particles and does not readily wash off during irrigations.  Sodium in large amounts is especially toxic to plants.

Soils saturated with sodium take on unique characteristics.  Because the alkali salt causes the soil particles to separate and act individually, the soil particles with the least disturbance can easily billow up into the air and create clouds of dust.  Water often sits on the surface of these soils without sinking in until it evaporates.  Sometimes the water will penetrate less than an inch into the soil.  The soil also becomes quite sticky and readily clings to shoes, hands, and clothing. Gypsum, as an amendment, is the solution for sodium problems.

Another common problem is soil pH.  Soil pH is a measure of the acidity or alkalinity of the soil.  It is measured on a scale of 1 to 14 with 7 being neutral.  A pH under 7 results in a progressively acid soil-water complex while a pH over seven is considered to be alkaline.  A soil or water pH that is over 8.0 or under 6.5 can be toxic to plants, as well as inhibit the uptake of nutrients by the plant.  Caliche and sodium are quite alkaline and are a leading cause of high pH soils in our area.

Soil compaction occurs when the soil is compressed by foot, animal, or vehicle traffic.  Anyone who has noticed the lack of grass growing under a child’s swing set or in the ruts of a dirt road will understand the difficulty in growing plants in compacted soil.

Finally, desert soils are notoriously short of available nitrogen, an essential plant nutrient.   Generally other nutrients like copper, potassium, and manganese are plentiful in our desert soils and we do not need to worry about feeding these nutrients when the plants are growing in native soils.  Without regular applications of nitrogen, however, most non-native, introduced plants will not thrive.

So, how do we know if we have these soil problems in our yards?  Here are six simple tests that anyone can do at home without having to be a soils expert or purchase a lot of expensive gadgets or materials. 

Dig a hole.  Using a pick and shovel, it is time to get a little exercise.  Dig a hole right where you are thinking about planting a tree or shrub.  The hole should be no deeper than the depth of the container and a little larger than it is wide.  Your actual planting hole will be much wider, of course.  Up to five times the diameter of the container in which the tree or shrub is planted is the recommendation, but loosening the soil for good root growth can come later.  For now, let’s keep the size of the hole manageable.

The next step is to fill the hole with water and then time how long it takes for the water to sink into the ground, that is, to disappear.  Most soils should drain within thirty minutes to one hour.   Suspect caliche, sodium, or compaction problems if it takes longer.  Do not plant anything at that site until you figure out and fix the problem.  Drainage problems are a major cause of tree death in our area.

Shake a jar.  Soil texture is a measure of sand, silt and clay in a soil. The more sand in the soil, the quicker a soil will drain.  That is good.  The more sand in a soil, the more frequently we have to irrigate.  That can be bad.

Likewise, the more clay we have in a soil, the slower it drains.  That can be bad.  The more clay in a soil, the more water it will have available for plants.  That can be good.  A soil with a mixture of sand and clay is generally the best kind of soil for gardens and landscapes because it resists compaction and allows water to move easily through the soil profile.

To perform the test, fill a clear glass quart jar half full of soil.  I like to take my samples about six inches deep.  It avoids a lot of problems and gives me a good estimate of conditions in the root zone.  Now, add water to the jar but do not fill it completely.  There needs to be enough room for the water to slosh around.  Filling the container up to the neck should be just fine.  Seal the jar with a lid and shake it vigorously.  Hint: do not do this over your expensive carpet.  Sometimes the lid does not seal tightly with the rim of the jar.

Once the soil is totally mixed with the water, set the jar down and wait a few minutes.  The sand in the soil will settle first, followed by silt and clay.  A good garden soil will be about one half sand and one half silt and clay.

Probe for water.  Using a soil probe, measure how deep the water penetrates the soil during an irrigation event.  I like to use a long screwdriver.  Where the soil is moist, the screwdriver will slip easily into the ground.  When it hits dry soil, it will stop abruptly.  With the probe in the ground, place your fingers at the soil level and remove the probe.  Measure the distance between your fingers and the tip of the probe.

How deep should the water go?  Use the”1-2-3 Rule”.  Water small plants, such as ground covers, cacti, and annuals to a depth of one foot.  Water medium plants such as shrubs to a depth of two feet.  Large plants, such as trees, should be watered to a depth of three feet.  Do not forget to soak the entire root zone.  Most roots will fall inside a circle drawn at the edge of each plant’s canopy.

Lose a shoe?  If you have a soil that resists water penetration and tends to pull the shoe off of your foot when you walk on it, think sodium.  It is probably a good idea to treat the soil with an application of gypsum.

Watch for the fizz.  Caliche is known to have a basic pH, that is, a pH well over the neutral point of 7 on the pH scale.  Because of this, any acid coming in contact with the mineral will cause the caliche to bubble and fizz.  Soil scientists use a drop of concentrated acid, like hydrochloric acid, placed onto soil sample to tell quickly and accurately whether caliche is present.

I personally do not like to carry concentrated acids with me because of the danger of caustic burns, so I use mild acids like vinegar, lemon, or lime juice.  Because they are not strong acids, I have to look carefully to see the fizzing action, but they do work.

Leaves turn yellow?  This is not specifically a soil test per se, but it is what we call in technical jargon, a bioassay.  We use the plant itself to conduct the test.  Look at the plant in question.  If the older leaves down below the tip of the branch turn yellow while the new, upper leaves stay green, chances are you need to feed the plant with nitrogen during, or just before, the next irrigation.  Ammonium sulfate, 21-0-0, is a good choice because the dissolved nitrogen will sink into the soil with water.

If the new leaves at the tips of the branches turn light green to yellow while the older leaves stay green, it may be an iron deficiency.  Nitrogen and iron are the nutrients most commonly deficient, or unavailable, in desert soils.

We have listed the most common soil problems found in the desert and simple diagnostic tests that anyone can do.  I hope that you will remember this basic rule of desert gardening:  Problem soils can, and do, cause significant stress to garden and landscape plants.  By knowing the more common soil problems and simple diagnostic tests that can be quickly done by the gardener, viable solutions to garden problems can be found.

If you have questions, you can reach one of the Master Gardeners at the Cooperative Extension office, 820 E. Cottonwood Lane, Building C, in Casa Grande.  The telephone is (520) 836-5221, extension 204.  The author’s email address is

The University of Arizona is an equal opportunity, affirmative action institution.  The University does not discriminate on the basis of race, color, religion, sex, national origin, age, disability, veteran status, or sexual orientation in its programs and activities.

Rick Gibson
Extension Agent, Agriculture
University of Arizona Cooperative Extension
820 E. Cottonwood Lane, Building C
Casa Grande, Arizona 85122
Voice:    (520) 836-5221
Fax:    (520) 836-1750

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