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Interpreting a Soil Test

Dr. Andrew Backhaus
Artistic Arborist, Inc, 4519 N. 7^th Ave, Phoenix, AZ 85013
602-263-8889 (office); rabackhaus@yahoo.com

Many times, when we are called into to diagnose a problem tree, we are asked to perform a soil analysis to determine the underlying cause of the problem. Soil tests are not always necessary. For example with trees having certain insect pests or diseases, soil tests won’t reveal much. However, when trees have yellow, ie., chlorotic, leaves then soil tests can provide clues as to what’s causing the symptoms.

Soil tests can be quite complex and difficult to interpret. Below is a list of items that are routinely given in a soil test to determine if anything is unusual. It takes a lot of experience to do this. I will try to explain what some of these terms are and what they mean.

 The chemical analysis of soils often consist of the following items:

 Chemical Analysis of Soils

1. pH
2. Electrical Conductivity of the extract (EC)
3. Total Dissolved Salts of the extract (TDS)
4. Sodium Absorption Ratio (SAR)
5. Organic Matter percentage (OM%)
6. Organic Acids content
7. Cation Exchange Capacity (CEC)
8. The quantity, in ppm, of the following:

What Do Each of These Numbers Mean?

pH

This refers to the acidity or alkalinity of a soil. The typical range we’ll see in our desert soils is pH 5.0 to 8.8. Anything out side this range can be harmful. The optimum range is pH 6.5 to 7.2. Changing soil pH is usually not recommended and is difficult to accomplish.

Electrical Conductivity (EC)

This is an important number on a soil report. It refers to the overall salt content in a soil. Excessive salt levels are toxic to landscape plants and often kill them outright. EC levels are determined by adding distilled water to a soil sample and then measuring the ability of that water sample to conduct electricity. Distilled water, with no salts whatsoever, does not conduct electricity, and has an EC of zero. Water with an EC between 0-1.0 is good for plants. Between 1.0-2.0 is fair. Between 2.0-3.0 is poor. Above 3.0 is bad and over 7.0 is toxic. Sea water has an EC of about 55.0.

 Total Dissolved Salts (TDS)

This is directly related to EC, except on a different scale. TDS values are determined on parts per million (ppm) or a percentage weight basis. Soil with an EC of 1.0 has about 640 ppm of salts.

 SAR (Sodium Absorption Ratio)

This number represents the proportion sodium out of total salts in the soil. This is important because sodium, unlike calcium, magnesium and potassium, is toxic to plants. For example, if the EC of the soil is 1.0, but contains only calcium and magnesium, the soil is fine. However, if the EC is 1.0 and all of it is from sodium, it would be toxic. The SAR gives an indication of this. Soil that has a SAR below 3.5 is acceptable. Soil with a SAR between 3.5 to 7.0 is poor. Soil above 7.0 is toxic to most plants.

 Organic Matter %

Organic matter % (OM%) is vital for soils, because OM helps a sandy soil retain moisture and prevents a clay soil from becoming cement-like. The OM% is also essential for maintaining soil microorganisms which are essential for plant growth. The OM% should be at least 1.5% - and, ideally be up to 5%. It can go up to 20% but that type of soil has to be managed differently. Native desert soils often have levels below 1% and these require lots of fertilizer and tend to have problems with moisture management. The best way to increase OM% is by adding a high quality compost (not peat or manure) to the soil.

Organic Acids