Soil Quality

Essentially the same parameters used to describe water quality are used to assess soil quality including salinity level, Na+ concentration relative to other ions, specific ion concentration or toxicity, and miscellaneous effects. In many cases water and soil quality analyses are the same, although acceptable use limits and potential hazards for each may vary. In order for water quality interpretations to be meaningful for a given plant species and management strategy, it will be necessary to have soil quality data for the site in question. For examples of analytical reports and their interpretation, see Testing and Interpreting Salt-Affected Soil for Tree and Shrub Plantings, Plant Materials Technical Note, MT-60.

When total soluble salts, Na+ levels and ratios, or specific ion levels are in question, either in irrigation water or in soil, soil testing should include all of the following tests:

1. Soil Salinity.   Soil salinity, like irrigation water salinity, is measured by the Electrical Conductivity (EC) test. The EC may be measured by the saturated paste extract or soil:water dilution methods. Although the two tests produce results expressed by the same units of measure, the values are not comparable. Soils are classified into five categories based on the concentration of ions present in a representative sample as measured by EC (see Table 2). Calcium (Ca++), magnesium (Mg++), and Na+ are the most common ionic salt components found in Montana soils.

Actual quantitative field testing of woody plant soil salinity tolerance is limited. Most woody plants adapted to climatic conditions in the northern Great Plains and Intermountain West survive and grow well on non-saline to very slightly saline soils. The number of woody species that will reach their full growth potential on soils with ECs >8 dS/m is very limited. A number of species will survive but grow at a reduced rate and vigor on soils with ECs between 6 and 10 dS/m. For approximations of tree and shrub soil salinity tolerances, see HortNote No. 6, Selecting Plant Species for Salt-Affected Soils. Also, see Montana Field Office Technical Guide (eFOTG), Section II, Windbreak Interpretations, Conservation Tree/Shrub Suitability Groups (CTSG), for a list of tree and shrub species adapted to salt-affected soils (see References).

The effect of irrigation water on soil salinity varies widely based on the water salinity level, concentration and ratio of the specific ions involved, method of irrigation, soil drainage properties, duration and frequency of irrigation, and other management factors. The long-term maintenance of soil salinity at or below a given level often requires using irrigation water significantly lower in salinity than the soil. As we approach irrigation water salinity levels comparable to the soil, additional management practices such as leaching, improved soil drainage, and other techniques will be needed. As noted earlier, leaching with salt-affected water will benefit a well-drained soil more than a poorly drained soil.

Note: There are multiple methods of measuring soil salinity. The values produced by various techniques may not be compatible, even when the units are identical. As an example, both the saturated paste and soil:water dilution ratio techniques report results as an EC value, even though the two values are not comparable. Additionally, field and laboratory equipment may need to be calibrated before a meaningful soil salinity value is produced. Always verify that soil salinity data has been presented as a saturated paste EC or TDS value before interpreting the results.

2. Soil Sodium Levels. As with irrigation water, the amount of Na+ in the soil is an important factor in determining its suitability for supporting trees and shrubs because Na+ strongly influences water infiltration and soil aeration.

a. Soil Sodium Adsorption Ratio – Like irrigation water, soil Na+ is best described by the SAR, an indication of the amount of extractable Na+ relative to Ca++ and Mg++. Soil SAR indicates the likelihood of reduced soil permeability (water infiltration) and aeration, especially on heavy-textured soils. A soil SAR >13 suggests a likelihood of reduced soil permeability and decreased plant survival and growth.

b. Exchangeable Sodium Percentage – Another useful indicator of potential soil Na+ hazards is Exchangeable Sodium Percentage (ESP). ESP measures the amount of soil exchange capacity occupied by Na+ and is calculated by the formula:

ESP = exchangeable sodium (meq/100 g or cmol/kg) / cation exchange capacity (meq/100 g or cmol/kg) X 100

One milliequivalent per 100 grams (meq/100 g) of soil equals 1 centimole per kilogram (cmol/kg) of soil. An ESP >15% indicates that soil Na+ will probably limit permeability.

In some cases, excessive Mg++ may also cause reduced water infiltration.

3. Soil pH.   Soil pH, although not a salt test, is often tested in a comprehensive soil analysis. It measures the hydrogen ion concentration in soil solution - an important indication of the chemical status of the soil. Since soluble salts affect soil pH and vice versa, it is often included in evaluations and discussions of soil saltiness. A main implication of changing the soil pH is plant nutrient availability, which is often a secondary response to microbial activity levels responding to changing soil pH. The availability of certain nutrients in soil solution begins to decrease above pH ~5.5 (Fe, Mn, zinc [Zn], copper [Cu], cobalt [Co]), above ~7.0 (phosphorus [P], B), and above 8.5 (Ca++, Mg++) (see Chart 2). The soil pH scale ranges from 0 to 14, with <7 considered acidic, 7 neutral, and >7 alkaline or basic (see Table 3). Each whole number represents a ten-fold change in both H concentration and OH, or a 100-fold change in the concentration of H relative to OH (since there is an inverse relationship between the two: as one increases the other proportionally decreases). Most arable soils in our region have a pH in the range of 7 to 9. Soil pH measuring 6.1 to 7.0 is considered ideal for most trees and shrubs, although various species will survive in a range from 5.5 to 8.0+. Listed in Table 3 are soil pH classes. It is more likely that a naturally salt-affected soil will have a high, rather than low, soil pH. Although it is possible to amend soil and water with acidifying products including ammonium sulfate and other sulfur containing products to decrease soil pH, it is often necessary to re-apply these substances in order to sustain the effect.

Chart 2.  Relationship between Soil pH and Plant Nutrient Availability in Soil Solution ⁽¹⁾

⁽¹⁾ From University of Minnesota Extension Service, BU-01731, Revised 2004.

4. Soil Texture Classification.  Although soil texture is not a salinity measure, it is often included with salt tests because texture greatly influences how salty soil can be managed. Soil texture indicates the relative amount of sand, silt, and clay particles in a soil sample. The proportions of these three particle sizes influences several soil properties, including water infiltration, percolation, soil aeration, moisture holding capacity, and others. Soils with a high percentage of small clay particles are called “heavy-textured” and are characterized by slow water infiltration into the soil, slow water percolation through the soil, low soil aeration, and a tendency for the soil to hold moisture with great tension. Soils with a high percentage of large sand particles are called “light-textured” and are characterized by rapid water infiltration and percolation, high soil aeration, but low water holding capacity. Light soils (sands and loamy sands) lend themselves to management practices designed to reduce soil salinity by leaching salts from the soil with applications of excess, low-salt irrigation water. Heavy soils (silty clay, sandy clay, clay) are generally more difficult to manage for salinity than soils classified as sandy or loamy. Medium-textured soils (sandy loams, loams, sandy clay loam, clay loam, silt, silt loam, silty clay loam) fall somewhere between light- and heavy-textured soils in terms of their properties and management.

Chart 3 graphically illustrates the relationship between soil EC, SAR, ESP and pH relative to soil classification, as well as plant performance.

Chart 3.  Relationship between Soil EC and SAR or ESP on woody plant survival and growth ⁽¹⁾

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Last Modified: 08/21/2008