Digging in the Dirt

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QUESTION: What are the soil factors used to determine the long term acceptance rate (LTAR) or soil-sizing factor?

ANSWER: This is one of the most common soils questions I hear. And specifically what is being asked is, "How can I identify soil texture, structure and consistence?''

The LTAR or soil-sizing factor is based on analysis of soil characteristics at the depth the soil treatment unit is installed. The factor is expressed in terms of gallons per day per square foot. Establishment of the LTARs is based on numerous research studies dating back as early as the 1950s evaluating acceptance rates of soils when septic tank effluent was applied.

On sandy soils, the rates were determined by what they could handle for treatment, as opposed to how much effluent the soils would accept. In the middle textures the rates related primarily to what happened when the biomat formed on the infiltrative surface, and for the clay textures by the amount of water that will infiltrate into the soil.


During a number of studies, it was determined that the LTAR varied within similar textures if there was soil structure and it was strong enough to be maintained when water was added. This led a colleague of mine, Dr. Jerry Tyler, to work on a series of tables relating LTAR to soil characteristics and wastewater strengths. One abbreviated example is provided in an accompanying graphic.

Many states use some variation of these LTAR tables in their codes to define how large the systems should be based on soil type. The idea is that soil analysis based on texture, structure and consistence could replace – or at least reduce – the reliance on percolation tests and sizing by texture alone. Minnesota is an example. Arizona has done this since the early 2000s and Colorado is considering adopting this approach.

To adequately evaluate soils requires the site evaluator/designer to have a working knowledge of soil characteristics and their identification in the field. This most often requires excavating multiple soil pits in the vicinity where the soil treatment system will be installed and making a detailed soil description including not only texture, structure and consistence, but also soil color and landscape position.

These methods of analysis also are provided as ASTM standard 5921: Standard Practice for Subsurface Site Characterization of Test Pits for On-site Septic Systems, published by the American Society for Testing and Materials. I would caution everyone to use this standard with care, however, since there are some discrepancies and errors in the document.

The field procedure is to take a quantity of the soil and moisten the sample. Then based on how that sample holds together, how it feels and the length of ribbon that can be made will determine the soil texture class.


Soil structure is the arrangement of the individual soil particles in a shape. The size and way the structural units, or peds, are arranged affects both acceptance and treatment of wastewater. Structure is described by three characteristics: the grade or expression of structure, the size of the individual peds and the shape.

There are four grades: Grade 1 is weakly expressed; grade 2 and 3 are well expressed, and grade 0 is structureless. Structureless soils can be of two types: loose, reflected in sandy soils usually breaking down to the single sand grains, and massive, where the soil is coherent and does not break down to defined units. Well-structured soils will accept effluent more rapidly as well as conduct oxygen through the larger cracks and voids, which explains why there are differences in acceptance among textural classes.

Size of the peds ranges from very fine to very coarse. Soils with finer structure typically result in better water and air movement, and therefore better treatment. For the coarser-structured soils, these pathways let water move rapidly but may not allow time for treatment. Shapes can be granular, blocky, platy and prismatic.

To determine shape, size and grade of structure, take a sample of the horizon being described out of the pit wall on a shovel. The shape can be observed both on the shovel and in the pit face. The size is estimated by examining the peds on the shovel and estimating using a sizing table. The grade can be determined by bouncing the soil on the shovel. If more than half the peds fall apart into individual particles, the structure is weak; if it is about 50/50, then it is moderate, and if less than half, the grade is strong.


Consistence is a measure of how easy it is to break up soil. Consistence is measured either dry or moist. The proper way to measure for comparison purposes is moist. This is because you can always moisten a dry sample but it is a longer process to dry a moist sample. Consistence measures range from loose and a term called friable, which means the peds break down easily, to very firm and rigid, which means the peds do not break down easily. Very firm and rigid consistence usually indicates the presence of significant clay particles in the sample.

This may sound intimidating and complicated. But if you ever have the chance to take a soil class, you will see that with a little practice and your already-considerable practical knowledge of soils in your area, you can become proficient at identifying these characteristics. A good reference source is to look at the soil descriptions for your area provided in the USDA soil surveys. This will put you in the right ballpark relative to your soils. You also can work with a professional soil scientist in your area.


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