I continually receive questions and comments about determining soil texture in the field. Soil texture, as hopefully everyone reading this knows, is one of the key components or indicators for how well the soil will accept septic tank effluent through identification of the soil sizing factor or long-term acceptance rate (LTAR), usually expressed in gallons per day per square foot (gpd/sq ft). Other soil components used to arrive at this number are identification of soil structure, consistence and rock fragment percentage.

Having the ability to identify soil texture allows service providers a way to check the accuracy of the site evaluation and design information in the field. If there seems to be a discrepancy between what you are seeing and what is seen in the design, a conference or discussion should be held with the designer to resolve the question before an installation, repair or inspection continues.

This discussion may also need to involve the local regulator if it leads to a change in design for an install or major repair. If you are a service provider, you will need to check the soil treatment area to see if the soil matches the type and size of final dispersal unit needed.

TEXTURE CLASSIFICATIONS

Soil texture is simply the expression of percentages of sand, silt and clay size particles (soil separates) less than 2 mm in diameter. Anything more than 2 mm in diameter is considered gravel or rock, the percentages of which can be used to provide a modifier of the soil texture designation to indicate the percentage of rock. In any case, texture is determined only with the fraction of soil less than 2 mm in diameter.

You will find there are different classification systems to describe soil separates. There is the Unified Soil Classification System used by civil engineers related to building foundations and the American Society of State Highway and Transportation Officials used by engineers when designing roadbeds. For wastewater flow through soil, the system that best relates particle size and pore spaces for water movement is the U.S. Department of Agriculture Soil Classification System.

The USDA system includes 12 soil texture classes determined by the percentages of sand silt and clay size particles in the less than 2 mm fraction, commonly expressed in a soil textural triangle. An exact measure of these percentages requires some laboratory analysis; running the sample through various sieves and using a pipette or hydrometer method to measure the rate of fail of particles in water in a graduated cylinder. This is not realistic for every instance in the field where you want to know the texture. In certain cases, it may be desirable to submit a sample to the laboratory to confirm your analysis.

The “feel method” is used to determine soil texture in the field. If there are rocks present, a sample can be run through a 2 mm (No. 10) sieve before using the feel method on the material passing the sieve. Research is conducted at colleges and universities to train soil scientists and demonstrates this method is very accurate in determining the soil texture class.

TOUCHY FEELY

Here is a brief description of the procedures to follow:

Take a 2-inch ball of soil in the palm of your hand; moisten (this does not mean saturate!).

• If the soil is sandy (feels gritty) and single grains can be easily identified, squeeze the moistened sample. If it forms a cast but falls apart when bounced in the hand or when pressure is released, it is sand; when the sample is squeezed between the thumb and forefinger, it will not form a ribbon.
• If single grains can be seen and felt but the cast can be handled without falling apart and will not form a ribbon, it is in the sandy loam texture class.
• If the cast feels gritty but smooth, can be handled freely without breaking apart, and does not form a ribbon, it is in the loam texture class.
• If the cast can be handled freely without breaking but feels smooth (almost like flour) and will not ribbon easily and the ribbon breaks apart and crumbles, the soil is in the silt loam texture class.
• If the moist soil is plastic, can be kneaded, rolled into a wire and a ribbon can easily be made that is longer than an inch, the soil is in the clay loam texture class.
• If the sample can form a ribbon more than 3 inches, is very sticky, and when rubbed or worked leaves a slick smooth surface, the soil is in the clay texture class. 

SHARPEN YOUR SKILLS

When I conduct classes either for college students or service providers, site evaluators, or installers, we have a set of standard samples available to practice these techniques. It does not take many times though before they can identify with ease at least the major textural classes. It is an invaluable tool to use in the field, and you and all your employees should become versed in how to determine soil texture.

I have noticed that some engineer and contractor suppliers advertise online and elsewhere that they have standard soil samples available with the laboratory analysis and comparison of where the samples would land in the USCS and USDA classification systems for comparison.

It may be worthwhile to have a set of these standards in your office so you and your employees can periodically keep your soil texture skills sharp. For a couple of hundred dollars, it can save a mistake in the field that could cost thousands!