Over the years, we have conducted hands-on classes on basic surveying techniques and the determination of U.S. Department of Agriculture soil texture class using the feel method. Also, in conjunction with some soil scientist colleagues, we teach a course in soil description, which involves determining soil texture as well.

We have always maintained that installers and local inspectors/regulators — with a little training — can determine soil texture class accurately enough to help determine the soil sizing factor to plan for system size. We expect that training on the techniques used to determine texture by feel, combined with a little practice on soils found in the areas they work, will allow them to make better decisions on system sizing and location.

As evidence, we have offered up information from a study conducted with University of Arizona students in a beginning soils class. With a few hours of training and practice, the students could identify the correct soil texture class approximately 70% of the time. There are 12 texture classes in the USDA textural classification system, so this is impressive for novices with no experience. We always highlight in our classes that installers and inspectors can do better than any college student.

A new comprehensive study looked at the ability to determine textural class by professional soil scientists, university students and seasonal employees trained to evaluate texture in their inventory work to gather rangeland data. There were no real surprises. When soil texture by feel estimates were compared with laboratory data, the results were often different — with those differences being more frequent for individuals with less experience.

IMPROVING ACCURACY

Professional soil scientists for the National Cooperative Soil Survey were able to identify soil texture with more accuracy than university students or seasonal workers. How well texture could be identified was directly related to the amount of training the individuals received.

The results showed that training increases accuracy. Professional soil scientists outperformed people who have had a day of training on how to describe soil profiles, and the lowest group were those who had 1.5 hours of training. With additional training and practice, the accuracy for all groups could be improved.

Researchers looked at the data a little closer and noticed something else. Even though the absolute accuracy in estimation fell with the less-experienced groups, they weren’t off by much. Accuracy improved from 40% to 80% if they used the texture classes next to the correct texture class in the textural triangle identified in the laboratory.

This caught our attention because the purpose of evaluating soil texture is to determine which of the four or five soil loading factors, depending on your state code, is used in the design of the soil treatment unit. For our purposes, identifying the soil sizing factor texture estimates need only distinguish the major textural breaks to assign the proper values. This research shows individuals with very little training can do this with 80% accuracy. With more training and practice, these numbers can be improved. 

There was one other interesting result: Apparently if the skill is not used regularly, there is a need to recalibrate. This was true for all the groups, even professional soil scientists! The authors suggest what they feel are necessary steps to ensure “citizen scientists” learn and maintain the skills to accurately estimate soil texture. As an aside, yes, installers and inspectors are citizen scientists!

PRACTICE MAKES PERFECT

They suggest continued practice with known samples to keep the skills fresh. In places like Minnesota, practicing during the long winter months is important. In the past, we have talked about having texture samples available to practice. If they are not obtainable from a nearby university or a geotech supply house, work with a professional soil scientist to put together a set of soils for practice.

Another suggestion is to go in the field in an area (the area you work) and look specifically at the local variability and ranges in texture. This is the reason we established regional soils workshops for septic professionals as part of our Minnesota training program. When installers and inspectors from a region evaluate the soils, they can discuss differences in interpretation in a nonconfrontational setting, which can help avoid disagreements in the future. We have also conducted these workshops for other states. If your state does not have a professional program featuring these activities, it would be a good idea to investigate how to start them with your state regulators.

Their final suggestion was that support systems should be developed with information-loaded cellphones and carried into the field to help the decision-making process. We expect to see these types of applications developed. We do know that some site evaluators and designers have created some of these applications for their own use.

With a little investment of time and effort, we believe professionals in our industry can be trained on how to evaluate soil texture and write soil descriptions to provide necessary information for system design. One does not need to be a soil scientist. However, working with a soil scientist in your area can improve your soil evaluation skills, leading to better system design and installation.