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Sodium Adsorption Ratio Screening Procedure for Soils Using pH and Electrical Conductivity

Dep. of Land, Air, and Water Resources, Univ. of California, Davis, CA 95616

*Corresponding author.

ABSTRACT

Soils with sodium adsorption ratios (SAR) <10 are generally considered to be below the threshold for potential Na hazard. The impetus for developing this screening procedure was to reduce the resources dedicated to determining precise SAR values in situations when simply knowing that the SAR is below the designated threshold is sufficient. The model was developed using 1000 samples and validated on a 24 000-sample data set from two different laboratories operating in the western USA. The screening procedure uses empirically derived combinations of saturated soil paste pH (pH_s) and extract electrical conductivity (EC_e) to predict if the SAR is above or below 10, or whether Ca + Mg and Na must be analyzed to determine the SAR. For some combinations of pH_s and EC_e, the ratio of Ca + Mg to estimated total cations [ETC = EC_e(10)] is used to decide whether Na must be analyzed. On average, the model incorrectly predicted that the SAR was <10 when the actual value was > 10 in only six out of 1000 samples. In addition, about 20% of the samples required Ca + Mg and Na analyses when the SAR was < 10. For the data sets tested, the screening procedure could have reduced the number of Ca + Mg and Na analyses by 50% with little loss of information about the potential Na hazard of the soils.

Contribution of the Dep. of Land, Air, and Water Resources and U.S. Dep. of the Interior, Bureau of Indian Affairs, Natural Resources and Engineering Lab.

Received for publication August 12, 1991.