PURPOSE OF SAR

The sodium adsorption ratio (SAR) is a parameter related to the sodium, calcium, and magnesium concentrations in a saturated soil paste[1][2] (concentrations in meq/L or mmolc/L).

 

 

 

SAR is a widely used index for characterizing soil sodicity.  Another index used to characterize sodicity is the exchangeable sodium percentage (ESP), related to the cation exchange capacity (CEC).  High SAR values indicate that sodium content in a soil has the potential to result in poor physical conditions, due to crusting, water-logging, or poor permeability.  High exchangeable sodium content can enhance clay swelling and dispersion, or disaggregation, of the soil.  The degree of clay swelling and dispersion depends on the levels of sodium and the salinity of the soil solution and applied irrigation water[3].

As a general guide[1], the classification of soils related to salts is shown in Table 1:

Table 1:  Soil Classification

Class	EC (mmhos/cm)	SAR	ESP
Normal	< 4	< 13	< 15
Saline	> 4	< 13	< 15
Sodic	< 4	> 13	> 15
Saline-Sodic	> 4	> 13	> 15

 

Electrical conductivity (EC) in this table refers to a saturated soil paste measurement.  In this scheme, SAR and EC are presented as they relate to the classification between a saline soil (EC >4, SAR< 13), a sodic soil (EC<4, SAR> 13), and a saline-sodic soil (EC>4, SAR>13).  As shown, exchangeable sodium percentage (ESP) can be substituted at 15% as a threshold value[2].

For a sodic soil, a potential management activity for restoration of permeability is to amend the soil with gypsum[3].  Gypsum (calcium sulfate) allows calcium to replace exchangeable sodium in the soil and subsequently mobilize the displaced sodium out of the rooting zone via leaching, which effectively increases the calcium content and lowers the SAR. Adding gypsum to a saline soil could increase the salt content further and aggravate the salinity problem[4].  However, gypsum may also be a beneficial amendment for saline-sodic soils.

 

RELATIONSHIP BETWEEN SAR AND ELECTRICAL CONDUCTIVITY

Examination of the SAR of a saturated soil paste is not sufficient to determine if a potential sodium problem is likely.  The SAR is also related to the electrical conductivity of the irrigation water.

The relationship between SAR and electrical conductivity of the irrigation water is a key factor in determining the potential problem of reduction in infiltration by soil sodicity.  This concept was introduced by Ayers and Wescot[1], and was further developed by Oster and Schroer[2] and Hanson, et al[3].

 

Figure 1:  SAR and EC of Irrigation Water, Effect on Infiltration

 

Figure 1, from Hanson, et.al., displays important information pertinent to any irrigation system.  Consider irrigation water with a high electrical conductivity in the range of 4.5 mmho/cm.  At that level of conductivity, there is no anticipated reduction in soil infiltration until the SAR is elevated to about 26. {Whereas irrigation water with a lower conductivity would experience a reduction in soil infiltration more generally at a SAR in the range of 10. This relationship between SAR and the electrical conductivity of irrigation water is the accepted basis of the irrigation water quality guidelines used in California since 1985.  This concept has been advocated by the Cooperative Extension Service of the University of California, since the publication of the Ayers and Westcot document referenced previously.

For effluent irrigation, a high electrical conductivity of the irrigation water helps to maintain good soil permeability, as shown in Figure 1.  However, under rainfall conditions, the rainwater will reduce the salinity of the soil water near the surface, and as a result, the infiltration of rainfall may be restricted.  This situation can be improved by the addition of gypsum or calcite to the soil surface, because the rainwater will dissolve either amendment.  More gypsum would dissolve than calcite because it is more soluble than calcite.  The dissolution of gypsum will increase the concentrations of calcium and sulfate and thereby also the salinity of the soil water near the soil surface, and of equal importance, dissolution of gypsum will reduce the ESP and SAR because it provides a source of calcium.

[1] Wu, L., C. Amrhein, and J. Oster, Salinity Assessment of Irrigation Water using WATSUIT, in W.W. Wallender and K.K. Tangi  (eds) , Agricultural Salinity Assessment and Management, 2^nd ed, ASCE Manuals and Reports on Engineering Practices, No. 71, 2012.

[2] Oster, J, Emeritus Soil and Water Specialist, University of California, Riverside, personal communication, 2017.

[3] Hanson, B., S.R.Grattan and A.Fulton, Agricultural Salinity and Drainage, University of California Irrigation Program, University of California, Davis, 1999.

[4] Davis, J.G., R.M. Waskom, and T.A. Bauder, Managing Sodic Soils, Colorado State University Extension, Fact Sheet No. 0.504, 2012.