Page 112 - Nutrient Management Planning Guide

107

Chapter 3.4

[Na + ]

[Ca 2+ ] + [Mg 2+ ]

2

Sodium Adsorption Ratio =

Excess soil salinity causes poor and spotty crop stands, uneven and stunted growth and poor yields. Salinity restricts plant water uptake, interferes with nutrient availability and can impair germination and root growth because of caustic salt effects. Saline areas also tend to have poor soil structure and are subject to water logging, both of which are harmful to crop growth.

Sodium Adsorption Ratio (SAR)

SAR is a less commonly requested analysis that

expresses the proportion of exchangeable sodium (Na + ) to exchangeable calcium (Ca 2+ ) and magnesium (Mg 2+ ) ions.

Soils with SAR values at 13 or higher are considered sodic. Crop growth on sodic soils is very poor. Excess sodium causes soil particles to repel each other, preventing the formation of soil aggregates. This results in a very tight soil structure with poor water infiltration and surface crusting.

As stated previously, SAR is not part of standard soil analysis packages for agricultural applications but is routinely done as part of most testing packages for environmental applications. Characterizing the proportion of exchangeable Na can be useful in identifying solonetzic soils.

The sodium hazard of a soil is determined by factoring in the EC and SAR of a soil. This results in a soil being classified as non-saline, non-sodic, saline, sodic or saline-sodic (Table 3.4.6).