Sodium
adsorption ratio (SAR) is a measure of the suitability of water for use in
agricultural irrigation, as determined by the concentrations of solids
dissolved in the water. It is also a measure of the sodicity of soil, as
determined from analysis of water extracted from the soil. The sodium
adsorption ration (SAR) indicates the effect of relative cation concentration
on sodium accumulation in the soil. SAR is a more reliable method for
determining this effect than sodium percentage. SAR is calculated using the
following formula:
SAR = Na/[(Ca + Mg)/2]1/2
Ions are expressed as milliequivalents
per liter (meq/L). The potential for a sodium hazard increases in waters with
higher SAR values. Although SAR is only one factor in determining the
suitability of water for irrigation, in general, the higher the sodium
adsorption ratio, the less suitable the water is for irrigation. Irrigation
using water with high sodium adsorption ratio may require soil amendments to
prevent long-term damage to the soil.
If irrigation water with a high SAR is
applied to a soil for years, the sodium in the water can displace the calcium
and magnesium in the soil. This will cause a decrease in the ability of the
soil to form stable aggregates and a loss of soil structure and tilth. This
will also lead to a decrease in infiltration and permeability of the soil to
water leading to problems with crop production.
Example of Irrigation Water
Classification :
If the sample of water from the 20 wells
was recorded on the Table 1. Recording data presented the value of EC, Cation
of Na, Mg and Ca. Then we can calculate the value of SAR using it’s formula.
After we get the value of EC (ᶣmhos/cm) and SAR, so we can make classification
of irrigation water quality using US Salinity Laboratory Classification by
drawing the lines at the diagram (Figure 1). The meaning of the irrigation
water classification symbol is defined below :
Salinity Hazard:
•
Low-salinity water
(C1) can be used for irrigation on most crops in most soils with little likelihood
that soil salinity will develop.
•
Medium-salinity water
(C2) can be used if a moderate amount of leaching occurs.
•
High-salinity water
(C3) cannot be used on soils with restricted drainage.
•
Very high-salinity
water (C4) is not suitable for irrigation under ordinary conditions, but it may
be used occasionally under very special circumstances.
Sodium Hazard:
•
Low-sodium water (S1)
can be used for irrigation on almost all soils with little danger of developing
harmful levels of sodium.
•
Medium-sodium water (S2)
may cause an alkalinity problem in fine-textured soils under lowleaching
conditions. It can be used on coarsetextured soils with good permeability.
•
High-sodium water
(S3) may produce an alkalinity problem. This water requires special soil
management such as good drainage, heavy leaching, and possibly the use of
chemical amendments such as gypsum.
•
Very high sodium
water (S4) is usually unsatisfactory for irrigation purposes.
Table
1. Classification of Irrigation Water Quality using US Salinity Laboratory
| No. of | Cation | SAR | EC | Classification of | ||||
| Well | Na | Ca | Mg | Na/√[(Ca + Mg)/2] | mmhos/cm | ᶣmhos/cm | Irrigation Water | |
| 1 | 2.94 | 1.90 | 4.20 | 1.68 | 0.72 | 720 | C2 | S1 |
| 2 | 4.06 | 2.40 | 4.50 | 2.19 | 1.04 | 1,040 | C3 | S1 |
| 3 | 3.08 | 3.40 | 3.30 | 1.68 | 0.64 | 640 | C2 | S1 |
| 4 | 7.71 | 1.30 | 3.40 | 5.03 | 0.48 | 480 | C2 | S1 |
| 5 | 7.70 | 1.50 | 3.40 | 4.92 | 0.40 | 400 | C2 | S1 |
| 6 | 7.41 | 2.10 | 4.90 | 3.96 | 0.72 | 720 | C2 | S1 |
| 7 | 5.68 | 1.50 | 4.10 | 3.39 | 0.44 | 440 | C2 | S1 |
| 8 | 6.65 | 1.20 | 5.00 | 3.78 | 0.48 | 480 | C2 | S1 |
| 9 | 6.91 | 11.90 | 8.40 | 2.17 | 0.68 | 680 | C2 | S1 |
| 10 | 14.08 | 0.90 | 5.00 | 8.20 | 0.84 | 840 | C3 | S1 |
| 11 | 5.04 | 2.70 | 33.30 | 1.19 | 1.04 | 1,040 | C3 | S1 |
| 12 | 32.34 | 15.80 | 35.20 | 6.40 | 3.96 | 3,960 | C4 | S1 |
| 13 | 14.56 | 6.30 | 39.70 | 3.04 | 2.48 | 2,480 | C4 | S1 |
| 14 | 5.04 | 1.30 | 37.70 | 1.14 | 0.96 | 960 | C3 | S1 |
| 15 | 15.82 | 6.60 | 47.40 | 3.04 | 2.64 | 2,640 | C4 | S1 |
| 16 | 6.44 | 2.60 | 41.40 | 1.37 | 1.00 | 1,000 | C3 | S1 |
| 17 | 15.68 | 6.40 | 40.60 | 3.23 | 2.28 | 2,280 | C4 | S1 |
| 18 | 43.54 | 24.30 | 44.70 | 7.41 | 4.60 | 4,600 | C4 | S1 |
| 19 | 25.22 | 5.40 | 53.60 | 4.64 | 2.40 | 2,400 | C4 | S1 |
| 20 | 19.90 | 5.90 | 52.10 | 3.70 | 3.12 | 3,120 | C4 | S1 |
Figure
1. Diagram for Classification of Irrigation Water According to The United
States Salinity Laboratory
Table 2. Characteristics of Irrigation
Water from Each Well
|
No.
of
Well
|
Characteristics
of Irrigation Water
|
|
1
|
Medium-salinity
water (C2) can be used if a moderate amount of leaching occurs and Low-sodium
water (S1) can be used for irrigation on almost all soils with little danger
of developing harmful levels of sodium
|
|
2
|
High-salinity
water (C3) cannot be used on soils with restricted drainage and Low-sodium
water (S1) can be used for irrigation on almost all soils with little danger
of developing harmful levels of sodium
|
|
3
|
Medium-salinity
water (C2) can be used if a moderate amount of leaching occurs and Low-sodium
water (S1) can be used for irrigation on almost all soils with little danger
of developing harmful levels of sodium
|
|
4
|
Medium-salinity
water (C2) can be used if a moderate amount of leaching occurs and Low-sodium
water (S1) can be used for irrigation on almost all soils with little danger
of developing harmful levels of sodium
|
|
5
|
Medium-salinity
water (C2) can be used if a moderate amount of leaching occurs and Low-sodium
water (S1) can be used for irrigation on almost all soils with little danger
of developing harmful levels of sodium
|
|
6
|
Medium-salinity
water (C2) can be used if a moderate amount of leaching occurs and Low-sodium
water (S1) can be used for irrigation on almost all soils with little danger
of developing harmful levels of sodium
|
|
7
|
Medium-salinity
water (C2) can be used if a moderate amount of leaching occurs and Low-sodium
water (S1) can be used for irrigation on almost all soils with little danger
of developing harmful levels of sodium
|
|
8
|
Medium-salinity
water (C2) can be used if a moderate amount of leaching occurs and Low-sodium
water (S1) can be used for irrigation on almost all soils with little danger
of developing harmful levels of sodium
|
|
9
|
Medium-salinity
water (C2) can be used if a moderate amount of leaching occurs and Low-sodium
water (S1) can be used for irrigation on almost all soils with little danger
of developing harmful levels of sodium
|
|
10
|
High-salinity
water (C3) cannot be used on soils with restricted drainage and Low-sodium
water (S1) can be used for irrigation on almost all soils with little danger
of developing harmful levels of sodium
|
|
11
|
High-salinity
water (C3) cannot be used on soils with restricted drainage and Low-sodium
water (S1) can be used for irrigation on almost all soils with little danger
of developing harmful levels of sodium
|
|
12
|
Very
high-salinity water (C4) is not suitable for irrigation under ordinary
conditions, but it may be used occasionally under very special circumstances
and Low-sodium water (S1) can be used for irrigation on almost all soils with
little danger of developing harmful levels of sodium
|
|
13
|
Very
high-salinity water (C4) is not suitable for irrigation under ordinary
conditions, but it may be used occasionally under very special circumstances
and Low-sodium water (S1) can be used for irrigation on almost all soils with
little danger of developing harmful levels of sodium
|
|
14
|
High-salinity
water (C3) cannot be used on soils with restricted drainage and Low-sodium
water (S1) can be used for irrigation on almost all soils with little danger
of developing harmful levels of sodium
|
|
15
|
Very
high-salinity water (C4) is not suitable for irrigation under ordinary
conditions, but it may be used occasionally under very special circumstances
and Low-sodium water (S1) can be used for irrigation on almost all soils with
little danger of developing harmful levels of sodium
|
|
16
|
High-salinity
water (C3) cannot be used on soils with restricted drainage and Low-sodium
water (S1) can be used for irrigation on almost all soils with little danger
of developing harmful levels of sodium
|
|
17
|
Very
high-salinity water (C4) is not suitable for irrigation under ordinary
conditions, but it may be used occasionally under very special circumstances
and Low-sodium water (S1) can be used for irrigation on almost all soils with
little danger of developing harmful levels of sodium
|
|
18
|
Very
high-salinity water (C4) is not suitable for irrigation under ordinary
conditions, but it may be used occasionally under very special circumstances
and Low-sodium water (S1) can be used for irrigation on almost all soils with
little danger of developing harmful levels of sodium
|
|
19
|
Very
high-salinity water (C4) is not suitable for irrigation under ordinary
conditions, but it may be used occasionally under very special circumstances
and Low-sodium water (S1) can be used for irrigation on almost all soils with
little danger of developing harmful levels of sodium
|
|
20
|
Very
high-salinity water (C4) is not suitable for irrigation under ordinary
conditions, but it may be used occasionally under very special circumstances
and Low-sodium water (S1) can be used for irrigation on almost all soils with
little danger of developing harmful levels of sodium
|
