Sodium Adsorption Ratio (SAR)

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