Use the tables on this page to assess the impact of irrigation water on soil water infiltration
Table 7. Assessing soils for sodium problems related to water infiltration. Use the following equation to determine the Sodium Adsorption Ratio of the irrigation water: SAR = Na + /(sqrtCa +2 + Mg +2 /2) = meq/L Example: Water sample reports the following:
Na + = 1,000 ppm Ca +2 = 200 ppm Mg +2 = 100 ppm 1. Convert ppm to meq/L.
Table 9. Assessing irrigation water for excessive carbonates (CO 3 -2 ) and bicarbonates (HCO s -) in relation to calcium and magnesium which may lead to soil deflocculation. Use the following equation to determine the Residual Sodium Carbonate (RSC) value of the irrigation water: RSC = (CO 3 -2 + HCO s -) – (Ca 2+ + Mg 2+ ) in meq/L Example: Water sample reports the following: CO 3 -2 = 6 ppm HCO s - = 183 ppm
Na + = 1,000 ppm /23 = 44 meq/L Ca +2 = 200 ppm/20 = 10 meq/L Mg +2 = 100 ppm/12.2 = 8.2 meq/L
2. Place values into SAR equation: 44/(sqrt10+8.2/2) = 20.6 3. Use the following table to determine potential sodium hazard SAR w values, categories, and precautions for irrigation sources with ECw > 1dS/m SAR (meq/L) Sodium Hazard Precaution 0-10 low Na water Little danger from structure deterioration to soil. 10-18
Ca +2 = 30 ppm Mg +2 = 18 ppm
medium Na water Problems on fine-textured soils and sodium-sensitive plants, especially under low-leaching conditions.
1. Convert ppm to meq/L CO 3
-2 = 6 ppm/30 = 0.2 meq/L s - = 183 ppm/61 = 3.0 meq/L
18-24
high Na water
Na accumulations on most soils. Salt- tolerant plants required with use of special amendments such as gypsum and leaching.
HCO
Ca +2 = 30 ppm/20 = 1.5 meq/L Mg +2 = 18 ppm/12 = 1.5 meq/L 2. Place values into RSC equation: (0.2 + 3.0) – (1.5 + 1.5) = 0.2 meq/L 3. Use the following table to determine potential residual sodium carbonate hazard RSC (meq/L) RSC Hazard Precaution <0 Minor Problems Little effect on infiltration. 1.25 – 2.5 Increasing Problems
> 24
very high Na water Unsatisfactory except with high salinity (>2dS/m), high Ca levels, and use of gypsum
Table 8. Interaction of sodium adsorption ratio of water (SAR w ) and electrical conductivity of water (EC w ) on soil water infiltration. Irrigation water influence on soil permeability SAR w & EC w No Restrictions Slight to Moderate Restriction Severe Restriction SAR w = EC w = 0 – 3 >0.7 0 – 3 0.7 - 0.2 0 – 3 <0.2
Infiltration may decrease with high sodium in soil or irrigation water. Consider flushing soils with calcium and magnesium amendments. Severe deflocculation will occur with high sodium soil or irrigation water sodium. Soil flushing with calcium and magnesium amendments is required to improve soil structure and permeability.
>2.5
Severe problems
SAR
=
3 – 6 >1.2
3 – 6 1.2 – 0.3 6 – 12 1.9 – 0.5 12 – 20 2.9 – 1.3 20 – 40 5 – 2.9
3 – 6 <0.3
w =
EC
w
SAR
=
6 – 12 >1.9
6 – 12 <0.5
w =
EC
Please contact your Harrell’s Representative for more information
w
SAR
=
12 – 20 >2.9 20 – 40 >5.0
12 – 20 <1.3 20 – 40 <2.9
w =
EC
w
SAR
=
w =
EC
w
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