# 2020 Harrells Balanced Approach Customer Agronomic Program …

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.

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

Table 9. Amounts of bicarbonate, carbonate, calcium, and magnesium in irrigation water which can lead to soil deflocculation, reducing soil water infiltration. Element Levels in irrigation water which may reduce soil water infiltration, mg/L Bicarbonate (HCO 3 ) 100 – 400 Carbonate (CO 3 ) 0 – 5 Calcium (Ca) 25 – 200 Magnesium (Mg) 20 – 40

medium Na water Problems on fine-textured soils and sodium-sensitive plants, especially under low-leaching conditions.

18-24

high Na water

Na accumulations on most soils. Salt- tolerant plants required with use of special amendments such as gypsum and leaching.

> 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

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

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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