LEVEL	ECw	HAZARD
C1	<0.25 dS/m	low hazard; no detrimental effects on plants, and no soil buildup expected.
C2	0.25-0.75 dS/m	sensitive plants may show stress; moderate leaching prevents salt accumulation in soil.
C3	0.75-2.25 dS/m	salinity will adversely affect most plants; requires selection of salt-tolerant plants, careful irrigation, good drainage, and leaching.
C4	>2.25 dS/m	generally unacceptable for irrigation, except for very salt-tolerant plants, excellent drainage, frequent leaching, and intensive management.

 

II. Sodicity

LEVEL	SARw	HAZARD
S1	<10	no harmful effects from sodium.
S2	10-18	an appreciable sodium hazard in fine-textured soils of high CEC, but could be used on sandy soils with good permeability.
S3	18-26	harmful effects could be anticipated in most soils and ammendments such as gypsum would be necessary to exchange sodium ions.
S4	>26	generally unsatisfactory for irrigation.

 

III. Bicarbonate and Carbonate Ions in Irrigation Water

High cencentrations of bicarbonate and carbonate ions in irrigation water can induce precipitation of calcium and magnesium ions to form calcium and magnesium carbonates, which have low solubility. The precipitation of calcium and magnesium allows sodium ions to dominate and, thus, increases the sodium hazard associated with the irrigation water.

IV. Residual Sodium Carbonate

The residual sodium carbonate (RSC) equals the sum of the bicarbonate and carbonate ion concentrations minus the sum of the calcium and magnesium ion concentrations, where the ions are expressed in meq/L. A negative RSC indicates that sodium buildup is unlikely since sufficient calcium and magnesium are in excess of what can be precipitated as carbonates. A positive RSC indicates that sodium buildup in the soil is possible. The degree of sodium hazard is as shown i the table:

RSC	HAZARD
<0	none.
0-1.25	low, with some removal of calcium and magnesium from irrigation water.
1.25-2.50	medium, with appreciable removal of calcium and magnesium from irrigation water.
>2.50	high, with most calcium and magnesium removed leaving sodium to accumulate.

The RSC value can be used to calculate the amount of gypsum or sulfuric acid to add per acre-foot of irrigation water to neutralize residual carbonates (i.e., to reduce RSC to 0). This can be calculated using the formulas:

RSC * 234 lbs. gypsum/acre-foot

RSC * 133lbs sulfuric acid/acre-foot

 

V. Other Salt-Related Hazards

pH - Where the pH of the irrigation water is greater that 8.4, bicarbonate and sodium ion concentrations are often high. Where the pH of the irrigation water is less than 6.5, iron and sulfate ion cencentrations are typically high. Attempts to adjust the pH of irrigation water should be based on RSC values, not pH alone.

Hardness - This is the sum of the calcium and magnesium concentrations, expressed as gypsum equivalents. To calculate gypsum equivalents from the calcium and magnesium concentrations, multiply each ion's concentration (in mg/L) by its molecular weight (Ca = 40 mg/mole; Mg = 24 mg/mole) divided by the molecular weight of gypsum (100 gm/mole). The hardness of the irrigation water is reflected in the following table:

GYPSUM EQUIVALENT	HARDNESS
<6	soft
6-12	moderately soft
12-18	average hardness
18-24	very hard
>24	extremely hard

The significance of hardness is in the difficulty presented in making soap suds, and with the evaporation of water, lime, or scale deposits in pipes, leading to clogging.

Sulfates - high concentrations could contribute to "black layer" problems under anaerobic conditions. Calcium and magnesium would react with some of all of the sulfate to form gypsum and epsom salts; the presence or addition of lime would prevent acidification while also forming gypsum.

Boron - can be toxic in irrigation water where its concentration exceeds 1 ppm.

Other Ions - Nickel, chromium, mercury, and selenium are typically found in industrial wastes and would constitute a haxard when present in irrigation water.