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soil science › ssac122 soil chemistry soil fertility and nutrient management

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🧫 ASSESSMENT OF

ASSESSMENT OF.

This lesson covers irrigation water quality assessment using salinity, sodicity, and related hazard indicators used in soil and water management.

ASSESSMENT OF IRRIGATION WATER QUALITY

Water quality parameters, management

Irrigation water quality criteria

Water quality is determined according to the purpose for which it will

be used. For irrigation water, the usual criteria include salinity, sodicity, and

ion toxicities.

Various criteria are considered in evaluating the quality of irrigation

water namely:

Salinity hazard
Sodium hazard
Salt index
Alkalinity hazard
Permeability hazard
Specific ion toxicity hazards

SALINITY HAZARD

The concentration of soluble salts in irrigation water can be classified

in terms of Electrical Conductivity (EC) and expressed as dS m [-1] .

There are four classes of salinity viz ., C1, C2, C3 and C4.

The classes C1 and C2 of water are considered suitable for irrigation

purposes (no problem). C3 and C4 classes of water are not suitable for

irrigation purpose (severe problems).

Water class	EC (dS m-1)	Remarks
C1 - Low salinity	0-0.25	Can be used safely
C2 - Medium salinity	0.25-0.75	Can be used with moderate leaching
C3 - High salinity	0.75-2.25	Can be used for irrigation purposes with some management practices
C4 - Very high	2.25-5.00	Can not be used for irrigation purposes

SODICITY HAZARD

High concentrations of sodium are undesirable in water because

sodium adsorbs on to the soil cation exchange sites, causing soil aggregates

to break down (deflocculation), sealing the pores of the soil and making it

impermeable to water flow. The sodicity hazard of irrigation water is usually

evaluated by:

11 (1/8)

Sodium Adsorption Ratio (SAR)
Adjusted SAR
Sodium to calcium activity ratio (SCAR)
Sodium ratio
Figure of merit

Sodium adsorption ratio (SAR)

United States Salinity Laboratory

(USSL) staff introduced the concept of

sodium hazard. It is calculated as

Where all the ions expressed as me L [-1]

The sodium hazard of irrigation water expressed through SAR does

not take into account the effect of anionic composition. Sodicity hazard also

classified as S1, S2, S3 and S4.

Water class	SAR	Remarks
S1 low sodium hazard	0-10	Little or no hazard
S2 medium sodium hazard	10-18	Appreciable hazard but can be used with appropriate management
S3 High sodium hazard	18-26	Unsatisfactory for most of the crops
S4 Very high sodium hazard	> 26	Unsatisfactory for most of the crops

11 (2/8)

Adjusted SAR : To predict sodicity hazard more correctly for those water

which contain appreciable amounts of HCO3 but no RSC. Ayers and Wescot

pointed out that sodicity hazard of these irrigation waters should be

determined by Adjusted SAR to be calculated as follows.

Adj. SAR = SAR (1 + (8.4 - pHc)

Where SAR = Sodium Adsorption Ratio

pHC = (pK2 - pKc) + pCa + p (Alk)

pK2 - pKc = conc. of Ca + Mg + Na in me L [-1]

pCa = Ca in me l [-1 ]

pAlk =from conc. of CO3+ HCO3 in me L [-1] .

The adjusted SAR should be evaluated for such water which have EC

higher than 1.5 and less than 3.0 dS m [-1] because only this group of water

are more likely to have twin problem of RSC and SAR.

Sodium to Calcium Activity Ratio (SCAR)

The application of SAR to the group of water, which have EC > 5 dS

m [-1] and Mg/Ca ratio > 1 is obviously questionable. For the ground water

having EC > 5 dS m [-1] and dominance of magnesium over calcium, the SAR

The classification of SAR/ SCAR ratio was given by Gupta (1986) by

following 6 classes of sodicity.

11 (3/8)

Non-sodic water (< 5)
Normal water (5-10)
Low sodicity water (10-20)
Medium sodicity water (20-30)
High sodicity water (30-40)
Very high sodicity water (>40)

Sodium ratio

Sodium ratio =

For good water, this ratio should not exceed one.

Figure of merit

This term was proposed by Cassidy to express the relative proportion

of divalent to monovalent cation and calculated by

Figure of merit =

Salt index

It is also used for predicting sodium hazard. It is the relation between

Na [+], Ca [2+,] and CaCO3 present in irrigation water.

Salt index = (Total Na) - (total Ca-Ca in CaCO3) x 4.85

Where all ions are to be expressed in ppm. Salt index is negative for

all good water and positive for those unsuitable for irrigation.

Alkalinity hazard is evaluated by

Residual Sodium Carbonate (RSC)

Residual Sodium Bicarbonate (RSBC) 11 (4/8)

Bicarbonates (HCO3

-) occur in low salinity water and its concentration

usually decreases with an increase in EC. The proportion of bicarbonate ion

is higher than calcium ions are considered undesirable, because after

evaporation of irrigation water bicarbonate ions tend to precipitate calcium

ions. Hence, the effect of bicarbonate together with carbonates evaluated

through RSC.

RSC = (CO3

-- + HCO3-) - (Ca2+ + Mg2+), all ions expressed as me L-1.

RSC (me l-1)	Water quality
< 1.25	Water can be used safely
1.25 - 2.5	Water can be used with certain management
> 2.5	Unsuitable for irrigation purposes

Since carbonate ions do not occur very frequently in appreciable

concentrations, and as bicarbonate ions do not precipitate magnesium ions,

Gupta suggested that alkalinity hazard should be determined through the

index called Residual Sodium Bicarbonate (RSBC) to be calculated as

below.

RSBC = HCO3- - Ca 2+, all ions expressed as me L-1.

Based on RSC/ RSBC ratio there are 6 alkalinity classes proposed

Non-alkaline water (-ve)

Normal water (0 me l [-1] )

Low alkalinity water (2.5 me l [-1] )

Medium alkalinity water (2.5-5.0 me l [-1] )

High alkalinity water (5.0-10.0 me l [-1] )

Very high alkalinity water (> 10.0 me l [-1] )

Permeability hazard

High sodium in the irrigation water can cause severe soil permeability

problem. Permeability is affected not only by high sodium but also by CO3

and HCO3- content in water. A part of CO3—and HCO3- is precipitated as

CaCO3 (or) MgCO3 removing Ca and Mg from irrigation water and leads to

increased proportion of solution. The effect on permeability has been

evaluated by the term permeability index, which is calculated as

Permeability index =

Where ions are expressed as me L [-1 ] . If permeability index value exceed 65,

water is considered suitable for irrigation.

SPECIFIC ION TOXICITY HAZARD

Sodium: Among the soluble constituents of irrigation water, sodium is

considered most hazardous. Excess of sodium ions characterizes the water

as saline or alkaline depending upon its occurrence in association with

chloride/ sulphate or carbonate/ bicarbonate ions. For some time in the past,

the quality of irrigation water used to be evaluated with respect to sodium

based on soluble sodium percentage (SSP) calculated as below.

SSP =

It has been useful in characterizing water, since a high value indicates

soft water and low value hard water . When water with excess of sodium

11 (6/8)

(SSP=66) is used for irrigation, part of it is adsorbed by the soil. Both, soils

and plants are adversely affected by high sodium irrigation water. Sodium

soils are relatively impermeable to air and water. They are hard when dry,

difficult to till and plastic and sticky when wet. These adverse physical

conditions prevent germination and are generally unfavourable for plant

growth. Even though, sodium is not as essential as other nutrients, it is

taken up freely by many plants and it may be specifically toxic to plants.

Magnesium : It is believed that one of the important qualitative criteria

in judging the irrigation water is its Mg content in relation to total divalent

cations, since high Mg content in relation to total divalent cations, since high

Mg adsorption by soils affects their physical properties. A harmful effect on

soils appears when Ca: Mg ratio decline below 50.

Mg Adsorption Ratio =

Chlorides : The occurrence of chloride ions in irrigation water

increases with increase in EC and sodium ions. Therefore, these ions are

most dominant in very high salinity water. Unlike sodium ions, the chloride

ions neither affect on the physical properties of the soil, nor are adsorbed by

the soil. Therefore, it has generally not been included in modern

classification system. However, it is used as a factor in some regional water

classification.

Chloride

Concentration (me L [-1] ) =

Chloride concentration (me l-1)	Water quality
4	Excellent water
4-7	Moderately good water 11 (7/8)
7-12	Slightly usable
12-20	Not suitable
> 20	Not suitable

Sulphate: Sulphate salts are less harmful when compared to chlorides.

This is because when both the ions occur in this concentration, only half of

the sulphate ions contribute to salinity due to the fact that approximately half

of the sulphates gets precipitated as CaSO4 while the other half remains in

soluble form as Na-MgSO4 in the soil. That is the reason, the potential

salinity of irrigation is calculated as Cl [-] + ½ SO4

Eaton proposed three classes for sulphate

< 4 me l [-1] - Excellent water

4-12 me l [-1] - Good to injurious

--.

- 12 me l [-1]    - Injurious to unsatisfactory

Potential salinity

It can be worked out by using the formula Cl + 1/2 SO4

are expressed in me l [-1] .

2- where ions

Potential salinity (me L-1)	Remarks
3-15 3-7	Can be recommended for medium permeability soils Recommended for soils of low permeability

Boron : It is evident that boron is essential for the normal growth of the

plant, but the amount required is very small. The occurrence of boron in

toxic concentration in certain irrigation water makes it necessary to consider

this element in assessing the water quality. The permissible limits of boron

in irrigation water are:

Boron class	Crops	Col3	Col4	Remarks
Boron class	Sensitive	Semi-tolerant	Tolerant	Tolerant
Very low Low Medium	< 0.33 0.33-0.67 0.67-1.00	< 0.67 0.67-1.33 1.33-2.00	< 1.00 1-2.0 2.0-3.0	For safely use Can be managed Unsuitable

Fluorine: fluorides are only sparingly soluble and are in only small

amounts. The concentration of fluoride ranges from traces to more than 10

mg L [-1] in natural water, and surface water do not exceed 0.3 mg L [-1] unless

they are polluted. Irrigation with fluoride saline water (upto 25 mg L [-1] ) has

not been found to affect yield of wheat. Therefore, it is doubtful if fluoride

requires any monitoring in India. At present, the average concentration of

fluoride has not been observed to be very high (10 mg l [-1] ).

Nitrate: Very frequently ground water contain high amount of nitrate.

When such type of irrigation water is applied on soils continuously, various

properties of soils are affected.

< 5 No problem

NO3 me l [-1] 5-30 Intensity of problem is moderate

30 Intensity of problem is severe

Lithium : Lithium is a trace element may be found in most of saline

ground water and irrigated soils. It has been found that 0.05-0.1 ppm of

lithium in water produce toxic effects on growth of citrus. It has also been

reported that saline soils of varying degrees found in India contain lithium

11 (8/8)

upto 2.5 ppm. Fortunately, the germination of majority of crops is not

affected with this level of lithium content.

Summary Cheat Sheet

Key Recall Points

ASSESSMENT OF is exam-relevant for SSAC122 and objective questions in soil science.
Use soil-test based interpretation with focus on pH, CEC, and nutrient availability.
Apply the 4R principle: right source, right rate, right time, and right method.

Exam Traps

Do not mix up soil fertility concepts with fertilizer quantity alone.
Numerical and term-based questions often test definitions, units, and threshold values.
In problem-solving, interpretation must follow soil reaction, crop stage, and management context.

References

3 sources • [1] [2] [3]

[1]

ICAR e-Course: Soil Chemistry, Soil Fertility and Nutrient Management

Official

[2]

Brady and Weil, The Nature and Properties of Soils

Book

[3]

Indian Council of Agricultural Research