IRRIGATION WATER ANALYSIS - WATER QUALITY GUIDE

Testing for Irrigation Suitability

Your water report can be separated into two distinct parts, quality and nutrient availability.

QUALITY

The pH expresses the acidity or alkalinity of water. A pH reading of less than 7 is acidic, and above 7 is considered alkaline. Most waters range form 7 to 8.5. Some stream waters may be as acidic as 6.5. Electrical Conductivity (EC) reflects the total salt content of the water. Chemically pure water does not conduct electricity, but water with salts dissolved in it does. EC can detect two distinct possible water-soil conditions. 1) Salinity or salt condition. One of the hazards of irrigation is the accumulation of soluble salts in the root zone when irrigated with waters with high EC. 2) Permeability problems or poor water intake caused by a condition in which the sodium proportion attached to the clay increased and the salt tends to disperse or run together, bringing about reduced rates of water penetration. See following table 1.

 

Table 1: EC, and General Water Quality
  No Problems
EC mmhos/cm		 Increasing Problems
EC mmhos/cm		 Severe Problems
EC mmhos/cm
For Salt
Conditions
<0.75
0.75-3.0
>3.0
For Poor Water
Intake Conditions
<0.5
0.5-2.0
>2.0


Table 2: Potential Bicarbonate Hazard (ppm HCO3)
None/slight
Moderate
Severe
Very Severe
0-120
121-180
181-600
>600



Table 3: Problems of Sodium in Irrigation Water
None
Moderate
Severe
Very Severe
0-69
70-180
181-200
>200


Table 4: The Potential of Problems Based Upon SAR Level.
None
Moderate
Severe
Very Severe
0-2
3-5
6-8
>9


Table 5: Boron & Chloride
Rating
Cl ppm
B ppm
Safe
0-70
<0.5
Increasing Problems
71-300
0.6-1.0
Problems Likely
>300
>1.0

COMPONENTS OF IRRIGATION WATER

Carbonate (CO3): Since calcium (Ca) and magnesium (Mg) carbonates are relatively insoluble, high carbonate waters mean the cations associated with them are likely to be sodium or potassium. They are almost always sodium. Upon drying in the soil, the carbonate ion will remove Ca and Mg from the clay similar to bicarbonate, and an alkali soil will develop. Carbonate content of water will usually be less than 10-15 ppm.

Bicarbonate (HCO3): Large amounts of bicarb ions in irrigation water will, upon drying in the soil, precipitate Ca thereby removing it. This leaves sodium (Na) in place of Ca. In this way, a Ca dominant soil can become a Na dominant soil by use of a high bicarbonate irrigation water. See Table 2.

Calcium (Ca): Ca is an essential plant nutrient and is usually found in soils as limestone (calcium carbonate), gypsum (calcium sulfate), and in various forms of calcium phosphates. A soil containing appreciable quantities of Ca is friable and easily worked, and it permits water to infiltrate into the soil. Excess Ca and Mg can be a hazard due to the effects of soil pH.

Magnesium (Mg): Mg is also an essential plant nutrient. It is similar in reaction to Ca and is generally abundant in soils. A common source of Mg is dolomitic limestone.

Sulfate (SO4): Sulfate is usually found in nature in the form of gypsum which is calcium sulfate. Sulfate is an essential plant nutrient and is readily available to plants.

Sodium (Na): Na is not generally considered an essential plant nutrient. All of the common salts of Na are water soluble. Na is the most troublesome of the major constituents in irrigation water. If Na is too high, the soil disperses or 'runs together' and becomes very tight and water will not easily penetrate. Problems of Na in irrigation water are reflected in Table 3 at left.

Sodium Adsorption Rate (SAR) adj: The SAR indicates the activity of the Na ions as they react with the clay. Adjusted (adj) SAR includes the added effects of the precipitation of Ca related to carbonates and bicarbonates. The potential of problems associated with SAR levels is shown in Table 4 at left.

Available Nutrients in Irrigation Water
Table 6: Plant Nutrient Availability - (results in ppm)
Rating Ca Mg K P N NO3 SO4 S
Low <20 <10 <5 <0.1 <1 <5 <30 <10
Normal 20-60 10-25 5-20 0.1-0.4 1-10 5-50 30-90 10-30
High 60-80 25-35 20-30 0.5-0.8 10-20 50-100 90-180 30-60
Very High >80 >35 >30 >0.8 >20 >100 >180 >60



Link to - "Proper Sampling Methods" to ensure best results from water analysis

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