Send your irrigation water to FAS for testing
Testing your water quality can improve soil health
Poor irrigation water quality, particularly salt-affected sources, can cause soil degradation and induce crop stress, resulting in a decline in crop yield. Most commonly, water quality is classed on its potential to lead to saline and sodic conditions in the soil. Excessive amounts of soluble salts (salinity) in water can cause an increase in soil salt content and increase crop water stress, while unfavourable Na levels can also lead to sodic soil conditions that lead to poor soil physical conditions. Remediation of salt affected soils can be costly and should be prevented from occurring. An essential requirement to prevent the buildup of salts in soil is good drainage and periodic flushing with good quality water.
To better manage potential negative impacts of irrigation water, it is good practice to regularly monitor irrigation water quality. Ideally, water sampling should occur in each season to gain an understanding of seasonal shifts in water quality and guide practice to improve scheduling and use of the irrigation water throughout the year. Where this is not possible, attempt to get at least a wet and dry season water sample analysed. Essential parameters, as assessed by FAS Agricultural Laboratory are:
· Water pH: This is an indicator of the acidity or alkalinity of the water. Ideally good irrigation water will have a pH of 6.5 to 7.
· Soluble base cations: The concentration of Ca, Mg and Na in the water is used to determine the sodium adsorption ratio (SAR).
· Alkalinity (HCO3): Excessive alkalinity is used to adjust the SAR value for the impact of the excess bicarbonate in solution.
· Electrical conductivity (EC): An indicator of the amount of total dissolved salt of water.
· Effective EC (EEC): This is an adjusted EC to account for the diluting effect of rainfall received in conjunction with the irrigation amounts.
· Sodium adsorption ratio (SAR): High SAR values indicate a sodicity impact risk of using this water.
· Adjusted SAR (ASAR): This accounts for the alkaline ions present that can precipitate Ca and Mg, effectively increasing the SAR.
The water quality is classed according to the relationship given in the figure below. It is important to note that the SASRI classification is based on the EEC and ASAR that account for effects of rainfall dilution and residual alkalinity, respectively, on the effective water quality. Depending on your soil type and water quality class, the water may be used with certain limitations.
The figure above shows the relationship between the Adjusted Sodium Adsorption Ratio (ASAR) and Effective Electrical Conductivity (EEC) on suitability of water for irrigation. The quality class determines the suitability for different soil types and conditions. The white lines in the graph define the assigned water quality class for reporting purposes, while the green-yellow-red gradient transitions highlight the increasing risk gradient across these categories (see Important considerations when interpreting the water class).
Further information and guidance on managing irrigation water quality and salt affected soils is available in Information Sheet 5.12 (Water quality for Soil Health) and 5.11 (Soil Salinity and Sodicity) (available from www.sasri.org.za and navigate to Knowledge Hub) or visit our website for sampling guidelines and submission forms www.fasagrilab.co.za.
Important considerations when interpreting the water class:
When a water sample is classified it is assigned to one of the water quality classes based on the discrete categories shown in the above figure. However, this can lead to confusion as to the suitability of the water for irrigation under different conditions, especially where samples lie near category class borders. To better guide the suitability and risk associated with using the water it is useful to evaluate where the sample lies relative to the colour gradients shown in the figure. Samples in the darker green regions indicate low risk except in very dispersive clay soils. Samples in the light green to yellow indicate increasing risk to all dispersive and poorly drained soils (and drainage is advised). The orange to red transition indicates very high risk to soil quality and the water should not be used for irrigation purposes without treatment.
