Knowledge of the nature and concentration of salts in irrigation water is essential for proper crop nutrition. To understand irrigation water quality, one must understand salinity. Salinity is the amount of dissolved salt in water. The amount of salt will determine the amount of energy a plant will need to use to pull water in. High salinity levels are directly correlated with greater risks to cultivation since it would present greater difficulties for plants to extract water from the soil.
To analyze the suitability of irrigation water, the total amount of salts dissolved must be considered. This is due to different salts differing impacts on water and soil.
Less soluble salts (calcium carbonate, magnesium carbonate, and calcium sulfate) occur when irrigation water is added to the soil. This results in a wet medium (soil solution) with differing salinity.
Understanding the concentration of salts in the soil would enable farmers to adjust nutrient balances to favor their absorption. That way, nutritional demands can be effectively and efficiently met throughout different plant stages.
For example, calcium and magnesium. They bare an antagonistic relationship with one another. When a plant needs calcium, equal parts of magnesium will need to be increased to balance the fertilizer solution. A balancing act occurs between the metals and salts in the soil. A high concentration of one element could cause problems with absorption. Since the crop needs must be covered exclusively through fertilization, balancing the solution should not be overlooked since this would greatly restrict the absorption of important elements such as calcium and magnesium.
Nitrogen is a synergistic macroelement it works with other macroelements, i.e., potassium, calcium, and magnesium. Therefore, it should be applied alongside these other compounds for better absorption.
Nitrogen is rarely found in irrigation water; however, Nitrates (a form of nitrogen) are. Nitrates compete with potassium, calcium, and magnesium absorption due to their electrochemical properties. As much as nitrates compete with the aforementioned chemicals, Nitrates increase phosphorus absorption. Close monitoring of Nitrates is advisable to achieve balanced nutrition.
Phosphorus is generally found in very low concentrations in irrigation water. When added to fertilizer solution, negative interactions with some microelements (copper, zinc, and iron) and macroelements (calcium) are advised to monitor due to alterations in plant transport mechanisms and soil pH.
Since chloride and sodium can accumulate in excess in plant tissues and soil, they must be monitored. Their excess can cause physiological disorders, especially in sensitive species. This effect must be counteracted by balancing calcium, potassium, sulfate, or nitrate concentrations. Likewise, it is important to monitor the natural presence of high concentrations of other elements that can give rise to phytotoxicity problems (i.e., heavy metals and microelements). Balancing the solutions is limited to pH correction.
Precaution must be monitored for the concentration of bicarbonate, chlorine, and sodium in the water when administered as a spray (in addition to high temperatures and relative humidity below 30%). Burns may occur under these conditions. This practice is common in avocado cultivation to counteract conditions of excessive transpiration.
Certain elements in irrigation water will condition the forms of fertilizers and influence production costs. Some soil solutions will have a higher condition for certain fertilizers due to the appearance of high concentrations of certain elements, i.e., chloride and sulfate.
Irrigation Water Qualities Influence on Soil
In addition to certain elements’ effects on crop nutrition, the effect on other important elements, such as the soil and its structure, must be monitored. Maintaining soil structure is critical for achieving efficient crop nutrition since it guarantees an adequate water-air balance, generating favorable environments for proper growth and functioning of the roots. Using water with inadequate sodium absorption ratios (RAS) favors soil compaction. The SAR values that determine the loss of soil permeability are linked to other parameters such as its texture or organic matter content (greater risk in loamy soils with little organic matter).
Take into account pH. It influences the availability of nutrients in the soil since it notably influences the definition of its pH values. In slightly acidic environments, adequate absorption of most nutrients is achieved.
For this reason, special care must be taken when using excessively acidic irrigation water. Once the soil’s buffering capacity is neutralized, an unfavorable environment can occur where nutrients are harder to absorb: the root system becomes toxic, and the solid structure becomes displaced (due to the displacement of calcium and magnesium). Water is an element that greatly influences soil structure; one must monitor the pH and dissolved salts to prevent its degradation.
Using water with a high pH also determines the absorption of nutrients. It may even modify the pH of the sap, reducing the availability of absorbed elements such as iron. To prevent these problems, it is advised to properly assess the crop requirements, the pH, and the content of active limestone in the soil, to adjust the pH of the final fertilizer solution. When an increase in pH is required, some type of caustic salt or carbonate can be used. Otherwise, the bicarbonate content of the water must be corrected by adding acids.
Consider irrigation water quality and its biological activity.
It is common for conditions to be influenced by the development of algae, which can later cause problems in filtration or irrigation emitters. Its influence is linked to pH and dissolved oxygen parameters in this case. Monitoring over time is crucial to anticipate algae development and implement preventive measures. On the other hand, it can be used to adjust the application dose of algaecide treatments and measure their effectiveness.
Another parameter to monitor is the redox potential. The redox potential is a good indicator of the water’s degree of disinfection. The colonization of the pipe network by bacteria is frequent, forming biofilm accumulations. These usually come off and generate blockages in the irrigation emitters. Therefore, preventing their activity through monitoring the redox potential is crucial.
In conclusion, irrigation water quality can be an important conditioning factor for the profitability of the crop, either because of the direct effects it may have on the plant or the environment where it grows. Or because of the associated costs of using poor quality water. Understanding irrigation water quality is important to take appropriate actions for agricultural projects and optimize them.