The quality and suitability of irrigation water are determined by the severity of potential problems of the system water-plant-soil after prolonged use
The most common risks, which form the basis for evaluating the effects of water quality, are the related to:
- Infiltration and runoffs
- Toxicity of specific ions
Irrigation water applied to soil partially infiltrates the soil, while the remaining irrigation water is retained by the soil itself. The retained portion is what contributes salts to the soil. This concentration of salts is due, on the one hand, to direct evaporation, and, on the other, to extraction of water by the plants’ roots. The magnitude of salt accumulation in the soil depends upon water quality, as well as irrigation management and drainage efficiency. Excess salt accumulation in the soil’s root zone (salinization), affects the crop’s absorption of water through osmotic mechanisms, increasing the plant’s consumption of energy for extracting water, and thus resulting in lower yields.
The concentration of these dissolved salts in the soil affects the suction effort that a plant has to exert in order to absorb water. In addition to salt concentrations in water, one must consider their concentration in fertilizers. If the water quality is poor, lower concentrations of fertilizer should be used, with more frequent applications than if the water quality were better.
When evaluating salinity, the quantity of ions in the water should be determined. Some of the factors that have been used for quantifying them have been:
- Total dissolved solids (mg/l)
- Total salinity (meq/l)
- Electrical conductivity (dS/m)
Water infiltration and runoffs
When the pace of irrigation water infiltration reduces appreciably, or when water remains on top of the soil for too long, or when water infiltrates very slowly, the water consumed by the crop does not replenish. In such case, the crop does not receive the water it needs to produce an acceptable harvest. Infiltration of water in the soil depends on water quality, as well as on the soil’s physical characteristics (such as structure, degree of compacting, organic matter content, texture, type of minerals in the clays) and on the soil’s chemical characteristics, such as exchangeable cations.
Losses occur due to:
- Losses due to runoff or drainage. When there is an intense precipitation, many soils lack the capability to absorb all the water that has fallen. If the terrain is on an incline, the water does not infiltrate, but runs along the surface. On the other hand, water that has penetrated the soil, if the soil cannot absorb it, is dragged down by gravity to deeper zones. That loss of excess water due to gravity is called drainage.
- Losses due to evaporation. A portion of the water held in the soil rises and is lost in the form of water vapor.
- Losses from transpiration of weeds.
Calcium and magnesium are the most abundant cations dissolved in the soil or involved the soil change process in arid and semiarid zones where irrigation is used. Frequently, irrigation waters contain high concentrations of sodium, which tends to partially replace divalent cations in the change process. Some physical properties of soils, such as particulate dispersion, soil structure, and stability of the aggregate, are closely tied to the types of exchangeable ions. Divalent ions (and calcium in particular) provide physical characteristics that are favorable to soils. Absorbed sodium, on the other hand, results in dispersion and swelling of clays, and when its proportion is too high, causes dispersion of the soil and reduces its permeability.
The quality factors that tend to have an influence on infiltration are total salt content (salinity) and sodium content in relation to the content of calcium and magnesium (sodicity). In general, infiltration increases with salinity and decreases as salinity diminishes or as sodium content increases in relation to calcium and magnesium.
The indexes proposed for evaluating problems related to water infiltration in the soil, geared toward predicting the risk of sodification or alkalization that irrigation waters might provoke, have undergone greater and more significant changes than what is seen in the case of salinity. These changes endeavor to also account for factors involving precipitation and dissolution occurring in the soil. They also seek to establish a relationship with the percentage of exchangeable sodium.
When soluble salts are present in the root zone, certain specific ions are absorbed that, accumulated in plant tissues, could end up provoking toxic effects for the plant. They hinder normal development, resulting in a reduction of yields, and in more acute phases, could even cause the plant’s death.
In irrigation waters, the most common ions that might cause toxicity problems are chloride, sodium and boron, although there is a notable variation in toxicity thresholds among the various plant species. Certain oligoelements (for example: Mn, Zn, Cu, Ni, Co, Cd) can also be toxic for plants and/or for human beings even in very low concentrations if they accumulate in the parts of the plant utilized as food. The presence of these oligoelements tends to be minimal in natural waters, but not so in wastewaters that, subsequent to various degrees of treatment, might be used for irrigation or be discharged into surface waterways and later used for irrigation.
Ions that affect irrigation water quality
- Chloride ion: Chloride ions favor chlorosis, which is accentuated in the plant parts that receive the most light, potentially degenerating into a necrosis of leaf edges. Concentrations in excess of 0.5 grams per liter are considered dangerous in most soils.
- Sodium ion: Sodium ions can replace caesium and magnesium ions in the clay-humic complex. The terrain can also lose its structure, becoming impermeable, and end up with a pasty consistency.
- Boron ion: The boron ion is highly toxic for most plants when concentrations exceed 2 mg/l. This pernicious effect is accentuated when sprinkler irrigation is used, given that the leaves are thus directly attacked. Boron is essential, however, for plant development, since it aids in cell wall formation.
- Sulfate ion: Irrigation using waters that have a high sulfate content restricts root development and production. This problem is aggravated if sulfate-rich fertilizers are used.
- Calcium ion: Calcium plays an important role in plant life, since it is involved in root growth, provides a better tissue consistency, participates in enzymatic activity, etc. A calcium deficiency can provoke chlorosis and restrict root growth. In combination with boron, it forms calcium borate complexes that have little solubility.
- Magnesium ion: The magnesium ion forms a part of chlorophyll. It is thus involved in carbohydrate formation, increases plant resistance to an adverse environment, and facilitates the fixation of atmospheric nitrogen. Magnesium insufficiencies cause yellowing of the leaves, making them less resilient.
- Potassium ion: The potassium ion regulates enzymatic functions in plants, is involved in photosynthesis, etc. A potassium deficiency in the plant results in delayed growth and diminishes the harvest’s quantity, quality, and conservation. If the plant absorbs too much potassium, it will suffer from deficiencies of magnesium and calcium.
- Phosphate ion: The phosphate ion is involved in processes of growth and synthesis of plant components. Its deficiency occasions weak plant development.
- Nitrogen: Nitrogen is the most basic element for plants, since nitrogen forms a part of proteins and other essential organic compounds. Plants absorb nitrogen in the form of nitrates and ammonium. A nitrogen deficiency affects growth, while an excess of nitrogen greatly increases plant development but dangerously lowers the quality of the fruit and creates deficiencies in other nutrients.
At AGQ Labs we are specialists in agricultural chemistry. The great value that AGQ Labs contributes to the agricultural sector lies in combining this knowledge of chemistry with specialized engineering. Our team of agronomists is comprised by professionals with experience at the world’s finest farms. All of our agronomical decisions are based on analytical information obtained from our internally developed monitoring and control procedures. We are experts in the control of soil-plant-water systems.
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