The Quality of irrigation water
The quality of water and its characteristics influence the plant’s growth, the soil structure, and also the irrigation system itself. The quality of irrigation water refers mainly to its physical and chemical composition, or more in details to the mineral composition of water and to the presence of solid and organic substances in it. Both chemical and physical properties (murkiness, presence of seaweed etc.) determin the suitability of water for irrigation. The parameters for irrigation water widely differ from the drinking water ones; moreover, quality parameters can vary depending on the crop: they react differently to specific minerals or water properties. To observe the characteristics of the water it is necessary to use laboratory chemical analysis. Parameters or chemical properties that determine the quality of water for irrigation are:
– Water hardness;
– Water salinity;
– Water alkalinity;
– The relationship between sodium, calcium and magnesium;
Water hardness is basically the sum of calcium and magnesium concentrations in water expressed in ppm (parts per million) of CaCO3. Calcium and magnesium are both essential nutrients for plants and their adequate concentration in water is beneficial. However, when the water hardness is too high, precipitation of calcium and magnesium salts may occur in the irrigation system. (A too low level of hardness could cause corrosion in the irrigation system).
Salinity of the water
Salinity is one of the water parameters with which farmers are most familiar. A too high level of salinity reduces the plant’s ability to absorb water. This could lead to reduced yields, withering and burnt leaves and other symptoms. Water salinity is measured as TDS (total dissolved salts) or as electrical conductivity (CE). Both measurements refer to the total concentration of dissolved salts in the water.
The pH of the water influences the solubility of the mineral salts. Undissolved minerals are not available for plants, as plants can only absorb minerals from an aqueous solution, IN IONS FORM, directly from water or soil solution. Most nutrients are available at a 5.5 to 6.5 pH range. Because of its infinite volume it is very difficult to influence the pH of the soil by controlling the pH of the irrigation water. Therefore, adjusting the pH of the water is important in the following cases:
– To avoid clogging of the emitters (e.g. in drip irrigation) due to mineral precipitation. For example, calcium carbonate;
– In hydroponics and soilless soils, where the pH of the irrigation water directly influences the availability of nutrients;
– When frequent irrigation is applied to soil. In this case, the pH of the water can affect nutrient uptake.
Alkalinity of water
Alkalinity is a measure of water’s ability to resist pH changes. It is calculated as the sum of carbonic acid (H 2 CO 3), bicarbonates (HCO 3 -) and carbonates (CO 3 2- ) in water. It is considered an important parameter of irrigation water quality, as it is much more difficult to lower the pH of water with high alkalinity than to lower the pH of water with low alkalinity, even if both have the same initial pH level. This can affect the availability of many nutrients. Alkalinity, like hardness, is expressed in ppm CaCO3.
Relationship between sodium, calcium and magnesium
SAR is an irrigation water quality parameter that helps to estimate the potential of sodium in the water to absorb soil particles, in relation to calcium and magnesium. Irrigation with water with SAR values equal to or greater than 10 may result in loss of soil structure and infiltration capacity. This is particularly true for soils with a relatively high clay concentration.