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Every irrigation water contains some quantity of dissolved substances or "salts” ranging from very low to very high. These dissolved materials come from the long-term weathering or break down of soil minerals. They dissolve and find their way into surface or ground water during the recharge process. The relative kinds and amounts of these dissolved substances affect the suitability of water for irrigation.
These dissolved materials are found only in solution as "ions". Examples of ions include: calcium (Ca2+), sodium (Na+), chloride (Cl-), and sulfate (SO42-). When the waters dry during evaporation, the ions combine and precipitate as solid "salts".
Examples of common salts that might precipitate from irrigation water are: CaCO3 (lime), NaCl (table salt), CaSO4 (gypsum), and MgSO4 (Epsom salt).
Various soil properties and crop growth are affected by both the amount and composition of ions in the water used as an irrigation source.
“Parts per million (ppm)” is a common way of expressing chemical concentrations. One ppm is 1 part of substance in 1,000,000 parts of water. One percent (1%) is equivalent to 10,000 ppm.
Liquid concentrations are expressed as “milligrams per liter (mg/L)”, which is 1/1000th of a gram in 1,000 grams of water, equivalent to 1 part per million. Trace concentrations may be expressed as “micrograms per liter (μg/L)”, equivalent to “parts per billion (ppb)”.
The concentrations of these substances as “pounds per acre-foot (lb/ac-ft)” is calculated by multiplying the concentration as mg/l by 2.72. Multiplying mg/l by 0.2267 gives pounds per acre-inch (lb./ac-in). Milliequivalents per liter (meq/l) is another way of expressing chemical concentration and is used in certain lab calculations.
Electrical conductivity (EC) is a common way to estimate the total “salt” content of a water sample. It is obtained by measuring the ability of the water sample to conduct an electric current. Conductivity may also be known as “specific conductance”.
Conductivity may be expressed in several ways. Common expressions are:
These units are related as follows.:
“Total dissolved solids” (TDS) is another way of expressing total salinity. TDS is measured by filtering and evaporating a water sample to dryness, then measuring the residue that remains. We commonly calculate a TDS equivalent from the conductivity measurement. TDS can be estimated as:
Calcium and magnesium ions on the exchange surfaces of clay particles help individual soil clay particles to “aggregate” or “clump together” to form “aggregates” or “granules”. Aggregation forms pore spaces, improves the soil structure, allows good soil permeability, and promotes water infiltration.
Excess sodium ions (Na+) that accumulate on those exchange surfaces can reverse the process, causing soil aggregates to “disperse” (or “deflocculate”), effectively breaking apart into their individual clay particles. This destroys soil pore spaces and breaks down soil structure. Soil permeability, infiltration, and percolation are reduced when this occcurs. Water percolation slows, water runoff increases, and crop growth can suffer.
Calcium and magnesium offset the adverse effects of sodium on the soil structure, so the ratio of calcium-plus- magnesium to sodium in the irrigation water is important. This relationship is commonly expressed in the calculation of the “sodium adsorption ratio (SAR)”.
Dissolved Mg and K are readily available as soil nutrients. The pounds per acre-foot of each nutrient can be subtracted from the fertilizer recommendation. Magnesium fertilizer is not normally needed if the irrigation water has over 8 ppm Mg.
Carbonates are nearly insoluble, so are found only in small amounts in most water sources. Bicarbonate is common in many waters and is found at higher concentrations than carbonates. Dissolved bicarbonates and carbonates combine with calcium and magnesium ions as an irrigated soil approaches dryness. They typically precipitate as calcium and magnesium carbonates (CaCO3, MgCO3) or “lime”.
This lime can increase the soil pH in some situations. We can calculate a "lime equivalent" from bicarbonate present in the irrigation water by multiplying the bicarbonate content (as mg/L) by 0.62.High bicarbonate levels contribute to the sodium effect on soil structure.
When calcium and magnesium ions precipitate as insoluble carbonates, they are removed from both the soil solution and surface of clay particles. Dissolved sodium ions then fills the clay surface sites previously occupied by calcium and magnesium ions, so soil structure deteriorates.
The SARa calculation includes the concentrations of calcium, magnesium, sodium and bicarbonate to more accurately predict the impact of irrigation water sodium on the soil.
Chloride is found in both the soil and irrigation water. It can be absorbed by either roots or foliage. Excessive accumulations of chloride in plant tissue can be toxic. Certain crops and varieties can prevent toxicity by blocking chloride movement from the roots to the foliage.
Under sprinkler irrigation, water evaporates from droplets on the leaf surface, concentrating chloride in those droplets. Chloride is absorbed through the leaf cuticle, accumulates in the leaf tissue, and may contribute to eventual plant toxicity.
Some waters contain large amounts of sulfate-sulfur (SO4-S). Sulfate is mobile in the soil and is the sulfur form readily used by plants. Irrigation water sulfate may be high enough to meet crop sulfur needs, especially later planted crops on heavy textured soils. Sulfur is needed by young plants, so crops growing before first irrigation may need fertilizer sulfur. These needs are best measured by soil testing.
Sulfate does not affect the soil except to contribute to salinity. In excess, it may have a laxative effect on humans or livestock that drink the water.
Some waters contain large amounts of nitrate-nitrogen. Like sulfate, nitrate is mobile and readily used by plants. The nitrate-nitrogen found in irrigation water can substitute for fertilizer nitrogen. In some areas the irrigation water nitrate can meet the entire crop nitrogen requirement.
Boron is an essential plant nutrient, but is needed in only small amounts. Boron is relatively immobile in soils. Soils irrigated with high boron waters can accumulate boron over time. If soil boron levels become excessive, certain crops can show toxicity symptoms.
Boron fertilizer is often not required for irrigated crops. Many Great Plains irrigation waters contain enough boron to meet crop needs.
(See also Crop File 4.03.014, "Toxic Ions in Salt Affected Soils" and Crop File 5.01.012, "Irrigation Water Quality: Interpreting Laboratory Results")
James, Hanks, & Jurinak. 1982. Modern Irrigated Soils. Wiley-Interscience Publishers. pp. 136-213.
Tanji, K.K. (ed.). 1990. Agricultural Salinity Assessment and Management. ASCE Manual and Report on Engineering Practice, No. 71. Amer. Soc. of Civil Engineers., New York. pp. 42-63, 138-160, 220-236.
Bresler, McNeal, & Carter. 1982. Saline and Sodic Soils: Priciples-Dynamics-Modeling. Springer-Verlag Publishers, New York. pp. 167-169, 181-185.
Hanson, Grattan, & Fulton. 1993. Agricultural Salinity and Drainage: A Handbook for Water Managers. Water Mgmt. Series, Pub. No. 93-01. Univ. of California Coop. Ext. Davis CA. pp. 1-36