PHOENIX, AZ July 2012 - It is projected that the world’s population will expand to over 8.5 billion by 2025.  To keep up with this growth agricultural production must increase by up to 50 percent.  The increase in agricultural production is expected to be met by these three ways:  1) increase in average yields, 2) increase cropping intensity (double or triple cropping fields), and 3) increase the amount of arable land.  It is estimated that the potential arable land in the world is over 8,000 million acres of which 46 percent or 3,700 million acres are currently farmed.  Bringing into production new lands will be expensive due to either high cost of irrigation development (most of the best land and water supplies are already taken), lack of available water, cost of new irrigation projects (world-wide some projects have had mixed performance results), and improper irrigation that degrades the quality of existing irrigated cropland.  The two most common processes that degrade cropland are waterlogging and salinity. To manage soil salinity there are a few things you need to know.

What is Salinity?

Salinity is defined as the total concentration of dissolved mineral solutes in water and or soils.  Solutes are comprised of positive charged (cations) elements and negative charged (anions) elements.  The more common cations include sodium, calcium, and magnesium.  While the more common anions include sulfate, chloride, bicarbonate, and carbonate.

Salinity can be measured on a concentration or chemical equivalent basis.  Concentration is sometimes expressed at parts per million (PPM) or milligrams per liter (mg/l) and often used for measuring salinity in water.  Be aware that 1 PPM equals 1 mg/l which also equals 1 mg/kg.  The more preferred measurement expression is on a chemical equivalent basis where the term Electrical Conductivity or EC is used to express salinity.  The units typically used for EC are decisiemens per meter (dS/m) or millimohs per centimeter (mmhos/cm).  However, it really does not matter which term is used because 1 dS/m is equal to 1 mmhos/cm—one of the more convenient features of the metric system.

Know Your Crops Salinity Tolerance

Plant salinity tolerance varies between crops.  Crops such as cotton and barley can produce acceptable yields at much greater soil salinity levels than crops such as corn and soybeans. This is because certain crops can make osmotic adjustments that enable them to extract more water from a saline soil. When soil salinity cannot be controlled at an acceptable concentration for the planned crop, an alternative more salt tolerant crop can be selected and that can produce economical yields. If interested, click on “crop salinity tolerances ” to view a table of various crops and salt levels where yield reductions occur.

Know Your Soils Salinity Level

It is important to know the electrical conductivity of your soils.  Electrical conductivity (EC) for soils is shown as EC_e where the e is for the saturated paste extract. Most labs report salinity (EC_e) in dS/m. Other soil important soil characteristics include cation exchange capacity (CEC) and pH.  Cation Exchange Capacity is an important measure of the soils fertility and potential productivity.  Typically, soils with higher silt and clay content have higher CEC values while soils with high sand content have low CEC values.  Soils with good CEC values (>10) have a greater ability to attract and hold positive charged particles (cations) which include the salt cations.  Soil reaction or pH is an important measure of nutrient availability, solubility of toxic elements, and microbial activity. 

Know Your Waters Salinity Level

Depending on the lab used water tests may be report salinity in parts per million (PPM), milligrams per liter (mg/l), or as electrical conductivity of the water (EC_w). However, some labs report salinity in Total Dissolved Salts (TDS). The table below provides some general interpretations for irrigation water quality. Use restrictions are dependent on the type of crop.

Knowing the salinity of your irrigation water is essential to determine how much salt is being applied with irrigation.  This is referred to as “salt loading”.  Salt loading is an expression of how much salt is being applied based on concentration (PPM) multiplied by the volume of water applied.  Determining the amount of salt applied with irrigation requires using the conversion factor of 1 mg/l = 2.72 lbs/acre foot of water applied.  Example: irrigation water has a salt concentration of 1270 PPM (remember PPM=mg/l=mg/kg) you multiply 2.72 X 1270 = 3,454 lbs or 1.7 tons of salt per acre foot of water applied.  Sounds like a lot but remember the soil has a tremendous buffering capacity.

NRCS Role

NRCS considers salinity to be a resource problem when the soil electrical conductivity exceeds the level at which crop yields decline by 10 percent or more. To determine if salinity is a resource problem the NRCS planner needs to know the crops you plant, typical yields, water test results, and soil test results. 

NRCS practices that help manage salinity properly includes: Irrigation Water Management-with the appropriate leaching requirement, Salinity & Sodic Soil Management, Irrigation Land Leveling, Irrigation Systems (Sprinkler, Surface/Subsurface, & Micro-irrigation), Conservation Crop Rotation, and Cover Crop.

For more information on Ag BMPs consult the Guide to Agricultural PM10 Best Management Practices.

 

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