Managing saline soil with drainage and irrigation
Saline soil is here to stay. The good news is that in many cases it can be managed by improving drainage and adapting irrigation to leach it out. Finding out the extent of the salinity is the first step, and the way to do that is with electrical conductivity (EC) mapping, says Cory Willness, a consulting agrologist with CropPro Consulting in Saskatchewan, Canada. EC is the ability of a material—for example, the water in soil—to conduct an electrical current.
“It’s the only way to identify salinity,” says Willness. “We can find exactly where saline soils are and the degree of salinity.” EC mapping is also especially useful for precision farming and for explaining puzzling yield variations.
Plants need salts in their cells and in the soil in order to function, says Mark Alley, professor of crop and soil environmental science in Blacksburg, Va., and as long as they’re in dilute concentrations, they’re almost always harmless.
“Nature will come to a balance,” he says. “Plants have concentrations of salts inside their roots and tissues that regulate metabolic processes. Salinity causes a problem when the solution around the plant roots is greater than in the plant. When it’s higher in the soil, water moves out of roots into the soil to reduce the concentration of salts around the roots, and the plant dries up.”
Salinity and soil
Salinity can affect the properties of soil. It causes fine particles to bind together in a process called flocculation, which can be beneficial for soil stabilization and aeration, as well as root penetration and growth. Too much, though, and plants have to work too hard to extract water from soil, and this causes plant stress.
Saline soils occur mostly in arid and semiarid areas of the West, where evaporation or transpiration exceed rainfall. It’s also found in greenhouse and container plant production when plants grown in small amounts of soil are overfertilized. The soil itself can be saline if it happens to be on marine deposits from a prehistoric inland sea, such as the one that once covered most of North America, from the Arctic Ocean to the Gulf of Mexico, because the salts dissolve in water and move up to the surface as the soil dries out.
The texture of the soil also affects salinity. Sandy soils flush saline water quickly through their rootzone, whereas clay soils drain slowly and are more likely to be high in salts.
The quality of the water affects salinity as well, and that fluctuates constantly. Groundwater quality can change over time, and surface water quality can change with the seasons. For example, says Alley, “You might find salt-affected soil after a hurricane, but it quickly washes away with rainfall.”
Immediately after irrigation or a rainfall, soil salinity is lowest. If the subsoil is very wet and there’s a lot of evaporation, salts come up to the surface, says Willness. As more water is taken up through transpiration and evaporation, salts become more concentrated in the remaining water. When the top 6 feet are dry and there’s a heavy rain, the salts wash back down again.
Electrical conductivity (EC) mapping
Understanding electrical conductivity and when and where salt levels may pose a problem to plant growth enables growers to monitor and manage salt problems, Alley says.
EC doesn’t show the exact degree of salinity or the condition of the soil. It does show the locations of saline soils, though, because they cause EC readings to spike, and EC does correlate to soil properties that affect crop yield, including topsoil depth, soil nutrient and pH levels, subsoil characteristics, water content and drainage conditions.
Eric Lund, president of Veris Technologies, which produces soil EC mapping systems, says, “The best times to measure EC are any time the field is smooth enough to drive over and has plenty of moisture, but it shouldn’t be saturated or close to freezing.” As water in the soil freezes, the soil pores become insulated from each other and soil EC declines. Also, because a truck pulls the Veris machine through the field, it can only be used where crops are not growing.
To do the mapping, a truck with a data logger and a GPS receiver pulls a cart, which holds coulter electrodes, through the field. One pair of electrodes injects an electric current into the soil, which runs continuously through moisture-filled pores between the soil particles, and other electrodes measure the voltage drop as the current goes through soil. The result is the EC reading, which is easily downloaded onto a computer.
“Most instruments measure two depths in one pass, and you can adjust the distance between electrodes,” Lund says. The most common spacing is between zero and 1 foot and zero and 3 feet, which provides a map of the topsoil and the subsoil.
This is very useful, says Willness, whose company has mapped approximately 40,000 acres with a Veris mapping system for EC and VRF (variable rate fertilization) in the past two years. Sometimes soil has no salinity on top, so the plants begin growing well, but they stop as the roots grow down into the saline level.
EC mapping helps explain yield variations, because it shows growers exactly where their sandy and clay soils are. Clay soils, which have numerous water-filled pores that are quite continuous, conduct electricity better than sandy soils because the more water in the soil, the greater the conductivity of the current. Soils with greater clay and organic content also usually yield better than sandy soils, which run out of moisture sooner.
“Compare soil sample maps with EC maps and look for areas of good soil that didn’t yield to their potential,” says Lund. “They may have problems that caused a lower yield.” If the maps match, the yield should be what you’d expect from the soil. If it isn’t, see if there are pests or other problems. If they don’t match, go to the area and take soil samples. There may be patches of different soils, which should be treated differently.
EC mapping also helps with precision farming, so growers can reduce their input costs by applying fertilizer, chemicals and irrigation to specific areas.
“You can’t just take a few soil samples for precision farming,” says Willness. “The soil in most fields varies significantly. In most fields, even a 2.5-acre grid still has a lot of variability—and you can’t take a 1/40-acre grid, the lab analysis would be too expensive.”
Because the truck makes closely spaced passes, EC mapping tells you exactly where the soil changes, Lund says. Once you know that, you can begin to take soil samples precisely where soils vary, so you can manage the different soils instead of the whole field.
As an example of how growers can use EC to target their use of pesticides, Lund says that one of the best locations for their sales is where growers use the fumigant Telone against nematodes. It’s expensive and powerful, so growers want to use it efficiently. Although EC doesn’t tell them where nematodes are, it does tell them where sandy areas are, which is where more nematodes are more likely to be found.
EC mapping also can help growers fine-tune Natural Resources Conservation Service (NRCS) soil maps, and improve the placement and interpretation of a variety of on-farm tests.
Growers can work with a consultant or local fertilizer dealer who provides the service, Lund says. The cost depends on how detailed the map is, but it’s usually around $5 to $10 per acre.
Growers may be able to reduce salinity in their soil. Irrigate with sufficient low or nonsaline water to flush saline water below plants’ rootzones. Improve the drainage to let it leach out more quickly. Monitor the soil for EC to track changes in salt levels.
How often soil should be tested depends on the value of the crop, the quality of the irrigation water and the quantity of soil in which the plants are growing, according to Alley.
Soils supporting high-value crops in greenhouse containers could be sampled every week. In fields with nonsaline soils, typically EC mapping only needs to be done once, Lund says. In fields where saline soils support low-value crops, testing could be done once a year using both the Veris and soil samples, especially where growers are taking action to reduce salinity and want to track the changes.
If possible, says Alley, plant salt-tolerant crops. “Salt-tolerant plants evolved in high-salt environments. They generally have low water demand and their root membranes don’t let water out as easily as other plants.”
According to the FAO (Food and Agriculture Organization of the United Nations), the most salt-tolerant fruits, vegetables and nuts include artichokes, asparagus, lima and winged beans, red beets, cowpeas, purslane, turnip, zucchini, coconut, date palm, fig, guava, jambolan and natal plum, Indian jujube, olive and pineapple.
“If the soil is heavy in salt, it might be worth abandoning,” Willness says, “but in a lot of cases it can be optimized.”
The author is a freelance writer based in Altadena, Calif.