Experts’ suggestions for soil salinity problems
PIERRE, SD – Excess salt in soil – sometimes referred to as “alkali spots” or sodic soil – is a growing problem across the United States and in South Dakota.
Salt can accumulate on the soil surface for a variety of reasons. Excess moisture, improper crop management and soil and geologic land characteristics may all contribute to salinity issues. Sodic soils typically have a pH of 8.5 or more, are poorly drained, tend to crust and have such a high percentage of exchangeable sodium that growth of most cash crop plants is reduced or completely inhibited.
Natural Resources Conservation Service (NRCS) District Conservationist for Davison and Hanson Counties, SD, Heidi Rients, says the problem is more intense in some areas than others.
“When you look at a map that pinpoints saline issues, it looks like the James River Valley from Aberdeen to Yankton has developed a lot of saline areas,” Rients says. “When soils become saline, crops may grow there, but plant quality isn’t as good as it should be.”
The transition to less diverse crop rotations over the past 15-20 years has also been a contributing factor to increasing saline issues, in part because seasonal plants don’t use as much water as perennial plants that used to blanket American soils. If soil water levels become high enough, salt can be leached to the soil surface, leading to poor soil structure due to sodium ions that cause soil particles to disperse (deflocculate). “The first sign of a saline issue is likely to be an area of a field where crops don’t grow as well as the rest of the field,” Rients says. “If the saline levels are high enough, crops may not grow in the area at all.”
Above average rainfall in some northern South Dakota counties, like Clark and Spink, has contributed to development of saline soils there. Soil characteristics in those counties also plays a role in salinity development.
“Over the past 20 years, loss of crop rotation diversity, particularly small grains and alfalfa, and the transition to more corn/soybean rotations has also been a factor in expansion of many salinity areas,” Shane Jordan, NRCS District Conservationist at Redfield and Clark, SD, says. “Soybeans don’t offer the root development or water usage demands of crops like alfalfa and small grain. Soybeans are also much less tolerant of salinity levels, resulting in less residue and soil surface cover.”
Inefficient management of water resources combined with increased evaporation potential draws salt to the soil surface. That’s why salinity issues are often first observed in watershed areas where naturally occurring vegetative buffers were located. Without the buffers to absorb excess moisture, salinity is much more likely to occur.
NRCS Soil Health Specialist Jeffrey Hemenway, Huron, notes that movement of water from one area to another can also result in depositing salt on the soil surface. Hydraulic conductivity (HC) – a soil property – determines the ease with which soil pores allow water movement. Soil type, porosity and soil pore configuration all determine HC.
“Water movement is slowed through soils like shale and clay because of their density,” Hemenway says. “If water moves quickly and carries salt, salt will remain on the soil surface once the water evaporates,” Hemenway says. “Standing water may also cause salt to rise to the soil surface. This problem has been seen since the 1990s during a wet period in the Jim River Valley. A couple of years ago it was estimated that South Dakota had nearly 300,000 saline affected acres. That’s millions of dollars of potential income loss due to salt.”
Recognizing saline areas – where salt is already visible or is affecting plant quality – is the first step in resolving saline issues. Soil sampling is the most accurate way to assess the level of a saline issue. “Extension services, NRCS or an private agronomist can assist with soil sampling,” Rients says. “An electrical conductivity meter may also give you an idea of how severe the saline issue really is.”
A high reading on the meter indicates a form of salt that could impact crop growth. A soil sample from suspect areas and help identify salt components and aid in development of a saline management plan. Meter readings also aid in selection of appropriate reclamation plant species since some are more salt tolerant.
Jordan says advancing areas of kochia and foxtail barley in a field can signal increasing salinity levels. “In the beginning stages of salinity development, soybeans especially don’t develop well,” Jordan says. “Kochia and foxtail barley may take over the area. At this point, it is critical to identify these areas in your fields and adjust management accordingly.”
Jordan and his colleagues have found that using targeted crop rotations that include small grains followed by planting high carbon based cover crop mixes or establishment of perennial covers can reclaim saline areas in a 1- to 3-year window.
“That’s along with a little cooperation of Mother Nature,” Jordan says. “Incorporating small grains and cover crops into rotations can be considered an investment into your pest and nutrient programs, with returns coming through increased production potential in subsequent crop yields. Annual management considerations and evaluations will continue to be key to successfully maintaining sustained profitability throughout these sensitive areas.”
When salt becomes visible on the soil surface, long term reclamation over a period of 3 to 10 years may be required. Perennial grasses have proven to be effective because they remove water from the soil profile form early spring to hard freeze. They also generate high levels of residue, enhance microbial functions and develop root systems advantageous to improving filtration.
“A perennial vegetative mixture we have found to be very successful is a mixture of Salinity Max Alfalfa – a newer salt-tolerant alfalfa – blended with Slender Wheatgrass, AC- Saltlander Wheatgrass, Western Wheatgrass, Tall Wheatgrass and Garrison Creeping Foxtail,” Jordan says. “Using at least 10-20% of the Creeping Foxtail in our mixtures to reduce drowned out gaps in establishment areas can be critical to overall water management of these areas.”
The key managing wet areas is insuring that a sufficient number of recharge acres are taken in so grasses can establish and encroach on higher salinity affected areas. Often, producers try to address saline issues by taking in only higher salinity areas. The result is a failed seeding, extended reclamation periods, and an unsuccessful reclamation outcome.” Garrison Creeping Foxtail is preferable to Reed Canarygrass when forage usage is a consideration. Garrison has proven to offer improved nutritional value and palatability over Reed Canarygrass.
Hemenway encourages producers to take a proactive approach to salinity—managing cropping systems to avoid saline development.
Tiling has been used to in some areas to reduce excess moisture issues. However, the cost and long term effect of tiling should be important considerations.
“Planting vegetation that can be harvested or used for forage or enrolling acres in the USDA Conservation Reserve Program are both economical ways to reverse a salinity issue,” Hemenway says. “Lowering a water table through use of plants works very well. Water quality can be affected by tiling because any leached nutrients move with the water.”
Experienced saline management personnel such as Extension agents or NRCS specialists can provide objective insight for individual situations. Removal of land from crop production should also be considered.
“Unproductive soybean and corn acres may be productive when it comes to forages,” Hemenway says. “You don’t need to drain land if you run livestock on it. Cover crops also help. Integrating practices that are good for soils and good for hydrology can be very good economically, too.”