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Confronting Soil Salinity: Kent Cooley's Strategies for Sustainable Agriculture

 


Matt Huber

 

By Cassidy Spencer

 

Kent Cooley is a lifelong resident of South Dakota, with a diverse background spanning soil science, environmental management, and agronomy. He obtained his bachelor's degree in Soil Science and Environmental Management, followed by a master's degree in agronomy with a focus on soils, from South Dakota State University.

 

He started his career with the Soil Conservation Service, mapping soils in the Northeast of South Dakota. He continued mapping soils in various counties and is now the area resource soil scientist in Rapid City. Through his experience and familiarity with South Dakota soils, Cooley has witnessed the unfolding salinity crisis firsthand.

 

We actually tried to figure out how many acres of saline soils were in the state back in the mid-2000s. We calculated about seven and a half million acres of saline soils,” said Cooley. “We've added almost another million more acres of saline soils in the state the last 10 to 15 years, and that's primarily due to management.”

 

 

Understanding Salinity

 

Excessive soil salinity occurs through the accumulation of salts, primarily in the upper layers of the soil profile, including the root zone and up to the surface. In extreme cases, salts may evaporate as they move up the soil profile leaving a visible white crust on the soil surface, which is the final degradation state of saline soils.

 

Several factors contribute to this issue, the two bedrock conditions being salt-rich parent material and an arid or semi-arid environment. Under these conditions, water will tend to move upwards in the soils due to the arid climate and the lack of deep-rooted vegetation to infiltrate or pull water further down into the soil profile. Due to the history of the land, South Dakotan soil makeup can be more prone to saline issues.

 

“About a hundred million years to 66 million years ago, this area was underwater. There was a great inland sea that stretched from the Gulf of Mexico all the way up to the Arctic Ocean,” explained Cooley. “It divided North America into two landmasses. Well of course, that water was salty, it was an ocean. And when that inland sea retreated, those salts were left behind in the sediments. That’s the source of most of our salts: most of South Dakota was underwater, except for the far eastern part of the state.”

 

 

Managing Salinity

 

Addressing soil salinity requires a multifaceted approach, focusing on prevention, management, and remediation. Cooley emphasizes the importance of diverse crop rotations and the integration of multi-species plant roots to utilize moisture more effectively and keep salts at bay. A stratified root system, mimicking that of native rangelands, draws moisture from various soil depths, reducing the upward movement of salts.

 

Saline issues can be exacerbated by tillage, which disrupts the soil structure and minimizes pore space, encouraging upward movement of water in the soil through capillary action. No-till practices allow larger, granular “cottage cheese” soil structure to form in the soil, allowing for small, medium, and large pore spaces which subsequently allow gravity to take water deeper into the soil profile. Salt-rich parent material and arid environments, when combined with poor land management techniques, can lead to these saline seeps—excessive upward movement of salt-rich water and the eventual development of a white crust on the soil, alongside severely degraded soils without necessary structure to effectively utilize water.

 

“When you look at the soil forming processes, natural pore space is built within the soil and those pores connect the deeper parts of the soil profile up to the surface. Now, if we have an area where we don’t have a lot of vegetation or that vegetation is shallow rooted, that allows the soil moisture that’s in that soil profile to move upward through evapotranspiration and capillary rise, which is basically water movement up those pore spaces when we have a dry period,” explained Cooley. “But if we’re using the water through the plants, the transpiration aspect keeps those salts deeper in the soil profile. That’s the biggest thing when we’re looking at salinity on our soils, we want to eliminate the evaporation off the soil surface and lose our water through transpiration, through the plants. That helps keep the salts down lower in that soil profile.”

 

Another effective strategy in the fight against salinity is the adoption of perennial systems. Perennials, with their deep and extensive root systems, offer continuous soil cover and utilize soil moisture throughout the year. This consistent use of water helps to maintain the salts within the deeper layers of the soil, preventing their accumulation at the surface. Perennial systems not only mitigate salinity but also provide other environmental benefits, such as reducing erosion, enhancing biodiversity, and improving carbon sequestration.

 

“When we harvest crops in our tillage systems or cropping systems, and we have a period of probably two months, up to four months, depending on whether it’s a small grain or a row crop, where we’re not utilizing soil moisture at all—that can actually build water tables that could potentially cause problems down the road,” explained Cooley.

 

 

 

Managing the Recharge Area

 

A critical aspect of salinity management is focusing on the recharge area, the upslope region that contributes to the water and salt flow towards the affected areas. Effective management of this zone can significantly reduce the volume of water moving downslope, thereby preventing the accumulation of salts in the lower lying areas.

 

“What actually starts becoming saline is downslope from those areas, either in a saline seep, which is typically on a side slope, or lower parts of the side slope, which actually can move up the slope if you don’t take care of the problem,” said Cooley. “We want to utilize that soil moisture in the uplands before it can ever get down to those lower areas if it’s moving laterally in that soil profile. You utilize it up there, then you’re way better off. And when you look at those saline areas across the state, it doesn’t matter if it’s a seep or if it’s a low-lying area where we’re starting to build salts on the soil surface—those are much smaller areas as compared to the recharge area.”

 

Cooley again praises the implementation of multi-species root systems being kept in soil year-round to begin to mitigate this issue. 

 

Once again, probably the most effective way to manage salt affected areas that I know is to put perennial systems in place in the uplands and the recharge area, while adding more salt tolerant species,” said Cooley. “When you look at trying to remediate the actual salt area, I've heard a lot of people want to use tile drainage, an interceptor drain to cut off the water with interceptor tile. But then you have the problem of where are you going to route that tiled drainage system, where's that salty water going to go? Is it going to stay on your place? Is it going to affect a neighbor and so forth on downstream? That's not a good scenario.”

 

 

Salinity and Sodicity

 

Salinity issues in soil are not to be confused with sodicity issues. As Cooley explains, the two issues are dealing with entirely different soil chemistry—salinity refer to an excess in calcium and magnesium salts in the soil profile. Sodicity refers to high sodium content in soils, which cause clay particles within the soil profile to disperse, developing a claypan. The claypan—a compact, dense layer of clay formed in the subsoil—is highly restrictive to root development and more difficult to remediate than saline seeps.

 

“When we're dealing with more of a sodic soil which can have an appearance of being saline, that soil test alone will actually tell us if there is a high sodium content in that soil, especially the upper part of that soil profile,” explained Cooley. “When that occurs, basically we're looking at either a sodium absorption ratio of 15 or greater, or an exchangeable sodium percentage of 13 or greater.”

 

It is much easier to alleviate a salinity problem than a sodicity problem. Once you have a sodic soil that has formed a claypan, necessary pore space in soil has been restricted and water cannot effectively travel or absorb in the soils.

 

 

Staying Aware of Irrigation

 

Cooley added, it is important to stay aware of the quality of water being utilized for irrigation, as well as the irrigation system and its compatibility with your soil makeup. If irrigation water is high in salts or sodium, there is the potential to imbalance chemical makeup of soils and encourage salinity or sodicity issues down the line. Closer to river systems, soils typically have higher sand and gravel content which helps to keep soil water moving down the soil profile. Further from river systems, farmers need be aware of the quality of water they are implementing through irrigation, as their soils with higher clay content are more prone to skewing toward excessive salinity and sodicity.

 

“In our irrigation guide, those heavy clay soils have a very low water intake and very low water movement through that soil profile because of the small pore space. And we are not supposed to move from a flood irrigation system to a sprinkler irrigation system on those soils. But we've been playing around with trying to implement appropriate management systems that will still allow us to convert to a sprinkler and avoid some of those salinity or sodicity problems by maintaining pore space and putting extra water in; especially in the last irrigation period during the year, we try to flush salts out or down that soil profile and keep them lower in the soil profile,” said Cooley. “Well, we had one producer that did not take our advice. He continued to grow continuous corn with tillage. And within a period of less than two years, he actually got to the point where he could not raise a corn crop because the soils were becoming so saline based on his management. So that's the situation that we're trying to avoid when we put sprinkler irrigation systems on these clay soils. We need a perennial system, or we need to try to keep salts pushed down by using a no-till system if annually cropped.”

 

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