Local scientists advance soil health assessment

In a September 2022 publication, two researchers from the Panhandle Research, Extension and Education Center (PREEC) proposed a natural land classification unit for comparing soil samples.

“First, the recognition, the understanding of the importance of the soil; it’s not just dirt, it’s a finite resource that sustains the life we ​​have on earth. And we have to preserve it, keep it, I think that’s the whole idea,” said Bijesh Mahargan.

Maharjan, associate professor and PREEC specialist, and Saurav Das, assistant research professor, both work in the Department of Agronomy and Horticulture at the University of Nebraska-Lincoln. They have collaborated with the USDA’s Natural Resource Conservation Service (NRCS) to promote soil health assessment.

The project began in 2020 when researchers teamed up to tackle soil conservation “from the ground up”. Topsoil, the top six to eight inches, has a limited resource of nutrients that can deteriorate over time.

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“It’s like a movement, suddenly people woke up and felt that we have to take care of our soil resources,” Maharjan said. “Topsoil is where the magic happens and soil as a resource must be conserved.”

Das said soil health has become a hot topic with growing concern over preserving topsoil for future generations and food security. The duo asked the question: how can a farmer determine if his topsoil is good or needs improvement?

“There are precise measurements for a pound of rice or a pound of something, but for soil we don’t have the same kind of measurement or reference,” Das said. “I spoke to (Maharjan), and he also had some ideas and we sat down to figure out how we can actually put numbers in to define the floor – is that good or bad?”

The concept, named Soil Health Gap, effectively defines the difference between undisturbed native soil and soil health in cropland systems. The soil health gap can be used as a baseline measurement tool to guide soil health management decisions and goals.

“Let’s say your homeland has 4% soil organic matter and your farmland has 2%. You know there is a 2% discrepancy, during different types of management practices we may have lost the 2% organic matter,” Das said. “That’s why we came up with the concept we call Soil Health Gap.”

The two researchers took the concept further by sampling soil organic matter from grasslands, no-till and conventional cropping systems, and exposed subsoil.

“We saw that there was an exponential decline in soil organic matter across different land use types,” Das said. “That gave us a set of numbers to define the natural state of soil health that hasn’t had any sort of farming practices and you can compare that to your cropland.”

Soil organic matter (SOM) is a carbonaceous component of soil made up of plant and animal tissues in various stages of decomposition. Maharjan said SOM was used as a consistent measurement tool because carbonaceous organic matter is central to the properties and processes that occur in healthy soil.

“Organic matter falls precipitously from unmanaged land to reduced tillage cropland, to no-till cropland, to exposed subsoil land,” he said. “The more you work the soil, the more you lose (SOM).”

Undisturbed native grasslands or pastures were used as the reference terrain.

“Wherever the minimum disturbance has occurred, so pastures and rangelands where there can be grazing, that’s what we call benchmark lands,” Maharjan said. “Now you compare yourself to that. Otherwise, in the past, people may say do this and do this and make outrageous and erroneous claims that you can improve your soil. Now we set a ceiling.

The Soil Health Gap study found that cropping systems should be compared to reference lands in the same soil type and climatic conditions. Localized differences in soil type and rainfall allow soil health to respond differently to management practices.

“We realized that healthy soil will be specific to the region. What is healthy in Lincoln may not be possible in the Panhandle,” Maharjan said. “So we’ve decided that the soil health should be compared – the Panhandle’s healthy soil will be referenced with the Panhandle’s native soil.”

The Soil Health Gap study encouraged researchers to pursue the development of a benchmark land classification system. The Cropland Ecological Reference Unit (CREU) would compare all cropland to the identified reference land found in the same ecological zone. The comparison system will establish a natural framework for comparing soil health in the same soil type and climatic conditions.

“Can I compare my cultivated land with native land near Lincoln? Most likely not, it must be very region specific,” Maharjan said. “The CREU speaks of a continental mass unit where soils from different sites can be compared. It shouldn’t just be geographical like Scotts Bluff County; it should be more natural boundaries.

The theory means that all soils in an identified land unit may have similar SOM potential.

Das further explained the importance of CREU development in Nebraska noting that the Panhandle receives, on average, half the amount of annual precipitation east of Nebraska and that the soil type of the Panhandle is sandier. If the SOM averages 7% near Lincoln, it is unreasonable to expect the same percentage to be achieved near Scottsbluff.

“All soils have a different potential, what (SOM) can be achieved, so you can compare each terrain within that natural boundary,” Das said. “Initially, the soil has all the same potential in the blocks of soil, then management practices or something happened that made (SOM) go down or up.”

Researchers worked with the existing NRCS land hierarchy of Major Land Resource Areas (MLRAs) as reference land sites. The CREU model was established by further separating MLRA sites based on soil and precipitation characteristics.

“The NRCS already has its own land classification system. We take that and split it even further,” Maharjan said.

Once CREU sites are designated and soil samples are compared to the identified reference soil or native range. Accordingly, management practices can be addressed.

Maharjan said cultivated land in Scotts Bluff County is typically 1-2% SOM, but native land has been found at 4% SOM.

“So we can do better,” Das said. “When the soil is healthy, it can store more water, more moisture for the crop and your production can be better compared to unhealthy soils.”

Maharjan said: “It’s hard to reach (4% SOM), it’s a ceiling but at least now you know that. If you manage the terrain correctly, you can shoot for four.

The researchers both stress the importance of being good stewards of the land.

“If the topsoil is healthy and rich, it has its own natural system that will better withstand the weather,” Das said.

The CREU concept can be used anywhere to promote soil health. Maharjan and Das move on to the next stage of CREU – demonstrate and validate.

With the help of funding and in cooperation with the NRCS, soil sampling in the identified land units and analysis will be underway. Maharjan said his PREEC lab is currently looking for an assistant to help with organization and the extra workload.

The researchers said the end goal was to establish a web-based tool accessible to everyone.

“Our end goal is an interactive map of the state where anyone can walk in and check their land against a reference land and find the gap (in soil health),” Maharjan said. “An interactive map of soil health, this will be unique to the nation.”

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