Measuring soil health vital for yield
by Deborah Jeanne Sergeant
If you don’t know your soil, you won’t know how to amend it – or what to expect for yield. That’s why Sieg Snapp, professor of Soils and Cropping System Ecology at Michigan State University, presented “Measuring Soil Health.”
“I was part of the early discussions about soil health in the early ‘80s,” Snapp said. “I’m very excited that it’s the flavor of the day. We all need to move on from treating it just like the dirt we walk on.”
She listed the topsoil health challenges as erosion, nutrient management, soil compaction, poor crop emergence/disease, poor water infiltration/moisture and poor pore structure/no tilth.
“We’re trying to prevent erosion and make sure we manage our nutrients properly,” she said. “It’s about trying to have sustainable crop yields.
“As we move into more and more challenges with intense rainfall, we need to be able to absorb that. Soil health is important for that. If you have intense rainfall at some point, you often have long dry spells. You have to hold that moisture. It all comes down to tilth.”
Since soil health is so complicated and performs so many functions, she said it’s essential for farmers to understand it.
She added that soil health affects crop yield over time as it responds to inputs like fertilizer and seeds, healthy root systems and nutrient supply for crops.
Effects include minimal nutrient losses through leaching or volatilization, minimal soil erosion, resilience to intense rainfall, fast moisture infiltration and capacity to store water for dry spells.
“Increasing organic matter is at the foundation of almost all these aspects of soil health,” Snapp said.
Aggregates can give farmers a good idea of soil health function. She added that aggregates offer a home for microorganisms.
“If you have sandy soil, it’s harder to build soil organic matter,” Snapp said. “Organic matter is key to make sand act in a way that improves soil health. In clay soil also, aggregates are key.”
She explained that soil organic matter allows soil some aeration.
“Much of the soil is almost a desert compared to what’s on the roots,” Snapp said. “This is one reason cover crops help us. They provide a home to microorganisms.”
Organic matter is made up of active, slow and stable portions. Snapp said active soil organic matter is from recent organic matter inputs and soil organisms. Slow organic matter is comprised of organic compounds derived from active matter and protected matter. Stable organic matter is physically protected humus and extremely recalcitrant charcoal.
“The largest amount is stable,” Snapp said. “It’s what you’re bequeathed by your ancestors. It takes so long to change it. You can change what we call the active and slow pools. The active part is where the microbes are based, the recent residue from cover crops and manure. Then they go through the slow pool and the stable pool.”
It’s possible to both see and measure soil health. Measurements may include weed types present, tillage ease, texture, surface crust, soil tilth/structure, soil fertility, soil erosion, soil compaction, earthworms present, drainage, decomposition of residues, crop yield and soil color.
“Tillage ease is rather subjective,” Snapp said.
She added that field observation and other testing may offer insights. These methods could include looking for earthworms, performing the tea bag decomposition test, using a penetrometer, using a soil profile characterization app and lab testing.
“Soil health is all relative,” she said. “You have to think about your benchmark and your plan and how you’re observing it over time.”
She encouraged farmers to define their soil goals before sending off soil samples to a lab.
“If you’re thinking more about crop health, you may want to do other kinds of measurements,” she added. “I think it’s important to look at change over time. Is soil organic matter increasing or decreasing over time? It’s not something that changes fast.”
Walk your fields to find the zones you’re going to sample, Snapp said. “Monitoring your soil is really important. Test your soil each time the same year. Set your goals and test your soil accordingly. When you walk your fields, you define your zones to sample.
“You can also use soil maps to decide where your fields are different so you can sample a representative area. You want to think what you’re aiming for.”
The soil map available at websoilsurvey.sc.egov.usda.gov/App/HomePage.htm can help farmers look up their soil type and enable them to take samples that accurately reflect their farms.
She said farmers should get samples from a fence row, to show their soil’s potential, and in a field in the same zone.
“You can see if it’s the same soil type using a soil map,” Snapp said. “Is this fence row a good benchmark? This will basically tell you if it’s the same soil type as your field you can use it as a reference. If you measure that at the same time as spots in your field, you can know what’s attainable for your field. You want to move toward that same soil organic level.”
Farmers need to sample consistently and to dig deeply in the soil.
“If you sample spring one year and fall the next, the soil is fluffed up to a different degree,” Snapp said. “How much is due to compaction of the soil? Sample it less frequently, but do it very well.”
Snapp recommended taking at least five samples.
“Mix them really well and then take a sample of that,” she said. “Dig a pit. Usually, you’re taking eight inches or so.” Correct depth can give a more accurate picture of what’s going on.
Farmers should also stay with the same lab as they can measure things a little differently. “Pick the one with measurements that are important to you,” Snapp said.
She emphasized the context of soil health tests.
“Soil health is influenced greatly by biology,” she said. “We have to have enough moisture – water – to have biology and have enough heat. If we measure in summer, you’ll get a lot of respiration. If you measure in the dead of winter, it will be sleeping. Even if you warm it up, you won’t know what’s going on relative to summer.”
She encouraged farmers to test in autumn because “spring can be tricky.”
A Solvita test provides a rough estimate of active carbon, but Snapp said it’s highly variable because of soil moisture. “Solvita you can do yourself in the field,” she added.
Soil samples are moistened, then incubated in a jar and the carbon dioxide that is mineralized is measured to provide a measure of soil activity as far as respiration of carbon dioxide.
Snapp prefers the tea bag decomposition test and it’s free. “It sounds too simple but you have to compare a green tea and a rooibos tea bag,” she said. “It’s from South Africa. It has a type of tea leaf that is hard to decompose and green tea is easy to decompose.”
Farmers bury the bags at the same depth for about a month and see how much they’ve decomposed.
“Don’t do it during a dry spell,” Snapp advised. “It’s turning out to be one of the more reliable tests out there.”
The active carbon POXC test shows the range of values possible with permanganate oxidizable carbon. The soil samples turn different shades in relationship to the carbon present. Lighter color means more active carbon was oxidized.
“These colors are measured with something like a reflective light system,” Snapp said. “It can be from someone holding it against a color chart or an actual measurement. It provides some indication if your soil is starting to obtain more soil organic matter.”