The Pennsylvania No-Till Alliance recently celebrated its 25th anniversary at the Jim Hershey farm in Elizabethtown, PA. One of the featured demonstrations was a soil pit, which was dug directly under recently planted sorghum-sudangrass.
Lisa Blazure, soil health coordinator, Stroud Water Research Center, and on the Board of Directors for the PA No-Till Alliance, discussed the benefits of looking beneath the soil surface and learning soil properties.
“Inherent soil properties are based on parent material – where it is in the geology, how it was formed, the rock parent material and the sand, silt and clay components of soil,” said Blazure. “Inherent properties cannot be changed.”
Dynamic soil properties, however, can be changed with management. Soil structure, organic matter, earthworm populations and soil microbial populations are dynamic and influence the soil profile.
Although the ideal soil pit is large, Blazure suggested farmers use a spade to dig an 18-inch hole. While this is a relatively small window to see what’s happening under the surface, it’s a start.
“Soil pits are like Christmas morning,” said Blazure. “You’re digging in and you don’t know what you’ll find. In a classic soil profile, there are distinct layers: topsoil, then the A horizon, or rooting depth. The A horizon soil, which includes the root zone, is darker due to higher levels of organic matter. The B horizon is more mineral rich with micronutrients. Depending on the soil, the B horizon has some water-holding capacity.”
The host farm had been inundated with heavy rain (32 inches over six weeks), and the effects of extreme moisture were obvious in the pit. Blazure noted that when the pit was dug, gray and red mottling became evident, indicating saturation. When water saturates soil, the iron that gives soil a red color dissolves and the same soil appears gray.
Although no-till management has traditionally been aimed at leaving surface residue to build organic matter, scientists have learned more about how organic matter accumulates. As the carbon present in above-ground residue decomposes with the help of bugs, fungi and bacteria, most of the carbon is respired by decomposition back up into the air as carbon dioxide. Only about 15% of surface residue becomes long-term, stable organic matter.
However, carbon is added via roots. “Everything green is photosynthesizing,” said Blazure. “Photosynthesis grabs carbon dioxide in the atmosphere, combines it with water and sunlight and makes simple sugars. Any time there’s a living, photosynthetic plant, it’s producing sugars, building the root system. Simple sugars and other compounds such as root exudate enter the soil. Carbon through that process is five times more likely to stay in soil and contribute to long-term stable organic matter.”
Nightcrawlers play a significant role in soil health by creating vertical burrows that allow water and roots to more easily infiltrate the soil. Above the soil, these hard-working worms construct middens above their soil entrance. Middens are a combination of plant residue and casts (nightcrawler feces). Blazure suggested learning to spot nightcrawler middens as one aspect of evaluating soil health.
“Seeing the residue is a great indication of the nightcrawler population,” said Blazure. “Nightcrawlers can live five to seven years, then when they die, if there’s no tillage in the system, the burrow remains in place for about 25 years.” Burrows provide easy access year after year for crop roots to reach the water table and micronutrients.
Smaller earthworms also play a beneficial role in ag systems. Rather than moving downward, these worms move in all directions through the top six to eight inches of soil. They come to the surface and also leave casts.
Penn State Soil Scientist Dr. Sjoerd Duiker discusses the critical role of nightcrawlers in opening up soil layers for plant roots. Photo by Sally Colby
To illustrate the power and value of worms in the soil, Blazure mixed some white paint with water and poured about nine gallons of the mixture into a ring above the soil. At the surface, where the soil structure is better with fine mottling and smaller pore spaces, the paint was concentrated where the water soaked in around the active root zone. As the paint spread and reached nightcrawler channels, it was easy to see the potential for root depth.
Dr. Sjoerd Duiker, associate professor, Department of Crop & Soil Sciences, Penn State, was also in the soil pit and explained the importance of no-till and crop residue.
“If we don’t use no-tillage, even with some reduced tillage, we will quickly reduce the crop residue cover to levels that are exposing soil to more erosion,” said Duiker. “We always need crop residue to build organic matter in soil. A lot of surface organic residue is just being decomposed, so it doesn’t really build organic matter as much as roots do. We need to get roots down into the ground.”
Duiker also provided more details about the role of middens. “Nightcrawlers pull residue toward their burrow, where it becomes soggy so they can eat it,” he said. “They mix it with soil in their gut and deposit casts on the soil surface, but they also coat the burrow with organic matter.”
Duiker described middens as close to size of a hand width. The presence of many middens indicates many nightcrawlers, and for each midden mound, there’s a tunnel about three feet deep that goes downward into the subsoil.
Nightcrawler burrows were evident in the soil profile. When subsoils are dense and compact, making it difficult for roots to penetrate, Duiker explained that roots grow predominantly through cracks and micropores. “All the roots are following nightcrawler burrows,” he said. “Nightcrawlers are very sensitive to tillage – they need crop residue at the soil surface.”
Many farmers have had a challenging year with excess rainfall. “There’s heavy equipment and there’s surface compaction,” said Duiker. “That is something we constantly battle, and we have to fix it. But how can we do it without tillage? Roots from cover crops take care of compaction by thrusting many roots downward into the soil.”
by Sally Colby
Leave A Comment