Biological activity | Bacteria produce mucus that glues particles together
MINOT, N.D. — Speakers usually grab their audience’s attention by telling a joke, but soil scientist Jon Stika likes to open his speeches with a lump of dirt.
Stika began his presentation at the Manitoba-North Dakota Zero Tillage Farmers Association annual conference by holding up two soil aggregates, both about the size of a tennis ball.
Stika, who works for the U.S. De-partment of Agriculture in Dickinson, N.D., said the samples were taken 15 metres apart and were both fine, sandy loam from North Dakota.
One sample came from a highly disturbed site with lots of tillage, no cover and nothing growing on it.
“The other has been in sod and a diverse mixture of perennial plants, always covered … a wonderful place to live if you’re a soil microbe.”
To illustrate the difference between the samples, Stika had audience volunteers drop the aggregates into separate tubes of water with steel mesh near the top to keep the lump in place.
After only seconds in the water, soil from the disturbed site broke apart and the soil particles seemingly dissolved, turning the water brown.
The other aggregate remained intact, with only a few particles breaking off and sinking to the bottom.
Stika said the demonstration shows that micro-organisms are a key factor in soil health. An aggregate that remains whole when immersed in water indicates that the soil has a high level of biological activity.
Stika said soil bacteria produce mucus, while soil fungi grow as long filaments. The outside of the filament is coated with a sticky substance.
“In the process of them exploring the soil, they leave the sticky substance behind. Those are the real basic sticky substances that glue the sand, silt and clay particles together,” he said.
“It’s the biology that makes the glues that holds aggregates together.”
Stika said a high level of biological activity is key for healthy soil because micro-organisms cycle nutrients and establish pores that permit water infiltration.
Kris Nichols, a USDA soil microbiologist in Mandan, N.D., agreed, noting the symbiotic relationship between soil organisms and plants.
“It’s sometimes talked about as microbial engineering,” she said.
“The microbes have engineered an environment that’s advantageous to themselves … but (also) advantageous to the plant.”
Dropping a lump of soil into water is obviously less formal than a laboratory test in which scientists record the amount and variety of micro-organisms in a soil sample.
Still, Nichols said Stika’s experiment is a valid method to assess soil biology.
“I really approve of the more informal soil testing,” said Nichols, who also spoke at the zero till conference in Minot.
“With soil biology (tests) … it’s very difficult to get some good soil biology data…. These more informal tests measure (soil) function, and function (is something) that biology helped to drive.”
Nichols said it’s difficult to accurately assess the population and types of soil organisms in a lab because factors such as soil moisture affect biological activity. Therefore, a sample taken at the same location but in different years could produce different biological results.
“With more formal tests, you’re sampling one spot at a point in time,” she said.
“The values you get at are really only applicable to that one spot at that point in time.”
Stika said growers could easily duplicate his experiment with an empty jar and water by dropping aggregates into the jar and watching how the soil responds.
“There’s a lot of functions that (soil) is supposed to perform, but don’t assume it’s performing these functions,” he said.
Stika also recommended another informal test to measure a key soil function: water infiltration.
He said producers can start by driving a six inch tall, six-inch diameter pipe three inches into the ground. They should then take a few sips from a half litre water bottle until the water drops to the shoulder of the bottle.
“What’s left is around 444 millilitres,” Stika said.
“That is amount of water that will produce an inch in that six inch (pipe). You can time it. OK, it took a half hour for that inch to go down. So that’s a rate of two inches per hour…. That’s easy to do. You can do it with stuff you have laying around the farm.”
By repeating the experiment, a grower can determine if the soil is becoming more permeable or not.
Another simple way to measure biological activity is to count the number of earthworms in a cubic foot of soil.
“If you want to dig a cubic foot of soil, we usually say that 10 (earthworms)… that’s pretty good soil health,” Stika said.
“If there’s less than that, there’s room to improve.”
The smell test is another option. An earthy, sweet smell is a sign of healthy soil, while a smell like a kitchen cleanser might mean bacteria is the dominant life form in the soil.
If there is no smell, the soil is either exceptionally dry or has a small population of micro-organisms.
It is an effective assessment tool, but Stika said developing a nose for soil is a skill.
“You almost need to have somebody who knows what they’re smelling, to say, ‘here, smell this.’ ”