Healthy Soils Are: well-structured
Soil structure, pore space key to proper soil function
“Soft and crumbly.” “Like cottage cheese.” “Like a sponge.” “Loose and full of holes.” Those and other common descriptions of what healthy soil looks and feels like refer to good soil structure.
Soil structure, the arrangement of the solid parts of the soil and the pore space between them, is critical to how the soil functions. When the solid parts—sand, silt and clay particles—cling together as coarse, granular aggregates, the soil has a good balance of solid parts and pore space.
Highly aggregated soils—those granular, durable, distinct aggregates in the topsoil that leave large pore spaces between them—are soils with good tilth and good structure.
Well-structured soils have both macropores (large soil pores generally greater than 0.08 mm in diameter) and micropores (small soil pores with diameters less than 0.08 mm that are usually found within structural aggregates).
An interconnected network of pores associated with loosely packed, crumbly, highly aggregated soils allows rapid infiltration and easy movement of both water and air through the soil and provides habitat for soil organisms.
Tillage destroys soil structure
Soil structure, the arrangement of the solid parts of the soil and the pore space between them, is critical to how the soil functions. However, management practices can reduce soil cover, disrupt continuous pore space, compact soil, or reduce soil organic matter, and negatively impact soil structure.
Since tillage negatively affects all of these properties, it’s high on the list of practices damaging to healthy soils.
When tillage loosens the soil, it leaves soil particles exposed to the forces of wind and water. Transported by wind and water, detached soil particles settle into pores, causing surface sealing, compaction and reduced infiltration. When this happens less water is available to plants and runoff and erosion increases.
By contrast, soils that are not tilled and are covered with diverse, high residue crops throughout the year have better soil structure, are highly aggregated, with high levels of organic matter and microorganism activity, high water holding capacity, high infiltration rates, and little compaction.
Can your soil pass “The slake test?”
Does your soil have good structure? Give it the slake test! Ray Archuleta, an agronomist with the USDA Natural Resources Conservation Service with a passion for soil health, has done the test scores of times. Anyone can do it, he says, and he predicts it will open your eyes.
“What happens with poor soil structure is that the pores collapse in water and the soil breaks apart,” Archuleta says. “Soil with good structure—the untilled soil—can still be intact for the most part even 24 hours later. The reason for the difference is soil structure. Biological cementing, the work of soil microbes, glues the aggregates of the untilled soils together.”
In a similar test, an infiltration or rainfall simulation test, Archuleta puts the two soil samples in wire mesh inserted into empty jars, then simulates rainfall onto them.
“When you put a tilled soil and an un-tilled soil in yarn jars and simulate rainfall onto them, you quickly see the untilled soil allows the water to infiltrate the whole profile. On the other hand, water stays on top of the tilled soil much longer,” Archuleta says.
“I think these tests are powerful visual tools to help explain and help people remember how soils function” Archuleta continues. “I used to think if I tilled the soil—fluffed it up—it would allow more water in. But that’s just not true. Tilling soil closes pore space and keeps rainfall from infiltrating. You’ve got to have pore space in your soil from top to bottom.”
“The tests tell me in our watersheds we have an infiltration problem, not a runoff problem,” he concludes. “What I mean is, if we focus on building healthy soils that result in more infiltration, we’ll do what we need to do to eliminate much of the runoff.”
How to do the Slake Test
Ray Archuleta, an agronomist with the USDA Natural Resources Conservation Service with a passion for soil health, has done the slake test scores of times. Anyone can do it, he says, and he predicts it will open your eyes.
The slake test compares two chunks of topsoil in water to see how well and how long they will hold together. Here are the steps:
Collect a chunk of topsoil––a size that would fit in your hand––from an area where you don’t till, like a fencerow, or a field you’ve no-tilled or had in grass for many years.
Get a second spade-full or chunk of soil from a field you’ve tilled consistently. It should be the same soil type as the first sample.
Find two glass jars, yarn jars or some kind of clear glass jars large enough to hold the chunks of soil.
Put together some type of wire mesh that you can hook at the top of each jar that will allow the soil to be submerged in the water, yet be held within the top half of the jar.
Insert the wire meshes into each jar.
Fill the jars with water.
At the same time, submerge the tilled sample in one jar, and the untilled sample in the other.
Watch to see which soil holds together and which one falls apart. The soil with poor structure is the one that will begin to fall apart.
Watch Ray do the slake test in one minute: http://bit.ly/2uq0qUX
Watch Ray do the infiltration test in one minute: http://bit.ly/2thrmHZ
To learn more about soil health, and to meet some of the farmers who are “Unlocking the Secrets in the Soil,” visit www.nrcs.usda.gov.