“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. 

Chemical and physical factors play a prominent role in small aggregate 

formation in clay soils, while biological processes drive development of large 

aggregates and macropores. Earthworms, for instance, produce both new 

aggregates and pores. Their binding agents are responsible for the formation 

of water-stable, macro-aggregates, and their burrowing creates continuous 

pores linking surface to subsurface soil layers. As they feed, earthworms also 

speed plant residue decomposition, nutrient cycling, and redistribution of 

nutrients in the soil profile. 

USDA is an equal opportunity provider and employer.

Give it 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. 

Continued on back
Soil organic matter also helps develop stable soil aggregates. 

Soil microorganisms that are fed with organic matter 

secrete a gooey protein called glomalin, an effective shortterm cementing agent for large aggregates. Organic glues 

are produced by fungi and bacteria as they decompose 

plant residues. Water-resistant substances produced by 

microorganisms, roots, and other organic matter, provide longterm aggregate stability from a few months to a few years. 

TILAGE DESTROYS STRUCTURE

Management practices that reduce soil cover, disrupt 

continuous pore space, compact soil, or reduce soil organic 

matter, 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. 

WANT TO LEARN MORE? 

VISIT WWW.NRCS.USDA.GOV

HEALTHY SOILS ARE: well-structured.

“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

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:

1. 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 notilled or had in grass for many years.

2. 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.

3. Find two glass jars, yarn jars or some kind 

of clear glass jars large enough to hold the 

chunks of soil.

4. 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.

5. Insert the wire meshes into each jar.

6. Fill the jars with water.

7. At the same time, submerge the tilled sample 

in one jar, and the untilled sample in the other.

8. 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.

If you want to see “Ray the Soil Guy” demonstrate the test or the infiltration test, checkout 

our online resources.