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The subtle science behind soil aggregates

How building soil structure pays long-term dividend.

On the surface, soils seem pretty simple – a medium in which farmers plant seed.  

Actually, soils are complex, filled with numerous substances and living creatures. A teaspoon of soil, for example, contains aggregates: particles of sand, silt, and clay bound together by organic matter. 

Aggregate stability is why some soils act as a sponge, quickly soaking up water after a hard rain. Soils with little aggregate stability may be muck and slop for days. Good aggregate stability gives soils the resilience to withstand adverse climatic conditions like high winds and torrential rainfall. Soils with poor aggregate stability are easily whisked away by wind and water even in minor weather events. That leads to soil erosion, compromised water quality, and ultimately, lost productivity, says Aaron Daigh, associate professor of soil physics and hydrology at North Dakota State University (NDSU). 

Native grassland has good aggregate stability. Tilled farm fields, on the other hand, do not. Tillage breaks aggregates apart and introduces air into the soil, stimulating microbes to increase the rate of organic matter decomposition. As soil loses organic matter to the atmosphere as carbon dioxide, less organic material is left to bind aggregates together.

Distribution or Stabilization?

Under a microscope, soils with adequate aggregate distribution will have numerous pores, where air, gas, and water mix with the aggregates and form productive soils. 

“Aggregate distribution is a function of stability,” Daigh says. “It’s like pouring concrete. By itself it’s strong, but not as strong as when you have rebar.”

In soil, microbes and roots start weaving the organic bits together, like adding rebar to concrete. As more roots, root exudates, and organic matter mix into the soil, the soil aggregates become stronger, more resilient, and able to absorb more water.

The cautionary tale, however, is that tillage – even a single tillage pass to smooth out a field – eliminates all that aggregate stability and distribution, Daigh says. Leaving soil undisturbed after one phase of the crop rotation is not enough to repair soils. 

Tillage upsets soil aggregate distribution and stabilization, and it should be used only as a last resort.

“The only situation where we recommend fixing ruts is when the ruts are deep or extensive enough across the field that it cannot be planted,” he explains. “If the ruts are 6 inches deep, or you can’t get in there, filling them is just going to have to be done.”

Shallow problems, like tread impressions, will be readily fixed by soil biology, he adds. 

Fix Broken Soils

Stopping tillage won’t immediately fix broken soils, but leaving a layer of crop residue on the surface is one component that will. Adding cover crops to a crop system helps repair problem soils quicker and boosts water infiltration – an important function of good aggregate stability, says Andrea Basche, assistant professor in cropping systems at the University of Nebraska - Lincoln (UNL).

Research performed by UNL at 17 sites across Nebraska shows that, regardless of soil type, adding cover crops increased water infiltration by 59% compared with plots with no cover crops. The research included a myriad of cover crop species and mixes plus cropping systems. Across all the sites no matter the treatment, a pattern emerged. “Cover crops, even after a year or two, can lead to an improvement in something as important as infiltration,” Basche says. 

During weather adversity, water’s ability to infiltrate soils that act as a sponge dramatically improves resilience, she adds. 

“We want these soils to be able to absorb water when it’s abundant, and squeeze it back out when it is not abundant,” Basche explains.

Not All Soils Are Equal

Soil texture has a major influence on soil aggregate stability, says NDSU’s Daigh. 

Clay soils have small particles with electrically charged surfaces. They form aggregates quicker than nonclay soils. 

Sandy soils have large particles with no electrical charge. They rely on the addition of charged organic matter to glue particles together. They will take longer to build aggregate stability. 

Silt loam soils have small to medium particles with no electrical charge. They hold a lot of water and are soft. It will take less time to build soil aggregate stability in silt loam than in sands, but silt loam is less forgiving than clay soils. 

Outcomes of Reduced Tillage

Commercial soil labs offer a litany of tests that can help measure fungi and microbial populations. These tests may be difficult to interpret by farmers, and they’re expensive. An alternative is to simply grab a spade and spend some time in the field looking for these characteristics: 

  • Reduced soil erosion, compared with when the fields were tilled

  • Healthy plants
  • Soil aggregate development, improving soil structure
  • Earthworm populations increasing 
  • Water percolating through the soil, rather than ponding. 

“I’m a big fan of spending time in your field and watching it change,” says Caley Gasch, assistant professor of soil health at NDSU. “Have faith.” 

In many cases, farmers who stop tillage will see improvement in soil structure in just a few years. Adding cover crops can jump-start that structure-building process, adds Anthony Bly, Extension soil field specialist at South Dakota State University. 

“It takes a while for you to see the earthworms and the biology improve in sandy soils. It took us 7 to 10 years to see significant changes with the silt loams on our farm [in South Dakota]. But our mistake was not including diversity. 

“Cover crops are important,” he says.

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