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PROBLEM SOILS

Problem Soils: Compaction

Compaction reduces yields by limiting root growth, but not Photo by USDA - ARS
for the reason most people think. It is assumed that root
growth is stopped because the soil is “hard”, but the real reason
roots do not grow into compacted soil is that there is a lack of
oxygen. One reason for this lack of oxygen is that compacted
soils have reduced pore space, as well as collapsed passages
between pores, so that it is hard for oxygen to diffuse down
into the soil. Since yield losses from compaction are caused
by a lack of oxygen, and the lack of oxygen is caused by a lack A microscopic view of mycorrhizal fungus growing on a corn root. The round
of pore space, it makes sense that the remedy for compaction bodies are spores and the threadlike filaments are hyphae. The substance coating
all of this is glomalin, revealed by the green dye.
is to create better pore space in the soil. worms don’t just magically appear in cropland soil. These
Historically, we thought the way to create pore space was organisms, like any other living thing, need to be fed. Some
with tillage, but pores created by tillage are not stable, and of them can be fed with manure or crop residue or any other
easily collapse when the soil is wetted or driven on. There decaying organic material; these organisms are called sapro-
is no tillage operation which will remedy compaction long phytes. Leaving crop residue intact on the soil surface also
term and all tillage will actually make compaction worse, helps prevent future compaction, as it helps hold up hooves
including subsoiling. This surprises many, as a subsoiler has and tires. Manure will feed many bacteria and fungi and pro-
been our “go-to” remedy for decades to break hardpans so mote the formation of aggregates. But the real compaction
roots can go down through it. But that is not the way it works, breakers are organisms that live on plant roots and are fed by
as subsoiling results in a massive loss of soil organic matter, root exudates. That is why cover crops are an essential link in
which makes compaction worse over time. eliminating compaction; they can keep these organisms that
live in the immediate vicinity of the roots (called rhizosphere
organisms) alive and thriving when there are no crop roots
to maintain them.
When breaking compaction is the primary goal, any growing
plant is better than no plants at all, but obviously some are
better than others. When putting together a mix for breaking
compaction, it is important to include both species that aggre-
Photo by Dale Strickler gate soil and species with deep taproots. Here are some of the

better species for breaking compaction, keeping in mind that
a diverse blend is better than any single species in most cases.

Compacted soil due to excessive tillage—note the crusting on top and the complete Cool-Season Species
lack of structure in the soil. How much rainfall do you think will infiltrate into this? The strong and deep tap root of the Nitro Radish has made it
So how do you remedy compaction, if not with a subsoiler? a staple item in the Green Cover Seed lineup for quite some
The answer lies with biology. Pore spaces are created by the time, while the Smart Radish is new and features a large
formation of what is called water-stable aggregates, or little number of branches off the main root as well. It also has a
soil balls that do not melt away with every rain. Water-stable higher percentage of its taproot below the soil surface, with
aggregates are formed when bacteria, fungi, and earthworms less of the root aboveground. Collards and Rapeseed feature
give off exudates that glue sand, silt, and clay together into deep taproots like radishes, though not as thick.
little balls, like popcorn balls are held together with corn Safflower has a very deep taproot and is very drought tolerant.
syrup. Some of these glues are more durable than others. Bac- Phacelia has a shallow root system but has highly branched sur-
terial glues are short-term in nature, while the most durable face roots that aggregate the soil surface and allow better infiltra-
glue is called glomalin, a compound secreted by mycorrhizal tion of rainfall and oxygen. Flax has an unimpressive root system
fungi. The problem is that most of our cropland soils are now but is one of the better cool-season mycorrhizal hosts and seems
devoid of mycorrhizal fungi, as these fungi must have a live to have a very strong aggregating effect on the soil. Sweetclovers
root for a host, and soon starve out with the fallow periods (yellow, white, and Hubam) have both strong taproots and high
common in our crop rotations. Bacteria, fungi, and earth- rate of exudates. They are also good nitrogen fixers.

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