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Measure Garden-Soil Compaction With a Carrot Test

By Anna Hess and Mark Hamilton

Tags: sustainable agriculture, soil, Virginia, Anna Hess and Mark Hamilton,

Carrot shapes

Carrot shapes are influenced by soil texture.

As you pull carrots out of your garden this fall, you can use the roots to get an idea about your soil's quality. You might have already noticed the differences in shape between carrots grown in different parts of your garden in year past. For example, did you ever dig up a bed of carrots and find that all of the roots had split and twisted into a jumbled mess? Sometimes, carrots curl around each other because you didn't thin the crop sufficiently. But splitting, gnarled carrots that aren't closely intertwined are generally a sign that your soil is either compacted or is full of pebbles and rocks.

Soil compaction

Compacted soil (on the right) lacks both the small and the large pores that allow roots, rain, and air to move efficiently through the earth. Often, a hardpan layer (darker brown in the drawing, but not distinguished by color in actual soil) develops just beneath the level that a plow or rototiller can reach.

What do I mean by compacted soil? Even though the earth seems solid when we're striding across it, as soon as you start peering closely at the dirt, you'll notice lots of air spaces between the grains. Unfortunately, it's relatively easy to mash your soil down so those air spaces disappear, a process known as compaction.

Simply walking on your garden soil can remove air spaces, which is why many gardeners create permanent aisles and beds, concentrating all of their foot traffic in certain sacrifice zones. Traditional tilling also creates compaction issues, especially if your soil is heavy or if you till when the ground is too wet or too dry. So your first step in dealing with compaction is changing your own habits so the problem won't come back.

What's next? You can physically fluff up soil with the broadfork, a tool that opens up spaces between soil particles without turning the layers of the earth. But before you rush out and buy expensive tools, I should tell you that moderately compacted soil often responds just as well to the action of biotillage cover crops like oilseed radishes. These deep-rooted plants easily push their roots through hard layers of soil, leaving biopores behind after they rot in place and increasing soil organic-matter levels in the process.

Okay, I know I just threw a technical term at you, but biopores are pretty easy to understand (and even to see in your soil). These large air channels start at the surface of the ground and run several feet into the earth, turning the openings into superhighways for soil-dwelling critters like earthworms. Meanwhile, biopores give roots quick access to other parts of the earth profile and also make it easier for rain to infiltrate deeply rather than running off during deluges. Finally, biopores promote faster carbon dioxide and oxygen exchange between the air in your soil and the air above, which helps encourage the aerobic microorganisms who do such good work decomposing organic matter and providing nutrients for your crops.

Side-view of a raised bed

The easiest way for a gardener to see soil-pore formation in action is to take away one of the boards supporting the side of a raised bed. You'll likely notice earthworm channels, smaller pores that follow roots, and the crumbly structure of good soil. Photo credit: Brian Cooper.

Biopores aren't the be-all and end-all of soil structure, though. In fact, much smaller channels between soil aggregates are just as important for healthy crops. These minuscule pathways do some of the same work as biopores, helping with air exchange and water management for example. But the smaller air cavities work a bit differently—rather than helping rain soak into the earth, mini-pores ensure that your soil can hold onto the falling water so all of the moisture doesn't drain away between storms. Small channels also allow water to move upwards from the groundwater into the root zone during droughts via capillary action, so they're doubly important for ensuring your crops find enough water to grow and thrive.

What can a gardener do to produce these essential, tiny channels between soil aggregates? The best solution is to add lots of organic matter and then beg your soil microorganisms to do the work for you. In fact, spreading mulches and other amendments directly onto the soil surface is like putting up a sign reading "Seeking earthworms—apply within." Worms will inevitably show up eat the tasty treats in situ, then they'll poop out high-nutrient castings deeper in the earth. And while moving between the two locations, the worms create—you guessed it—holes in the soil for roots and air to follow.

A third type of even smaller pore is created when minuscule soil particles are chemically bound together into aggregates, which range in size from nearly too small to see all the way up to several inches in diameter. These aggregates usually begin forming when roots or fungi increase in girth while thrusting their way through the soil, an act that pushes soil particles together on either side of the roots or fungal hairs. This slight compression of the soil is then cemented into more long-lived aggregates when microorganisms eat nearby organic matter and create gummy secretions to bind the soil particles in place. Next, calcium ions in the soil merge small aggregates together into larger particles known as peds.

Okay, that got a bit technical, but the bottom line is simple. Tiny air channels in soil form between soil aggregates, and soil aggregates form due to living things like roots and fungi doing their job deep in the earth. Larger pores form along earthworm channels, and yet more massive channels are due to the work of deep-rooted crops.

In the end, promoting healthy critters promotes healthy soil. And healthy soil means straight, unbranched carrots—gotta love it when you can eat your report card!

Personality Tests For Your Soil

Did you enjoy this excerpt from Personality Tests For Your Soil? If so, you can learn more easy ways to gauge the health of your garden earth in the ebook, on sale for 99 cents this week.

Anna Hess enjoys writing about her adventures, both on her blog at Walden Effect and in her books. Her first paperback, The Weekend Homesteader, helped thousands of homesteaders-to-be find ways to fit their dreams into the hours leftover from a full-time job. Hess is also the author of The Naturally Bug-Free Garden, Trailersteading, The Ultimate Guide to Soil, and several ebook-only titles. She lives with her husband in the mountains of southwest Virginia.

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