Growing roots exert a pressure of up to 25 atmospheres – equal to the pressure you’d feel 800 feet under water. Yet compacted soil will stop roots dead in their tracks. This article will discuss how soil gets compacted, and how to mitigate and prevent compaction in lawns, vegetable gardens, and landscape areas.
A brief primer on soil structure
The fundamental particles of soil are tiny bits of various minerals – essentially, weathered rock. Organic matter holds these soil particles together, forming aggregates. Tiny aggregates themselves associate into larger aggregates, again held together by organic matter (see picture). Fungal hyphae and fine root hairs also help to generate aggregates.
The amount of aggregation determines the structure of the soil. If a soil has many large aggregates, there will be many large pores between the aggregates because the aggregates can’t fit tightly together. In a healthy, well-structured soil, pores make up half the volume of the soil. Soils with poor aggregation have less pore volume because their small aggregates pack tightly together (see picture).
Pores are essential for plant growth:
(1) Pores are filled with air and water. Pores usually contain about equal amounts of each, depending on how wet the soil is. Roots absorb nutrients dissolved in the water, for example potassium, calcium, and iron. Roots also absorb the air in the pores, providing the oxygen that roots need to grow.
(2) Pores provides pathways for root growth.
Compaction, on the other hand, destroys soil pores by squeezing aggregates together – which, as discussed above, is easier when aggregates are small. This starves plants of nutrients and creates a physical barrier to root growth. Two practices can prevent, and to some extent reverse, compaction:
(1) Promote aggregation by giving soil plenty of organic matter. 5% organic matter is a typical level for a healthy soil. A soil test can measure organic matter. Soil test kits can be picked up at the VCE office at the Albemarle County office building at 460 Stagecoach Road in Charlottesville.
(2) Avoid physically compressing soil by minimizing foot and machine traffic. If traffic is unavoidable, wait until the soil is dry before walking or driving on it.
Below are recommendations for mitigating and preventing soil compaction in a few locations around the home.
Vegetable gardens
Preserve organic matter by eliminating or minimizing tillage. No-till is counterintuitive (and controversial). Tillage certainly softens the soil’s top layer in the short term. In the long run, though, tillage depletes the soil’s organic matter by over-aerating the top few inches of soil. Soil microbes respond to the increase in oxygen and consume the soil’s organic matter. This, in turn, causes soil aggregates to fall apart. (Some experts do suggest one-time tilling of compost into new beds to speed up soil improvement.)
Instead of tilling, add organic matter by spreading compost or mulch over the surface of the soil. The organic matter will work its way into the soil over time. Accelerate the infiltration of organic matter by loosening the soil with a spading fork, broadfork, or garden rake. This opens up small gaps in the top few inches of soil, but without excessive aeration and aggregate breakdown .
Cover crops are another effective way to add organic matter and loosen the soil. There isn’t space here for an-depth discussion of cover crops, but Piedmont Master Gardener Cleve Campbell has written a great introduction to cover cropping. Cover crops’ roots add organic matter to the soil. Clovers, and legumes like hairy vetch, boost soil nitrogen as well. Cover crops’ roots also break apart compacted soil. Forage radishes are particularly good at punching holes in heavy soils.
Terminating cover crops is another complex topic, covered in detail here. It’s important to terminate cover crops before they go to seed and become weeds! Don’t till cover crop residue back into the soil, for the reasons mentioned above. Rather, cut the cover crop at the soil’s surface using a stirrup hoe or hand sickle – or even a lawnmower. Compost the cover crop residue or simply leave it on the soil surface, a practice termed green manure. The residue will decay and release organic matter into the soil.
Finally, minimize foot or machinery traffic on the cultivated parts of the garden. Paths between the garden beds are a good way to keep traffic off the cultivated beds. If you must stand or kneel on the beds, support your weight on a board to spread the pressure out.
Lawns
Aeration is the best way to alleviate compaction in lawns. Usually this is done with a rented machine. Find a device with hollow tines or spoons that bring soil cores up to the surface. Spikes or blades that cut into the soil aren’t as effective: they let some air into the soil, but also create compaction around the holes they cut. Spiked shoes also are not a good solution, for the same reason.
After aerating, it’s not necessary to remove the cores. If you don’t mind the appearance, simply wait for them to fall apart. A faster method is to wait a few days, until the cores have dried a bit, then rake them into tiny pieces and compost or leave in place. If desired, follow up aeration with overseeding, fertilizing, and liming. (Don’t apply lime without first doing a soil test to measure soil pH.)
The best time to aerate is when the grass is growing at its maximum rate. In other words, aerate in spring or fall for cool-season grasses and summer for warm-season grasses. Also, don’t aerate when the ground is very dry; this will decrease the effectiveness of the aerator and may even damage the tines. Also avoid aeration when the ground is very wet; this can make compaction even worse.
How often should you aerate? A home lawn shouldn’t need aeration more than once a year. If you notice water puddling, this is an obvious sign that compaction is a problem. Similarly, heavy thatch – the layer of mixed living and dead material between the grass blades and the soil – is a sign of compacted soil. If the thatch is more than an inch thick, it’s time to aerate.
Prevent compaction by minimizing foot and machine traffic. Of course, foot traffic is a main reason for having a lawn! Paradoxically, one solution is to concentrate traffic in a small area, for example by making pathways of mulch or stepping stones in high-traffic areas. Similarly, sometimes machine traffic is unavoidable. Don’t drive machinery across lawns when the soil is very wet. Drive machinery on boards or sheets of wood to spread out the weight.
Landscapes
Compaction in landscaping is tricky because these areas don’t lend themselves to invasive methods. Moreover, trees and shrubs are particularly sensitive to compaction because their roots extend quite far from their trunks.
Alleviate compaction by adding organic matter to the surface, in the form of mulch. The mulch will break down and organic matter will, slowly but surely, fall into the soil below. Wood chips, shredded bark, and mulched leaves are good choices, inexpensive (or free) and easy to obtain. Allow leaves to decompose in place under trees. If leaving leaves in place is for one reason or another unacceptable, rake the leaves onto some grass, shred them with a lawnmower, and return them to the area under the trees.
Under trees, mulch should extend to the outer extent of the branches (see picture). This perimeter is called the drip-line of the tree. The mulch should be only 2-4” deep. Any deeper and it will impede decomposition and release of organic matter into the soil. Don’t mound any mulch against the base of the tree trunk – the notorious “mulch volcano” (see picture). If mulch covers the bark, moisture can cause the bark to rot, and animals may gnaw on the covered bark. The root flare – the widening of the trunk where it begins to divide into roots – should be visible.
A professional arborist may use an air spade to loosen heavily compacted soil around a tree. An air spade uses supersonic jets of air that remove soil – even when heavily compacted – but leave roots largely undamaged. After removing a few inches of compacted soil, higher-quality soil, with plenty of organic matter, is added in its place. Needless to say, this is an extreme, and expensive, solution.
Given the difficulty of remedying compacted soil in landscape areas, prevention is critical. Mulch works very well for this because it discourages foot traffic. Direct foot traffic away from trees and shrubs using pathways of mulch or stones. Don’t drive or park heavy machinery near landscaped areas.
Closing thoughts
Soil compaction is a serious problem anywhere humans have disturbed the soil, whether in a yard, a cityscape, or a farm. Prevention is by far the best way to combat compaction because compaction is hard to reverse, particularly in a short time. Adding organic matter to the soil, in one way or another, is the primary way to alleviate compaction. This, as discussed in the “primer” section above, promotes aggregation of soil particles and a robust network of pores that allow root growth and infiltration of air and water into the soil. The other key practices are an observant eye, for signs and symptoms of compaction, and, of course, patience.
References and further reading
Elements of the Nature and Properties of Soils, 4th Edition. Ray R. Weill and Nyle C. Bandy
Air trench digging or loosening soil. University of Florida
Assessing and addressing soil compaction in your yard. New Jersey Agricultural Experiment Station
The Biology of Soil Compaction. Ohio State University Extension
Cover crops. Cleve Campbell, Piedmont Master Gardener
Forage radishes, a hard-working cover crop. Cleve Campbell, Piedmont Master Gardener
How to control thatch in your lawn. University of Minnesota Extension
Management of compaction – coring. University of Massachusetts Extension Turf Program
Managing soil compaction. Oregon State Extension
Mulching landscape trees. Penn State Extension
Physical Root-Soil Interactions Physical Biology 2017 (PDF). Evelyne Kolb et al.
Problems caused by compacted soil. University of Maryland Extension
Restore compacted soils. University of Colorado and U.S. Botanic Garden
Scraping: scuffle hoes. University of California Agriculture and Natural Resources
Strategies to avoid and manage soil compaction. University of California Agriculture and Natural Resources
Understanding soil compaction. New Jersey Agricultural Experiment Station
Virginia Tech Soil Testing Lab
Walkways in the landscape. University of Florida
What to do about compacted soils. Michigan State University Extension
Featured image: Volker Prasuhn, Wikimedia Commons (public domain image).
