Soil Compaction

Gardening by the Yard : Episode GBY-1107 -- More Projects »

Rototillers fluff up the top layer of soil, but below that, the tines create a slick and often impenetrable surface known as hardpan.

Soil compaction is the primary cause of many garden problems, says master gardener Paul James.

Drainage problems. If the surface of the soil is extremely compact, water can't penetrate into the subsoil, which means that water can't reach the root zone of plants. And as many gardeners know, if plants can't get a drink of water, they won't be plants for long! The soil compaction can be so bad that water literally runs right off the soil surface.

But soil compaction can also occur deeper in the soil, resulting in waterlogged soil, and waterlogged soil can cause all kinds of plant problems--from nutrient deficiencies and stunted growth to an overall lack of vigor and ultimately the demise of the plant.

Stunted root growth. Plant roots have a tough time penetrating through compacted soil. Roots in a nice, fluffy soil have no problem probing the depths of the ground. However, in compacted soil, roots can't go anywhere.

Causes and Cures

Causes of soil compaction vary. In some cases, soils are just naturally compact, and that's particularly true of heavy clay soils. But in most cases, compaction is due to heavy equipment such as lawn mowers--especially riding lawn mowers. Rototillers can also lead to soil compaction, especially deep down in the soil. The tines turn and fluff up the top eight to ten inches of soil, but beyond that depth, the tines create a slick and often impenetrable surface known as hardpan.

Figure A

Believe it or not, though, foot traffic is very often the No. 1 cause of compaction in most yards and gardens, and it's the easiest type of compaction to control. A board laid on the soil while you work in the garden can reduce compaction by distributing your body weight evenly along the board (figure A). Simply thinking twice before walking in garden beds will go a long way towards controlling compaction, as well. Of course you can't avoid walking in the lawn nor should you want to. There, the surest way to avoid compaction is to aerate at least once a year. A good gas-powered aerator will pull plugs from the soil, which in turn allows water and nutrients to reach the root zone of turf grasses.

Figure B

Even spiked shoes such as these will help (figure B), although they can also cause other problems--notably, sprained ankles. A light (1/4" to 1/2") layer of sifted compost or shredded leaves spread out over the lawn every few months will help loosen compacted soil because, as those materials decompose, they break up the soil. The same is true of mulching mowers; tiny grass clippings and shredded leaves go a long way towards loosening up the soil.

In established garden beds, a spading fork stabbed into the ground and rocked back and forth does an excellent job of loosening up the soil. And a broadfork does an even better job, although the weight of the tool makes the task a tad tougher. And, again, a light layer of compost, shredded leaves, grass clippings or any combination of those materials--essentially any form of organic matter--topped by a two- to three-inch layer of organic mulch, such as a nice shredded bark product,will slowly but surely make soil compaction a thing of the past.

Figure C

In the case of trees growing in compacted soil, use an auger made for planting bulbs (figure C). Drill several holes to a depth of six to 12 inches at random locations from the base of the tree to well beyond the drip line. Then fill the holes with compost.

"Loosening compacted soil is best viewed as an ongoing process rather than a quick fix," says James. "But believe me, folks, it's something that can and should be done."

Also in this Episode

Gardening by the Yard : Episode GBY-1107
•	Diagnosing Plant Problems
•	Soil Compaction
•	Cubicle Landscaping
•	Arum Scarem

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