Compacted Soil

Compacted soil creates problems in the garden because water and air do not flow freely through the soil. Water stands on the surface because it can’t soak into the hard ground. Plant roots have trouble penetrating the hard earth, and when they do penetrate it, there isn’t enough oxygen for them. As a result, only shallow-rooted plants thrive on compacted soils. For instance, compacted areas in bluegrass lawns often are filled with annual bluegrass, a shallow-rooted weed.

Compacted soil is caused by repeated traffic, either foot or vehicular. The weight of the constant traffic compresses the soil, packing it so that air can’t penetrate. Foot traffic usually compresses only the top couple of inches of soil. Vehicles compress much deeper, and heavy construction equipment can compress soil as much as two feet deep.

Sandy soils are least likely to become compacted; heavy and loamy soils are the most susceptible. Wet soils compact far easier than dry soils. To avoid compaction at times when you are planning to have many people on your lawn, as for a party, don’t water for a few days beforehand. Dry soil has greater structural strength and compacts less than wet soil.

Solving Compaction Problems

Compaction can be corrected most easily by tilling the soil. Tillers dig down about 6 inches; if the compaction is shallower than that, a tiller will solve your problem. If the compaction is deeper than a tiller can reach, have a farmer with a chisel plow break it up. Chisel plows dig deeply into the soil without turning it over.

If this isn’t practical, rent a posthole digger or soil auger and dig holes through the compacted layer every couple of feet. Put the soil back in the holes as they are dug. This will allow drainage and root penetration.

In areas that are planted and can’t be tilled, dig holes through the compacted layer. In lawns and shallow ground covers, this is most easily done with a rented aerator.

Most aerators dig holes about 3 inches deep. This is enough to penetrate surface compaction. Where aerators can’t be operated, drill holes with a soil auger. Augers that drill holes two or three inches in diameter and attach to a power drill are sold for planting bulbs. Drill the holes as close together as possible, perhaps 6 inches to a foot apart.

Resist the temptation to poke holes with a crowbar or other tool. Because poked—rather than dug—holes compact the sides of the hole, they don’t solve the problem very well.

If you can wait, nature will relieve the compaction over time. Soil is a dynamic matrix, constantly being stirred by earthworms, insects, plant roots, and alternations of temperature and moisture. Eventually, the compaction will lessen, but it may take a few years.

Preventing Compaction

Once compaction has been relieved, you need to deal with the problem of preventing it from happening again. Here are some ideas:

Compacted soil often occurs along a regular traffic route, an area of easy access. If this area lies in the pathway of traffic, the best approach is to construct a regular path with stepping-stones, pavers, or some other ornamental surface.

Another approach is to alter the traffic pattern. Change traffic patterns with fences, hedges, or plantings. Prickly plants and shrubs with stiff branches are strong deterrents to both people and animals. Ground covers that are more than ankle deep are difficult to walk through and can be used to re-route traffic. A temporary fence will divert traffic until a more aesthetic hedge or ground cover can establish.