Background
Soil compaction around trees is a serious and persistent problem. It can happen when the earth is compressed by vehicle treads and foot traffic.
Procedure
• Find out if your soil is compacted by digging a hole about 1 m deep. Fill it with water and see how long it takes for the water to drain. If it takes 12 hours or more, it probably means the soil is compacted or contains a lot of clay. Well-drained soil holds little water, even after a heavy rainstorm.
• If the soil is compacted, break up the upper layer of earth with a hoe or pick, then lay a thick bed of mulch over it. Mulch is a mixture of organic materials like leaves and wood chips that can be spread around or over a plant to enrich and insulate the earth. It stops soil erosion and often prevents the growth of unwanted vegetation. Rather than working it into the earth, just let it sit and rot.
• Do not use material such as straw that encourages mice to nest or provides them with cover in winter. They will remove the bark from seedlings or saplings.
• You may also want to add some earthworms, although they'll probably arrive by themselves. The mulch will rot from above while the tree's roots break up the earth and the earthworms mix it all together.
Some easily available mulches are:
• Hay: The best type is spoiled hay (hay that has been ruined by rain and is unfit for animal feed).
• Leaves: You can keep them in place with sheets of plywood held down by a few stones.
• Lawn clippings: Don't lay them on any thicker than 15 cm.
• Deciduous wood chips and dry weed stalks.