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Improve compacted soils

A trencher can be used to dig four or more trenches out from the planting hole.

A trencher can be used to dig four or more trenches out from the planting hole. Loosely fill the trenches with the same soil that came out of the trench, or amend it with organic matter if you wish. This provides loosened soil channels for root growth in a compacted site. Vertical trenches can be cut along a slope to slow runoff water.

Because roots grow poorly in compacted soil, it should be tilled or broken up with specialized heavy equipment prior to planting. Avoid doing this beneath the canopy of existing trees. Significant root damage could occur and the trees might die.

A coring machine can be used to enhance growth in compacted soil. It pulls soil cores out of the soil creating holes for water and air penetration. The soil cores are left on the surface of the soil. This is sometimes referred to as vertical mulching.

A different technique pulls tines over the site to create holes in the soil but they compact the sides of the holes and may not improve drainage and plant growth appreciably. A more sophisticated machine manufactured by Verti-Drain from Holland drives 16-inch-long tines into the soil. When the tines are fully inserted into the soil, they are deflected laterally several inches. This breaks up the soil in a manner similar to inserting a garden pitch fork into the soil and displacing it several inches.

These techniques can increase water percolation and reduce water runoff (figure above). Air excavation tools are useful for soil around existing trees (See: air excavation tools). A pitchfork can also be used to loosen and aerate compacted soil on a small site.

Several two- to three-feet-deep trenches can be dug from the planting hole like spokes in a wheel. A backhoe or trenching machine can dig trenches quickly. Some people refer to this technique as vertical mulching. Amended or original soil can be placed back into the trench although there is no evidence that amended soil increases root growth more than backfilling with original soil.

Although this may not provide all the benefits of loosening the soil around the entire planting hole, it may be less expensive and roots should be able to grow well in the loose, aerated soil in the trenches. This technique can also increase root growth on existing trees in some circumstances.

Gypsum applications can reduce effects of compaction only if it was caused by sodium-saturated exchange complexes in the soil. Gypsum will not reduce compaction if compaction is caused by other factors. A specialized soil test performed by a soil lab can reveal if sodium has saturated the exchange complexes.