Mother Nature mulches in the fall by dropping leaves from her trees and by laying the blades of grasses or the leaves of herbaceous perennials over the soil. She covers the ground only with the waste she produces.
We, on the other hand, buy bags of ground bark, chipped wood scraps or colored wood waste from only God knows where, pile it over the soil and call it mulching. I see mulch piled so deep trees seem to emerge from volcanic cinder cones. Roots of shrubs gasp for air and die from suffocation. Dense mulch absorbs most of the rain before it can penetrate to the soil, and plants suffer in drought.
The leaves that fall to the forest floor in autumn serve as a blanket of insulation, allowing the soil to remain warmer longer and roots able to absorb water longer. The longer roots absorb water, the more resistant they become to damage by freezing temperatures. The leaves will decompose during the growing season, allowing nutrients to return to the soil for roots to absorb.
Ground bark sold as mulch, on the other hand, contains very few nutrients. Decomposing, the mulch leaves behind clay-like particles called colloids. As colloids accumulate from repeated applications of ground bark, a slime-like layer forms over the soil, reducing air movement. Roots need oxygen, and they generate carbon dioxide. A thick layer of mulch over a colloidal layer can cause a toxic accumulation of carbon dioxide.
Hardwood bark decomposes faster than pine bark, creating a colloidal layer sooner. Double-shredded hardwood bark mulch decomposes within a year, leaving behind fine organic colloids.
Repeated applications of hardwood bark and especially double-shredded hardwood bark also raise the pH of soil and accumulate manganese. Since manganese is not very soluble, it accumulates to toxic levels within seven to 10 repeated applications. When the manganese levels in the soil exceed the levels of iron, copper and zinc, roots are unable to absorb iron for photosynthesis. Thus repeated use of hardwood bark mulch is a double-edged sword.
Novice home gardeners like hardwood bark mulch because it is dark, keeps that color and does not easily wash away. But out of sight, trouble is brewing. Early signs of manganese toxicity are a gradual decline in growth, iron-deficiency symptoms on the newly emerged leaves, stunted growth and extensive branch dieback.
Often, the only solution is removal and replacement of plants and soil.
Repeated application of hardwood bark and composted wood chips recently forced one commercial blueberry grower to dig up an acre or more of plants. Manganese had accumulated to nearly 400 pounds to the acre, killing the formerly well-established and productive plants. Lowering the manganese from toxic levels took plowing the fields to a depth of a foot to dilute surface soil by blending in sub-soil.
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