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John Lloyd recently completed his Ph.D. degree
in entomology at The Ohio State University,
Ohio Agricultural Research and Development Center,
in Wooster, and is currently an assistant professor
in the Department of Plant, Soil, and
Entomological Sciences at the University of Idaho in
Moscow, Idaho;
Dan Herms, Ben Stinner, Harry Hoitink, |
Mulching is one of the most widely used cultural practices in ornamental landscapes. Our research has shown that organic mulches can have major effects on soil fertility and plant growth. Mulching with low C:N composted yard waste increased plant growth by increasing soil organic matter, microbial biomass, and nutrient availability, which demonstrates that composted yard waste serves as high quality organic fertilizer as it decomposes.
On the other hand, high C:N mulch derived from recycled wood pallets induced nutrient deficiencies and decreased plant growth. The high carbon content of the ground wood stimulated the growth of soil microbes, which competed more successfully than plants for the limited supply of nutrients.
The nitrogen-depleting effect of mulch diminishes over time as it decomposes. As microbes die and decompose, the nitrogen they contain is released for use by plants unless the carbon source is replenished by adding fresh mulch. Nitrogen immobilization by microbes will probably have a greater impact on herbaceous plants and newly transplanted woody plants than on well-established trees and shrubs with extensive root systems.
Nevertheless, it may be best to reserve mulches with a high C:N ratio for use away from plants, such as on paths. Alternatively, these products can be blended with composted materials with a low C:N ratio, such as yard waste, animal manure or sewage sludge.
Soils in landscapes surrounding new homes and other buildings are often nutrient deficient with little organic matter because topsoil is removed and soil profiles are inverted during construction. We've shown that mulching with composted yard waste decreases soil compaction while in creasing organic matter, microbial biomass, nutrient availability, and plant growth. Yet, ironically, yard trimmings are often collected and removed from ornamental landscapes. Composting them instead for use as mulch offers great potential for rehabilitating degraded soils, while diverting a valuable natural resource from landfills.