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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, |
Our research has focused on whether this model of nutrient cycling, developed primarily through studies of forested and agricultural ecosystems, can explain effects of organic mulch on soil fertility and plant growth in ornamental landscapes. We compared two organic mulches that differ dramatically in their C:N ratios recycled ground wood pallets with a C:N ratio greater than 100:1, and composted yard waste (a blend of wood chips, leaves, and grass clippings) with a C:N ratio less than 20:1. The availability and use of both products as mulch is increasing dramatically because of increased recycling in an effort to divert organic wastes from landfills (Glenn, 1999; McKeever, 1999).
Because these products vary widely in their C:N ratios, we predicted that they would have dramatically different effects on nitrogen availability and plant growth. Composted yard waste, with its low C:N ratio, should release nutrients at optimal rates in slow release form, thereby increasing soil fertility and plant growth. Conversely, we predicted that the high C:N ratio of ground pallets would induce nutrient deficiencies and decrease plant growth by stimulating the growth of carbon-limited microbes, resulting in high rates of nutrient immobilization.
A field study was conducted in replicated plots at The Ohio State University's Ohio Agricultural Research and Development Center in Wooster from 1998 - 2000. Plots were mulched with composted yard waste, ground wood pallets, or were left untreated as bare soil controls. Mulch was applied to the soil surface in a layer two inches thick. Each spring, any residual mulch still remaining was removed and replaced with fresh mulch.
To determine how fertilization might interact with mulch to affect nutrient availability, half of the replicate plots from each of the three treatments were fertilized, and the other half were left unfertilized as controls. The fertilizer used was 18:5:4 NPK, with 56% of the nitrogen in slow release form (methylene urea), and 44% of the nitrogen in fast release form (17% ammonium nitrate and 27% water soluble urea). Fertilizer was applied at a moderate rate of 3 lbs N / 1,000 ft2 / yr (2 to 6 lbs N / 1,000 ft2 / yr is the recommended rate for trees and shrubs), with half of the annual amount applied at budbreak in spring and half in early October.
To determine how the experimental treatments affected soil parameters, soil was periodically sampled through the growing season to a depth of 6 inches (the zone in which most fine root activity and nutrient uptake by woody plants occurs). Soil samples were then analyzed for organic matter content, microbial biomass, total extractable nitrogen, immobilized nitrogen, nitrogen mineralization rate (the rate at which inorganic nitrogen is released from decomposing organic matter), and nitrogen in forms available for plant uptake (dissolved organic nitrogen and mineral nitrogen, including ammonium, nitrate, and nitrite).
The data reported represent the average of five sampling dates over the course of the 1999 growing season. A river birch (Betula nigra 'Cully Heritage') and rhododendron (Rhododendron 'Pioneer Silvery Pink') were planted in each plot to determine how these soil treatments impacted the growth of ornamental trees and shrubs. We also quantified flower production of rhododendron.