The Q&A Archives: Plant Fertilizer W/nitrogen

Question: I have a flower bed and I mulch it. I have been told that any kind of bark mulch depletes the soil of nitrogen, thus depriving my perennial and shrubs of nitrogen, is this so? If yes, what should I purchase to relenish the nitrogen in my soil. I usually use Miracle Gro to fertilize my plants.

Answer: That has been a gardening "truism" for years. You may have heard from fellow gardeners, gardening books, or articles that using mulches with a high carbon-to-nitrogen (C:N) ratio, such as wood chips, may take soil nitrogen out of plants' reach. It's true that microbes use soil nitrogen while they decompose materials with a high C:N ratio. But recently researchers from the Royal Horticultural Society's Wisley Garden in Surrey,England, did a number of studies on mulch. They found that very little nitrogen was actually tied up, and plant growth was not significantly affected. The study compared cocoa shells,
coarse bark, chipped wood waste, compost, horse manure
mixed with sawdust, and fine bark mulch to black plastic
mulch and bare ground. After 12 months, they measured soil
nutrient levels, pH, and organic matter content and compared
them with initial readings.

All plots with organic mulch showed an increase in soil
nutrient levels, even under wood waste and coarse bark,
which have relatively low nutrient content. Soil acidity
was reduced, even under acidic mulch (coarse bark; pH 5.2). So mulch on!

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