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Setting the record straight on mulch

Q: I’ve heard that some mulches can adversely affect garden soils by either making them too acidic or depleting them of nutrients. Is this true?

Colin Williams, Harrisburg, PA

A: Vincent Drzewucki, horticulturist at  Hicks Nurseries, Inc. , in Westbury, New York, replies: The benefits of using mulch in garden beds far exceed any adverse affects they might have on soil. Mulches applied to the soil’s surface control weeds, conserve water, moderate fluctuations in soil temperature to prevent heaving and frost damage, and keep roots cool in summer and warm in winter. Organic mulches improve soil structure and add nutrients to soil as they decompose, while any mulch around the bases of trees and shrubs creates a barrier that helps prevent injury to the bark from lawn mowers and trimmers. Mulches also enhance the look of the garden. 

Organic mulches such as leaves, wood chips, bark, and pine needles decompose over time and produce humic acid which has an acidifying effect in the soil. Also, the microbes in the soil use the available nitrogen in the process of breaking down mulches with high carbon contents such as wood chips, leaves, or sawdust. Any available nitrogen is thereby tied up by the microbes, making it less available to your plants. The longer it takes for the microbes to break down a material, the longer the nitrogen will be unavailable.

Both conditions can be corrected by simply adding soil amendments. Lime can be applied if you find that certain mulches have made your soil too acidic. And, if you tend to prefer using mulches that are high in carbon content such as wood chips or sawdust, counteract the unavailability of nitrogen by adding a high-nitrogen fertilizer like dried bloodmeal or milorganite.

Although composting organic mulches before using them would help avoid these problems, compost is not necessarily a good mulch by itself. A layer of compost in a garden bed would invite weed seeds to germinate in it immediately, and the well-decomposed compost would disappear quickly once the soil microbes got a hold of it, leaving your beds unprotected. 

From Fine Gardening 65, pp. 72