Mulch and Soil Fertility

Mulch is the gardener's greatest time saver—just make sure it doesn't monopolize all the nitrogen.

August 26, 2013

Like the decaying leaves and twigs that blanket forest soils, garden mulch benefits plants by reducing moisture loss through evaporation, insulating soil from winter cold and summer heat, minimizing erosion and compaction, and suppressing weeds.

Depending on your location, you probably have access to a wide range of organic mulches—those derived from plant matter—including hardwood and softwood barks, compost, hay, lawn clippings, leaf mold, pine needles, sawdust, straw, wood chips, and the hulls of plants such as buckwheat, cocoa beans, or peanuts. All of these will decompose over time, feeding soil microorganisms and releasing small quantities of nutrients.


But mulches are not equal when it comes to their effects on soil fertility. An Ohio State University research team demonstrated this clearly when they compared the growth of river birches mulched with either pulverized wooden pallets or compost made from yard waste; trees grown in bare soil served as experimental controls. Their findings: Trunk growth and foliar nitrogen were greatest in the trees mulched with compost, and lowest in those mulched with the shredded pallets.

This research highlighted the interactions between plants and the invisible multitude of bacteria and fungi that feed on the same pool of essential soil nutrients as plant roots. Their numbers increase when carbon is readily available—such as when carbon-rich mulch is layered on top of soil. As microbe populations explode in the presence of carbon, they take up and immobilize nitrogen, making the nutrient inaccessible to plants. As mulch decomposes, carbon levels gradually decline and the microbes release—or mineralize—nitrogen back to the soil, making it available again to plants.

The key to finding and using the best mulch for your circumstances is to understand the carbon-to-nitrogen (C:N) ratio of various materials. In mulches with a C:N ratio greater than 30:1, not enough nitrogen is present in the mulch to support microbial growth, so microbes scavenge what they need from surrounding soil, outcompeting plants in the process. In mulches with a C:N ratio less than 30:1, the amount of nitrogen in the mulch exceeds microbial requirements, leaving more for plants.

Gardeners can decrease the competition between plants and microbes by adding fertilizer. In an extension of their study, the Ohio State researchers applied 17 pounds of synthetic fertilizer with an 18-5-4 analysis (the equivalent of 3 pounds of actual nitrogen) per 1,000 square feet of mulched area. In the plots mulched with compost, additional nitrogen did not increase the trees' growth, indicating that compost alone fully met the nutrient needs of both microbes and plants. But when they added fertilizer to the plots mulched with shredded pallets, plant growth was substantially greater because the trees no longer had to compete with soil microbes for nutrients.


The bottom line: Use mulches with a high C:N ratio on paths and in other areas away from plants. If you want to use these high-carbon mulches around plants, increase their fertility by blending them with materials with a low C:N ratio or by adding nitrogen fertilizer. Use mulches with low C:N ratio on degraded soils, new landscapes, or in spots where you want to spur plant growth. If it's possible to analyze the nutrient content of the products you use, blend them to the point that the C:N ratio drops to about 30:1 or lower.

Organic Materials and their C:N Ratios
Recycled pallets   125:1
Ground pine bark   105:1
Fresh wood chips   95:1
Hardwood bark   70:1
Fresh wood chips with foliage   65:1
Pine straw   64:1
Freshly fallen leaves   55:1
Composted wood chips   40:1
Composted yard waste   17:1
Composted manure   12:1

Originally published in Organic Gardening Magazine, Oct/Nov 2013