Making Mulch Of An Old Rumor

Wednesday November 30, 2005

The Republican

Making mulch of an old rumor
Wednesday, November 30, 2005

Rumor has it that mulching your garden beds or your trees and shrubs could starve your plants. You haven't caught wind of this rumor?
It's been in the air and in print for the past 30 years or so, ever since mulching surged in popularity as a way to quell weeds and conserve water. You may wonder what mulching could have to do with feeding plants. If anything, you'd think that nutrients in mulches would help nourish plants, not starve them.
Well, the logic goes like this: The two nutrients needed in greatest quantities by soil microorganisms are carbon and nitrogen.
Wood chips, straw, sawdust and most other organic mulches are high in carbon but low in nitrogen. When soil microorganisms chew away on such mulches, decomposing them, they have to balance their diet with extra nitrogen that they find somewhere. They find this nitrogen in the soil itself, and they are a lot better than plants at getting it.
The result is that plants are starved of nitrogen. This nitrogen starvation, mind you, is only temporary. As soil microorganisms die, the nitrogen in their bodies is released back into the soil. There, it becomes available to plants once those microorganisms have used up enough soil carbon, "breathing" it out of the ground as carbon dioxide.
The above scenario does hold true whenever you mix a load of any high carbon, organic material into the soil; lay that same material on top of the ground as mulch, though, and you have a whole new ball game.
Then, decomposition occurs mostly at the thin interface where the mulch touches the soil and the rate of decomposition is much slower. Decomposition is so slow, in fact, that a steady state is reached where nitrogen is re-released at about the rate at which it is being used for decomposition. The microorganisms are happy and the plants are happy.
Still, that rumor that plants will suffer from high carbon mulches keeps going round and round. This, in spite of the above explanation and "in the field" experience of agricultural researchers and many gardeners, myself included.
A garden, like any biological system, represents a complex interaction of energies, so sweeping generalizations don't always hold. Yes, there are situations - rare - where that old mulch rumor may hold true.
One such situation would be where you mulched with a very high carbon, very low nitrogen material (sawdust, for example) on a soil very, very low in nitrogen.
Another situation would be where you planted a seed right into a high carbon mulch. The young seedling would be starved of nitrogen until its roots hit the soil below. No need to forsake the benefits of mulch in either of these situations; just sprinkle on some nitrogen fertilizer, such as soybean meal, to make up the deficiency.
In just about all situations, though, there's no need to do anything more than spread organic mulch right on the ground. In the coming months, it will insulate the soil against cold and then, when warm weather arrives, insulate the soil against heat, soften the impact of raindrops, enrich the soil with humus and nutrients, conserve water and quell weeds.
Don't pay attention to those ugly rumors.
Any gardening questions? E-mail them to me at lee@flying
beet.com and I'll try answer them directly or in this column. Lee Reich, Ph.D, is an avid gardener who, after more than a decade in plant and soil research with the U.S. Department of Agriculture and Cornell University, turned to writing, lecturing and consulting. He is a master gardener and author of books including "A Northeast Gardener's Year."