Ig­nore those ugly ru­mors about mulch

The Progress-Index - At Home - - NEWS - By Lee Re­ich

Ru­mor has it that mulching your gar­den beds or trees and shrubs could starve your plants. It’s a ru­mor that has cir­cu­lated for the past 40 years or so, ever since mulching surged in pop­u­lar­ity as a way to quell weeds and con­serve wa­ter.

Is there any­thing to it? If any­thing, you’d think that nu­tri­ents in mulches would help nour­ish plants, not starve them.

The logic be­hind the star­va­tion ru­mor goes like this: The two nu­tri­ents most needed by soil micro­organ­isms are car­bon and ni­tro­gen. Wood chips, straw, saw­dust and many 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 car­bon-rich diet with ex­tra ni­tro­gen, which they must find some­where else. So they pull this ni­tro­gen from the soil, and are a lot bet­ter than plants at get­ting it. The re­sult: plants starved of ni­tro­gen. All true. How­ever, this ni­tro­gen star­va­tion is only tem­po­rary, for starters. As soil micro­organ­isms die, the ni­tro­gen in their bod­ies is re­leased back to 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.

Also, this sce­nario — plants be­ing starved for ni­tro­gen — holds true when you mix a load of high-car­bon, or­ganic ma­te­rial into the soil. But lay that same ma­te­rial on top of the ground as mulch and it’s a dif­fer­ent ball game. Then, de­com­po­si­tion oc­curs mostly at the thin in­ter­face where the mulch touches the soil, and the rate of de­com­po­si­tion is much slower. 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 ecom­po­si­tion. The micro­organ­isms are happy and the plants are happy.

Still, that ru­mor that plants will suf­fer from high car­bon mulches keeps go­ing around, de­spite the field ex­pe­ri­ence of agri­cul­tural re­searchers and many gar­den­ers.

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 soil that is 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.

Still, there’s 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, 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 it against ex­ces­sive heat. An or­ganic mulch also soft­ens the im­pact of rain­drops, so wa­ter can per­co­late into the soil rather than run off. Th­ese mulches also en­rich the soil with hu­mus, that witch’s brew of nat­u­ral com­pounds that helps feed plants and ben­e­fi­cial soil or­gan­isms to fend off pests, quell weeds and im­prove wa­ter use by plants.

So don’t pay at­ten­tion to those ugly ru­mors.

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