Vapors from sour mulch actually kill grass cause yellowing or even whitening of shrub leaves nearby.

Vapors from sour mulch actually kill grass and cause yellowing or even whitening of shrub leaves nearby. With the Solvita compost test, the vapors cause a bright distortion.

Have you heard the term sour mulch before? Washington state plant pathologist Dr. Ribeiro has often warned about how to spot it – and avoid it. But a mulch that “kills at a distance”? A northeast mulch producer submitted his product to Woods End lab with the caveat -“I’m not sure you can figure out what’s wrong”.

A quick Solvita compost test showed a powerful bright yellow indicative of extremely high volatile acid levels. Our lab ran a scan for organic acids and found 37,000 ppm of formic acid (among others) – an unusually high level of a single organic acid, and at that, one of the most plant-toxic forms.  Formic acid generation is partly linked to splitting of pyruvic acid in wood during decay. But such high levels are unusual and may be associated with an additional factor: huge piles that undergo high heating and partial pyrolysis in the absence of air. That’s a virtual “factory” for acetic and formic acids.

Wood pyrolysis chemistry is complex and has not been fully elucidated. Research on wood subjected artificially to partial pyrolysis with heating shows formation of methanol, acetic acid and formic acid. “It’s not as unusual at it sounds”, says Brinton who points out that OSU’s emeritus professor Harry Hoitink frequently warned wood chip and bark producers to avoid huge piles that cause partial pyrolysis.

Our tests of the northeast mulch so far appear to confirm this. It is the 2nd mulch sample recently that showed toxicity with extreme foliar damage from vapors coming off the mulch.  The mulch was so toxic that 1:100 dilutions into soil still showed some inhibited plant growth. The cure? – further treat the mulch so that the organic acids naturally biodegrade. The Problem with the cure? With a pH driven down to 2.2, there’s virtually no biology at all. A slow stepwise process of mediating pH with lime and allowing ample access of air, may remove the toxic natural agent – “we just don’t know how long it will take”. Dr Ribeiro has recommended blending manures into sour mulches to improve biological activity.