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  • Writer's pictureAnneliese Abbott

Perils of Permaculture: Why the C:N Ratio of Mulch Matters


Learning Garden at Nature Center
My "pizza garden" at the Nature Center looked pretty good the first year--but not the second

Fresh out of my first year of agronomy classes, I was excited to do my first internship at an organically managed CSA. My job at the Kalamazoo Nature Center was a Farm Educator, and I was excited about to spend my summer teaching kids about organic gardening.

 

My supervisor was very creative and had all sorts of fun ideas. The centerpiece of the educational program was the Learning Garden. The year before, he had laboriously created an intricate and beautiful raised bed system. My favorite was the pizza garden—eight wedge-shaped beds with narrow paths between.

 

I was pretty much free to plant whatever I wanted in the Learning Garden, but my supervisor laid down a few rules. He was a strong advocate of permaculture and told me that the soil was not to be disturbed for any reason. Rather than pulling the dead stalks of last year’s plants up by the roots, I was supposed to cut them off at ground level. Perennial and annual plants were all mixed together, and the tops of the beds were covered with a thick layer of straw to control weeds.

 

For my first summer, this plan worked pretty well. I conscientiously avoided disturbing the soil at all except to dig small holes for transplanted plants. Everything grew well, though the veggies weren’t quite as big as the ones in the nearby CSA fields. The pizza garden was a big hit, with wheat on the outside of the circle and oregano and thyme plants at the tip of each slice. I enjoyed it so much that I decided to work at the Nature Center my second summer of college, too.

 

But the second year didn’t go nearly as well as the first. I followed the exact same management system, but the plants looked noticeably bad. They were stunted and yellowish, a sure sign of nitrogen deficiency. I was back to that same dilemma as my “organic” pot experiment—how do you add nitrogen to an organic system without tillage? I still didn’t fully understand organic nutrient cycling, and neither did my supervisor. The garden that year was an almost total failure, and we didn’t really know why.

 

It wasn’t until I carefully studied my soil science book that I found the answer. The culprit was the straw we were using for mulch. Straw has a very high carbon-to-nitrogen ratio—about 80 parts carbon to 1 part nitrogen. That means that, in order for soil microorganisms to decompose the straw, they have to suck nitrogen out of the soil. There’s no nitrogen left for the plants—hence the acute signs of nitrogen deficiency. And when we mulched with fresh straw each year, we were inadvertently continuing to tie up all that nitrogen and keeping it unavailable for the plants.

 

I was never able to make permaculture work at the Nature Center, but I have since learned that a permanently mulched system can indeed work. The key is that the mulch has to have a much lower C:N ratio than straw. In our CSA garden at Abbott Farms, we mulch with first-cutting hay, which decomposes rapidly and actually adds nitrogen to the soil instead of tying it up. I mulched my community garden plot in Madison with composted leaves, which also had a low enough C:N ratio to keep nitrogen available to the plants.

 

So it is possible to have an organic no-till vegetable garden! But only with low C:N ratio mulch.

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