Nitrogen-Fixer Fail! Adding Mimosa Leaves Decreased Plant Growth

Adding mimosa leaves as a fertilizer to a bed didn’t increase plant growth. Instead, it seems to have stunted it.

A couple months ago we set up a test where we added mimosa (Albizia julibrissin) leaves to one bed, with a control bed next to it. Then we planted a variety of brassicas, including mustard, pak choi and cauliflower. Interestingly, the bed with the mimosa leaves has done worse than the one without.

We got the original idea for using mimosa leaves as fertilizer from a book titled Restoring the Soil by author Roland Bunch. In that book, Bunch describes using fresh Gliricidia sepium leaves as a fertilizer by burying them next to growing crops.

We decided to do the same thing with mimosa.

Here’s what we found:

While watching that video, James asked in the live comments section:

Albizia julibrissin might have some use as a nitrogen-fixer due to the species below-ground nodulation; however, using the high-protein leaves to fertilize a garden bed was not effective. We have used alfalfa as fertilizer before and planted right away, which gave us good results, so the problem isn’t because we were “burying leaves” and soaking up the nitrogen in the soil. The nitrogen in these leaves would preclude that anyways. Burying high-carbon materials can be a problem, not buying “green” composting materials.

There is another issue going on here. My guess is that mimosa is allelopathic and suppressed the growth.

Allelopathy in Mimosa

There is a noticeable difference when the beds are compared side-by-side. It seems there is an allelopathic effect from the leaves which dwarfs brassica growth and induces earlier bolting.

A reader shared a study claiming A. julibrissin is allelopathic to the invasive “Tree of Heaven:”

“The current study evaluated the allelopathic potential of the Persian silk tree (Albizia julibrissin) leaf residues on seed germination and biomass attributes of the Tree of heaven (Ailanthus altissima) seedlings. An experiment was conducted based on a completely randomized design by eight replications. Experimental treatments consisted of different levels of leaf residues at concentrations of 0%, 2.5%, 5%,and 7.5%.Germination index was used in order to investigate the effect of allelochemicals on seeds germination. In addition, the effect of allelopathic materials on seedling biomass was measured by calculating the parameters of collar diameter, fresh and dry weight of seedlings, root dry weight, number of leaves, seedling weight vigor index and percentages of seedling water content index. The results revealed that seed germination index, seedling dry weight, seedling fresh and dry root weight and seedling weight vigor index significantly decreased at different concentrations of leaf residues compared to control, but the increase in concentration had no significant effect on these attributes. But higher concentrations of leaf residues had stronger inhibitory effects on seedling collar diameter, fresh seedling weight and the number of leaves per seedling. The present study clearly proved the allelopathic effects of the Persian silk tree (Albizia julibrissin) leaf residues on biomass attributes of the Tree of heaven (Ailanthus altissima) seedlings. In addition, the Persian silk tree (Albizia julibrissin) leaf residues can be used to control the irregular growth of Tree of heaven seed as an invasive species, as well as weed management in agroforestry systems which require further studies.”

Note that it reduces biomass, and that it was the “leaf residue” which did so.

I’ve used mimosa as a chop-and-drop tree for a decade or more, yet it appears this might not be a good idea. This is why we do backyard science experiments, instead of just assuming that a method or a species will work.

WITH MIMOSA:

WITHOUT MIMOSA:

Remarkable!

Looks like I’ll just have to plant more comfrey.

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