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Stump dirt and mushroom compost

Beech treeI consider stump dirt to be a miracle planting aid.  But what is it?

Stump dirt spilling out of a hollow tree

The obvious answer is --- that moist, dark, earthy-smelling organic matter found inside decaying trees or logs.  Different trees create stump dirt of varying quality; my favorite source by far is our ancient hollow beech halfway up the hillside, while box-elders product lower grade stump dirt.  Maybe hardwood stump dirt is better than softwood?

The analytical side of me started nibbling away at what stump dirt actually is a few weeks ago, and the best idea I've come up with is that stump dirt is pure organic matter created when fungi decompose wood.  The closest mainstream garden ingredient I could find is mushroom compost, but that is the result of fungi growing on higher nitrogen substrates like straw and manure, so any comparisons should be taken with a grain of salt.  One study of mainstream mushroom compost showed that it consisted of:

  • 28% organic matter and 58% moisture
  • 1.12% nitrogen, 0.67% phosphate, and 1.24% potatssium (aka NPK of 1.12-0.67-1.24)
  • 2.29% calcium, 0.35% magnesium, and 1.07% iron
  • C:N ratio of 13:1

Handful of stump dirtNaysayers on the internet report much lower NPK values for mushroom compost, though --- closer to 0.7-0.3-0.3 --- and I suspect our stump dirt is at the lower end of the fertilizing spectrum.  That would explain why the garden beds I treated with stump dirt last year didn't show much growth --- stump dirt isn't a replacement for compost.  Instead, it makes a great ready-made potting soil and can also be used like peat moss to fluff up organic-matter-poor soil.  If we ever had enough to apply stump dirt to our garden in large quantities, I suspect it would act a bit like biochar, providing spots for microorganisms to grow unhindered.  And stump dirt from deep-rooted forest trees is probably even higher in micronutrients than the analysis above portrays.

All of that said, you can't buy stump dirt, and you only find it in middle-aged to old forests.  I mine a couple of five gallon buckets every year out of our beech tree, but save it for extra-special occasions.  Another reason to have a mature woodlot on your property, perhaps?

Never see another chick drown when you switch to our chicken waterer.


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