my front lawn is 50-feet wide x 100-feet deep. The soil underneath is very RED CLAY. So I've sheet-mulched pretty much the entire yard and buried it all with a thick layer (4" or more) of woodmulch. I have two raised beds (4-feet x 40-feet); I have one bed 8-feet x 40 feet; and I have a large bed 20-feet x 40 feet. In between all of these beds, I have planted fruittrees.
An apple tree was delivered the other day and when I dug the soil, it smelled like sewage and was very wet. I planted my tree higher than the heavy moisture to keep from drowning it. We've had some wet weather. Is the wood mulch causing the ground to sequester TOO MUCH water?
Is the sewage smell a problem? or should I just ignore it (not that I would have any idea what to do about it).
my soil amendments are horse manure, pig manure, bio-char, wood mulch (mostly hardwood), and a few buried logs here and there. Most of the fruit tree are starting to throw off some new growth.
Anyways. long story short. The soil smells like ass. Is that a problem? or is that a good thing?
When I get a long stretch of rain, my garden gets a strong earthy smell. My husband finds it offensive, though I don't. But the odor goes away as soon as things dry out some. So possibly the excess moisture is causing the same effect for you, but since you've incorporated non-composted manures, your smelling the anaerobic by-products off of the manure decomposition. Just guessing. I've never worked with non-composted horse and pig manures.
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What part of the world are you in? What's the weather like lately? Snow, rain, dry, hot, cold, warm...? What's the temperature like under the mulch compared to the top?
Probably just the air-hating invisible beasties doing their thing. Perhaps inoculate with a bit of bokashi to populate it with friendly air-hating invisible beasties (anaerobic bacteria).
But then again, sewage smell can also mean sewage or other nasty things. Is it sulphuric smell, like rotten egg? There is usually a distinctly different smell from anaerobic manure issues to human sewage system issues, but it takes some experience learning the two.
Do you have a septic tank/field? If so, how close to your garden/mulch area? Are you on city sewer - and if so how is it maintained? Are you on a well? If so, where is it in relationship to the area you're talking about? If not, where does the city water run? What else is underground in your neighbourhood - what else has been underground in the past? There could be something there that hasn't been maintained properly, I know there is a lot of trouble with our city and their funding to maintain the underground infrastructure - I imagine other municipalities have this problem too.
When you are dealing with clay soil it is important to open the substrate up so that organics can filter in, this is best done with a broad fork. What has happened in your situation is that the sheet mulching created an anaerobic system, in effect smothering the soil beneath.
While this is not always a completely bad thing (it can eventually start to get the sweet earth smell once all the anaerobic bacteria have completed their work) it is not normally what folks are seeking when getting ready to plant.
I would start opening up the clay substrate so amendments can filter in. I would also plant deep, big root crops (daikon radish for example) to chop and drop so they can decompose and add organic material into the substrate so it will start to open up and be able to respire the way good soil should.
This is not a situation that can't be remedied, you do have a good start laying on top, it just needs a way to get down into the substrate.
For a quick fix that is by no means ideal, you could go around with a rake and fluff everything up. This would allow some portion of the mulch to dry out in the sun and wind, so that sow bugs and other agents of decay can do their job in an aerobic environment.