Can you have too much organic matter in your soil?
My findings that adding
lots of organic matter actually helped equalize cation levels in the
forest garden leads me into a discussion of the most controversial
part of Steve Solomon's book. He agrees with the organic status
quo to a certain point, recommending that we add organic matter until
our soil contains 4% organic matter in the south and 7% in the north,
and until our CEC is at least 7, levels that are achieved through
topdressing half an inch of compost per year. However, Solomon
argues that after you reach your organic matter goal, you should back
off to topdressing with only a quarter inch of compost per year since
adding too much organic matter unbalances your soil.Tables
Organic matter is
closely linked to nitrogen, and that was my first concern when I
considered Solomon's low-compost policy. Each 1% organic matter
in your soil releases about 15 to 25 pounds of nitrogen per acre per
year, and high-demand vegetables need around 200 pounds per acre.
If you run your garden hard, like I do, you might end up growing two
low-demand crops and one high-demand crop each year, for a total
requirement of 400 pounds of nitrogen per acre, but a more average
yearly demand would be 200 to 300 pounds.
Time for some
math. The 8.95% organic matter already in my front garden (which
is my best soil) would release 179 pounds of nitrogen per acre
annually, but I need another 100 pounds or so. How do I make up
the nitrogen deficit if not with compost? Solomon uses purchased
additions of seed meals, feather meal, and fish meal to bring his
nitrogen levels up to par.
I'm not adamantly opposed to buying nitrogen amendments if it would
make my garden healthier, my soil tests bring that assertion into
question. In the table above, higher organic matter levels seem
to match up with sweeter soil and with a better ratio of
exception is the mule garden, which I think must have had the organic
matter levels mismeasured this year. My
previous year's results showed the mule garden as
having the highest organic matter levels, and my eyes back up that
Granted, there is a
limit to how much nitrogen vegetables can use each year, but I estimate
you'd have to raise your soil up to 20% organic matter before nitrogen
would start washing out into the groundwater. I think I'll stick
to my Rodale approach to nitrogen and organic matter until someone
presents a stronger argument for seed meals.
Trailersteading is a look at how several
homesteaders used old mobile homes to achieve their dreams.
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