Here's a simple farmer's (me) understanding of this.
Anytime you remove crop from the soil, you deplete nutrients, nitrogen
being only one. Seeds are especially rich in nitrogen (protein), so when
you harvest a soy crop, whether conventionally grown or organically grown
with innoculant, you are removing nitrogen from the soil. Because legumous
crops, especially if they are innoculated, fix nitrogen from the air, the
deficit is less than say, corn, which doesn't fix nitrogen. But, it's still
a net loss.
Legumous cover crops add nitrogen because they are totally returned to the
soil where bacteria release the fixed nitrogen as the nodules die. Nothing
is removed. Net gain of N (OK don't go into carbon cycle here)
Your friend with the critters is setting up a straw man argument. His
critters remove crop -grass- from the land along with the nutrients the
soil used to produce that crop. He harvests his secondary crop -critter
flesh(protein)- thus removing those nutrients from where they originated.
The fact that his critters return some of those nutrients while they are
grazing is moot since the form of the nutrients is changed and only those
not used to produce flesh(protein) are returned. I suspect he didn't
mention soil compaction either...
Marcie A. Rosenzweig
Full Circle Organic Farm
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