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Re: Soil equilibrium
Most of the time, when folks calculate the N in compost, they forget about the N in the biology.
When an N analysis is performed in a lab, they usually float off the organic matter. If you are analysing compost, how much of it is organic matter that will float off, and thus ignored?
When you go to the fact that the C:N of decent compost (the stuff that hasn't gone anaerobic and lost the N, S, P as gaseous forms) is about 15:1 or 20:1. So compost should contain 25 lbs of actual N per 1000 lbs. Someone could check my calculation, but that's a hefty amount of N you are actually putting on in a ton of compost.
What amount of N do you actually have to replace each year? You should only have to replace the N you take off in your crop. The residue should stay in the field and be incorporated back into the soil. So only count as gone the N actually in your removed crop.
The biology is needed to decompose those residues, and hold those nutrients in non-leachable forms in the soil, of course. If your crop residues are still present the next spring, you know you've lost the needed biology. You need to fix it. I know fields where corn stalks from the corn grown four years ago are still not-decomposed. They need some fungi in that soil. You don't have to send in a soil sample for us to analyze to know you are missing the biology, and you need to fix it!
So, get the organisms to hold nutrients in the soil over the winter. You only have to replace the N removed in the... well, corn seed, for example. How much N is present in a 150 bushel an acre yield? That's what C:N ratio? About 75 to 80 lbs of actual N per acre? That's 3 tons of compost per acre to replace the removed N. Should therefore also be more than enough S, P, Fe, Ca, etc. Given that it is good compost.
And actually, if you do a cover crop of legumes UNDER your corn, or a green manure N-fixer over the winter, you need to add even less compost to replace the N removed at harvest.
I'd appreciate people thinking through this approach and KINDLY pointing out where I'm not getting something straight, if in fact there is a logic error in there anywhere. Getting other minds to chew on my thought processes would be a good thing. Maybe other people have already thought of this too, but if you have, I haven't seen it written up anywhere, and people should be thinking this through, as long as it makes sense.
Elaine Ingham
President, Soil Foodweb Inc.
www.soilfoodweb.com