A friend, Rodrigo, asked me some basic questions about fertility for organic corn. I emailed a response to him and then thought it was worth putting it on the site as well.
Copy and Paste =>
It turned out that I had more to say on the subject... You might already know all this. But if nothing else I find that it clarifies my thoughts when I explain what I do to other people.
This is what I do to figure out how much fertilizer I need. A rough estimate is 1 pound of N for every bushel/acre of corn you want to grow. In my case I estimate 140-150 bushels/acre. So I need 150 lbs/acre of N for that year.
First I subtract any nitrogen credit that I get from growing clover the previous year (See this on our rotations). I conservatively estimate that I’m getting 60 lbs/acre of nitrogen from the clover that I underseed my wheat with in the growing season before I plant the corn. A good chart on legume nitrogen credits is this one, from U of Wisconsin. It's in a pdf called "Nutrient Management Fast Facts", linked here -
I found out from Jennie-O what the nutrient content of the litter they provide is. Every year it’s slightly different, but it’s usually about the same.
Last year they told me this -
The average for 2017 was:
- Nitrogen 51 lbs per ton
- P2O5 41 lbs per ton
- K2O 30 lbs per ton
- Turkey litter runs about 3 pounds S per ton, and averages ~0.11 pounds Boron per ton
The other final piece is that only a certain percentage of the N is available to the plant the first year. It’s somewhere between 45-70%, depending on how soon you incorporate it into the soil after it’s been spread.
(Chart below is from Calculating Manure Application Rates | UMN Extension)
I want 150 lbs/acre N to grow 150 bushel/acre corn.
I reduce that by 60 lbs/acre N, the credit from the clover.
So I still need 90 lbs/acre of N from turkey litter/other sources of fertilizer that aren't "synthetic".
I decide to put on 3 tons of turkey litter/acre, and this (from the info provided by Jennie-O) has:
3 tons x 51 lbs/ton = 153 lbs of N
I take the middle value of how much is available the 1st year, 60%.
60% of 153 = 91.8 lbs
So I’m getting 91.8 lbs from the turkey litter plus a 60 lb credit from the legume, giving me roughly 150 lbs of N.
If you didn’t have the clover you’d have to add more turkey litter. BUT, many places already have too much phosphorus in their soils and turkey litter has plenty of phosphorus in it. Jennie-O is legally required to have soils tests done, that the farmer pays for, that shows that the soils that are going to take the turkey litter don’t have too much phosphorus, which is a pollutant, running into the water.
I’ve been told that for organic rotations the best thing to do is continue to grow legumes in a rotation to build up N naturally as well as increasing the ability of the soil to “mineralize”, or create, more organic matter that feeds the microbes that in turn excrete more plant available N. A kind of manure from bacteria that the plants then eat.