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| ISU extension bulletins have come and gone. Some said figure a pound of N per bushel of bean yield. Some said figure nothing. Some have said the benefit is enough N to make the bean straw and leaves rot without taking any from the corn crop. Likely it depends as much on weather, soil conditions, and soil type as on the bean crop. That too much repeated rainfall drowns the nodules and little excess N is made. There was an ISU extension bulletin that said corn had no response to added N the first year after a good alfalfa crop was plowed under, and that for the second year, a spring N test should be made with the corn up 6" to see if any N was needed the second year. Its been removed from the collection of bulletins.
In the 1990s, I did a 6 year crop rotation on my place, oats/alfalfa, two years alfalfa production, corn, beans, corn, planning for the alfalfa to leave enough N for the corn and the beans to do that again. To the extent that there was alfalfa hay to sell, and the corn and beans didn't die of malnutrition, it worked, but the grain crops were never spectacular.
Soil tests showed P and K stayed high, probably from the long roots of the alfalfa bringing it up from the subsoil, but selling alfalfa was a pain, got me plenty of exercise which was part of my plan for having the place, so eventually I figured out that the return on the alfalfa was so much poorer than the return from fair corn that I built a liquid side dresser and took alfalfa out of the rotation (to no long be shafted by crooked horse hay buyers) and bought my N. My corn improved, and got best when I went with adding considerable P and K along with putting N down with the planter and side dressing. E.g. the cost of the low income from the alfalfa compared to the potential income from corn was a lot more than the cost of purchased N.
I'm beginning to suspect that high Ca soil tied up much of the soil test's indicated P and K though after a year of golden rimmed corn and another of golden rimmed beans, the K tests were very low and the following crops did respond to the application of potash in profusion. I had been applying "removal" rates of P. I hope to get my tenant to try some test strips of excess P and K for next year.
Gerald J. | |
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Soybean N Carryover???
