You have probably decided on your cropping pattern and crop rotation for the year, but some of those follow up issues need some attention. Such as fertility and nitrogen issues. With the high cost, your decision needs to either pay in more yield or save unneeded expense. The fact that many producers have opted out of a 50-50 corn and soybean rotation means more problems to solve.

Scott Murrell Northcentral Director of the International Plant Nutrient Institute at West Lafayette, IN, says many nitrogen issues arise when switching between a corn-corn rotation and a corn-soybean rotation. His presentation at the 2008 University of Illinois Crop Protection Technology Conference indicated several factors to consider:

1) Agronomists in most Cornbelt states use a constant figure of 30 to 45 lbs of nitrogen per acre left from the prior year soybean crop. Other states use a credit of one-half pound to one pound per bushel of yield. And without a prior soybean crop the credit is lost.
2) Since soybeans scavenge nitrogen from the soil, the absence of a soybean crop means a corn-corn rotation will have some residual nitrogen to use, and the drier the year the more that is present. A soil test can confirm the amount and save money on over application.
3) University research indicates a corn-corn rotation will yield less than a corn-soybean rotation with similar management. And if yields have decreased, nitrogen rates will need to be adjusted downward if yield is used as the factor to determine application rates.
4) Tests can be conducted on the corn crop to determine nitrogen use, such as a chlorophyll meter to test for nutrition problems, and a stalk nitrate test at the end of the season to assess appropriateness of the nitrogen rates used.

Shifting rotations can also impact soil pH levels, since nitrogen has a long term acidifying effect on the soil. (The initial reaction is alkaline, but eventually switches.)

1) In nitrification, ammonium-N converts to nitrate-N, and the source of the ammonium, which is usually anhydrous ammonia, is more acidic.
2) The leaching of nitrates leaves more acidic cations in the surface of the soil.
3) The volatilization of anhydrous ammonia creates an acid.

Murrell says the soil will determine whether acidification will occur when rotational shifts are made. Some Iowa tests found no change after 23 and 48 years. Nebraska tests found a change after 14 years. So the lesson is to closely monitor soil pH after shifts are made in rotations. Another consideration is the balance of other soil nutrients. Murrell says, “A CC sequence removes more phosphorus (P), magnesium (Mg), and sulfur (S) than a CS sequence but less N and potassium (K). Just how large such differences are depends on the yield levels of each crop.”

Summary:
With soybeans supplying nitrogen and corn using nitrogen, Cornbelt farmers who are shifting cropping patterns due to market prices need to monitor nitrogen more closely than in their normal cropping pattern. Shifts in the amount of nitrogen used will possibly chance the soil pH and change the balance of other soil nutrients. However, the major factor is to avoid an over-application of nitrogen that is unneeded, or to ensure enough nitrogen is being applied to maintain optimum economic yields.