Between 1995 and 2000, the number of acres planted to soybeans in the eight counties that the Maquoketa River flows through has increased  from 389,000 acres to 668,000 acres.

Growers have added soybeans to their rotation in response to the farm program and commodity prices, and some use the rotation to break weed, insect and disease cycles associated with continuous corn production. An added benefit is the reduced amount of nitrogen needed for corn following soybeans in the rotation.

During the last two crop seasons nine Maquoketa River Watershed producers have hosted various rates of nitrogen (N) and phosphorus (P) demonstrations on their farms. The corn following soybeans were fertilized with N rates of 0, 30, 60, 90, 120 and 150 pounds per acre. These sites also had replicated treatments of P2O5 at 0, 46 (crop removal) and 92 (twice crop removal) pounds per acre. Two demonstrations in the Onslow area, a garden spot this season, had treatment averages more than 200 bushels per acre up to 229 bushels per acre as indicated in figures 1 and 2.

The average return to dollars spent for N fertilizer (at 20 cents per pound of N) and corn at either $2.00 or $2.40 per bushel is shown in figure 3. The largest return to N fertilizer was the 90 pounds N per acre rate due to the increasing total cost of N and declining yield response with additional N input.

Residual N in the cornstalks following maturity was measured for all N treatments as shown in figure 4. The check, 30 and 60 pounds N per acre treatment did not have enough N available to optimize corn production. Beginning at the 90 pounds N per acre rate and increasing to 1,618 parts per million (ppm) nitrate N with 150 pounds of N per acre applied, the stalk nitrate results were within the 700 to 2,000 ppm optimum range.

There was a very small yield response to the one- and two-year crop removal rates of added P, as shown in figure 5. The cost of added P was significantly greater than the income from the additional corn yields, as shown in figure 6.

Iowa State University recommends fertilizing with P when soil tests fall below the high range (less than 21 ppm P), with an option to use a low rate of P in starter fertilizer when P tests are in the high soil test range.

One producer has said, "The days of wholesale fertilizer application are over, and we need to take a real look at the effective rates of fertilizer under various conditions."

A recent University of Illinois study showed that soil test levels as low as 20 ppm P with any commonly used tillage system will result in P in runoff water that is two times greater than the benchmark limit of 0.035 ppm set for lakes in Environmental Protection Agency Region 7.

Maquoketa Watershed Project staff extend their appreciation for hosting these demonstrations to Jule Brown, Joe Wingert, Gary Soules, Pauline Antons and Alan Jacobs, Neal and Kirby Paulson, Gary and Dan Bockensedt, John and Pete Kalb, and the Glen Janssen family.