Recent field studies (Karlen, 2007) indicated that nitrogen, phosphorus, and potassium removal with the cob and upper portion of the corn plant averaged 10, 2, and 13 lb/ton of dry stover, respectively. The baseline assessment is based on the assumptions that soils from which crop residues would be removed would have optimal phosphorus and potassium soil test levels and that nitrogen was being applied at the MRTN. Therefore, for a stover harvest rate of 1.5 tons/acre, the annual fertilizer requirements of nitrogen, phosphorus, and potassium would be 190, 27, and 57 lb/acre, respectively, for continuous corn or 140, 27, and 57 lb/acre for corn rotated with soybean. For 2020 projections (2.5 tons of dry stover per acre), the panel increased fertilizer input to account for nutrient removal and assumed only a slight increase (10 percent) in MRTN because of better efficiency of nitrogen use. That resulted in estimated fertilizer requirements of nitrogen, phosphorus, and potassium of 218, 29, and 69 lb/acre for continuous corn.
Inputs would be much lower for dedicated perennial biomass crops. Woody crops are rarely fertilized. Herbaceous perennial crops would be harvested when senescent, so loss of nitrogen, phosphorus, and other nutrients would be minimized because plants retranslocate such nutrients to roots in fall. The panel envisions a process in which biofuel processing facilities capture those nutrients, which can then be periodically returned to the soil. Given the ability of perennial grasses to maintain yields for many years with little or no fertilization, it might be feasible to return removed nutrients once every 3–5 years and thus reduce energy requirements for fertilizer transport and application. Commercial nitrogen fertilizer is energetically expensive but could be replaced, if needed, by growing one or more legume species with a biomass crop. Competition between the crop and the legume could be minimized, if competition occurs, by using a legume that has a different season of maximal growth from the biomass crop.
Karlen, D.L. 2007. Balancing bioenergy opportunities on your natural resources base. Paper read at Indiana Crop Advisors Conference Meeting Proceedings, December 18–19, 2007, Indianapolis.
Mallarino, A.P., D.J. Wittry, and P.A. Barbagelata. 2002. Iowa soil-test field calibration research update: Potassium and the Mehlich-3 ICP phosphorus test. North Central Extension–Industry Soil Fertility Conference 18:30-39.