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Fertilizer contributions to crop yield
Some have estimated that nutrient inputs are responsible for 30 to 50 percent of crop yield in the U.S. Making these estimates presents significant challenges. Certain assumptions are required regardless of the approach taken. One difficulty that arises is that crops respond differently to different fertilizer elements. For example, corn response to nitrogen (N) fertilizer is much greater than that of a legume such as the soybean or peanut. This effort is further confounded by many other factors such as variable soil fertility levels, climatic conditions, and changes in production practices that affect nutrient use efficiency. Nevertheless, meaningful estimates of the contribution of inorganic fertilizer to crop yield can, and have been made.
 Smith et al. (1990) investigated the impact of chemical use reduction on yields for eight major crops in the U.S. The authors analyzed the impact of eliminating the use of pesticides (herbicides, insecticides, fungicides) and inorganic N fertilizer on corn, cotton, rice, barley, sorghum, wheat, soybean, and peanut yields. Other crop nutrients were not taken into account. The estimated effect of eliminating N fertilizer alone (i.e., no pesticide elimination) on U.S. crop yields is shown in Table 1. Average U.S. corn yield was predicted to decline by 41 percent without N fertilizer, or in other words N fertilizer was responsible for 41 percent of corn yield. The elimination of all pesticides and N fertilizer resulted in an estimated 53 percent decline in corn yield. Therefore, N fertilizer was responsible for the majority of corn yield among the inputs analyzed. The elimination of N in cotton production resulted in an estimated yield reduction of 37 percent, the largest of any single input group analyzed. The average estimated reduction in yield from elimination of N fertilizer of the six non-leguminous crops analyzed was 26 percent.
 One approach to estimating the portion of crop yield attributable to fertilizer is to select studies for major crops where zero fertilizer controls have been employed and calculate the portion of yield attributable to fertilization. At Oklahoma State University scientists have studied wheat fertility since the late 1800’s (OSU Soil Fertility Research Highlights, 2000). The Magruder Plots were established in 1892 and are the oldest continuous soil fertility wheat research plots in the Great Plains, and are among the oldest continuous soil fertility plots in the world. As one would expect, the fertility treatments have changed since the plots were established, with yearly inorganic fertilizer applications commencing in 1930. The inorganic N source was sodium nitrate (NaNO3) from 1930 to 1946 when it was changed to ammonium nitrate (NH4NO3). Nitrogen rates have ranged from 33 to 60 lb N/A. The early inorganic phosphorus (P) source was ordinary super phosphate (0-20-0-12S), this was replaced by triple super phosphate (0-46-0) in 1968. The P rate throughout the study has been constant at 30 lb P2O5/A. Figure 1 shows that when averaged over 71 years N and P fertilizer have been responsible for 40 percent of wheat yield.
 Another long-term study is being conducted at the University of Illinois. Various crops, rotations, and fertility treatments have been evaluated in the well-known Morrow Plots since 1888. Early fertility treatments included manure, rock phosphate, bone phosphate, and limestone. In 1955 commercial fertilizer treatments were imposed that combined N from urea, P from superphosphate, K from muriate of potash, and lime. An evaluation of grain yields for continuous corn (Figure 2) from the no fertilizer control and the N+P+K+lime treatment revealed that on average from 1955 to 2000, 57 percent of yield was attributable to the fertilizer+lime treatment (derived from data provided by Harold Reetz, personal communication).
 A long-term irrigated study in western Kansas has examined the effect of various N rates (0 to 200 lb N/A in 40 lb increments) and P fertilization (0 and 40 lb P2O5/A) on yields of corn and grain sorghum. Over 40 years (1961 to 2000) of this study, N and P fertilizer on average produced 44 percent of corn yield and 31 percent of sorghum yield (derived from Schlegel, 1990, 1991, and 2000). The data presented in Table 2 summarize the 40-year average yields for both crops for each fertility treatment and percent of yield attributable to fertilization. These data clearly illustrate the importance of balanced fertility in crop production. A somewhat more realistic approach to estimating the amount of yield attributable to fertilizer would be to examine the contribution to yield that fertilizer makes at the optimum rates of N and P. The economic optimum N rate for corn in this study was 160 lb/A, and in most years for sorghum it was 80 lb/A (Schlegel 2000). Phosphorus fertilizer (40 lb P2O5/A) was necessary to maximize profit for both crops. The forty-year averages for percentage of yield attributable to fertilizer at the economic optimum rates for N and P for corn and grain sorghum were 60 and 38 percent, respectively.
The data from the long-term studies discussed herein represent 157 years of crop production. Although significant variability in crop response to fertilizer inputs occur among crop species, soil conditions, climate, and other factors, these data and the results of the chemical use reduction investigation (Smith et al., 1990) tend to support the generalization that somewhere between 30 to 50 percent of crop yield in the U.S. is attributable to nutrient inputs.
References
OSU Soil Fertility Research Highlights. 2000. The Magruder Plots: Environmental Production History 1892-2000. Oklahoma State University, Stillwater.
Schlegel, A.J. 1990. Effect of nitrogen, phosphorus, and potassium fertilization on irrigated corn and grain sorghum. Kansas Fertilizer Research. Kansas State Univ. Agric. Exp. Stn. and Coop. Ext. Serv., Manhattan.
Schlegel, A.J. 1991.Effects of nitrogen, phosphorus, and potassium fertilization on irrigated corn and grain sorghum. Kansas Fertilizer Research. Kansas State Univ. Agric. Exp. Stn. and Coop. Ext. Serv., Manhattan.
Schlegel, A.J. 2000. Nitrogen and phosphorus fertilization of irrigated corn and grain sorghum. Kansas Fertilizer Research. Kansas State Univ. Agric. Exp. Stn. and Coop. Ext. Serv., Manhattan.
Smith, E.G., R.D. Knutson, C.R. Taylor, J.B. Penson. 1990. Impact of chemical use reduction on crop yields and costs. Texas A&M Univ., Dep. of Agric. Economics, Agric. and Food Policy Center, College Station.
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