By Richard B. Alexander and Richard A. Smith Reston, Virginia 1990
The purpose of this report is to document the compilation of machine-readable estimates of nitrogen and phosphorus fertilizer use by county for the period 1945 to 1985. The technique used to disaggregate state-level fertilizer use data to the county level is described in detail. Numerical values of nitrogen and phosphorus fertilizer use by county are not presented in the text of this report because of the size of the data file, but may be obtained in machine- readable form upon request (digital data are available in several forms). Several graphical summaries of nitrogen and phosphorus fertilizer use are provided to give a general impression of the spatial and temporal variability that exist in the data.
For certain years and states, fertilizer use data is either incomplete or reflects a fundamental change in the estimation method which may affect the comparability of the data from one year to another. Fertilizer use data for the states and years listed in Table 1 reflect these types of problems. For states with annual fertilizer use data that were incomplete, the SRS estimated current use from the previous year's use data as well as from any monthly data that may have been available for the current year. Annual survey data (as described above) were also used by SRS to supplement state fertilizer use data that was found to be incomplete.
Table 1. States and years with fertilizer use data that required additional estimation or are not comparable with previous data. Data on fertilizer use are from the U. S. Department of Agriculture (1966, 1976, 1977-1985)
_______________________________________________________________ YEAR STATES DESCRIPTION OF PROBLEM _______________________________________________________________ 1980 LA Complete data not available. Current data estimated from previous year's data. 1981 LA, CONN, ND 1982 NH, VT, DEL, MONT 1983 DEL, SD 1984 CONN, MD, NH, VT, DEL, MINN, ND, SD 1985 Utah, MD, DEL, MINN, SD 1985 MO Previous data not comparable (new reporting method used) 1985 ND Data includes sources not previously considered 1985 TEX New fertilizer year used (June 1 to May 31) _______________________________________________________________
FCik = FSi FACik / FASi (1)
FCik is county-level fertilizer use for the ith state and kth county, FSi is state-level fertilizer use for the ith state, FACik is county fertilized acreage for the ith state and kth county, and FASi is state fertilized acreage for the ith state.
Because of the availability of only three annual estimates (1974, 1978, 1982) of county fertilized acreage, state-level fertilizer use was disaggregated to the county level by using fertilized acreage data that most closely corresponded in time to the fertilizer use data. The temporal correspondence of fertilizer use and fertilized acreage data that was used to estimate county-level fertilizer use is shown in Table 2.
Table 2. Annual fertilizer use data and corresponding annual fertilized acreage data used to estimate county-level fertilizer use
__________________________________________________________ Fertilizer Use1 Fertilized Acreage2 __________________________________________________________ 1945 - 1976 1974 1977 - 1980 1978 1981 - 1985 1982 __________________________________________________________ 1 U. S. Department of Agriculture (1966, 1976, 1977-1985) 2 U. S. Department of Commerce (1974, 1977, 1982)
Although state-level fertilizer use data reported by USDA (1966, 1976, 1977-1985) should generally include farm and non-farm uses of fertilizer, this data was disaggregated to U. S. counties in proportion to only reported county farm uses of fertilizer chemicals. Consequently, this disaggregation method underestimates fertilizer use in those counties with large non-farm fertilizer uses.
In general, the accuracy of the county-level estimates of fertilizer use is potentially affected by differences that may exist in fertilizer application rates among counties. For a given state, the degree of proportionality between county fertilized acreage and fertilizer use is dependent upon the magnitude of differences in fertilizer application rates among counties. Intrastate variability in fertilizer application rates, which should generally reflect variability in nutrient requirements of different crops, would be expected to be smaller (and estimates of county fertilizer use more accurate) in those states where crop types are not very diverse or where crop types are uniformly distributed throughout the state.
Evaluation of the accuracy of the disaggregation method in light of these factors requires information on county-level crop types and acreages as well as estimates of typical fertilizer application rates for various crop types. Because this information was not readily available during the preparation of the report, this analysis was not undertaken. Improvements in the accuracy of county fertilizer use could probably be attained in future studies with the use of an algorithm that disaggregates state-level fertilizer use according to estimated fertilized acreage and fertilizer application rates in counties.
Several graphical summaries of nitrogen and phosphorus fertilizer use are provided as examples of the spatial and temporal variability that exist in the data. County maps of estimated nitrogen and phosphorus fertilizer use are shown in figures 1 and 2, respectively, for the year 1985. County nitrogen fertilizer use for 1985 is mostly between 640,000 kilograms per year (25th percentile) and 4.7 million kilograms per year (75th percentile) with a median of nearly 2 million kilograms per year (see Figure 3 ). County phosphorus fertilizer use for 1985 is typically about 15 percent of that estimated for nitrogen. Phosphorus fertilizer use for 1985 ranged mostly from 128,000 kilograms per year to 833,000 kilograms per year with a median of 272,000 kilograms per year. Nationally, the spatial pattern of fertilizer use for 1985 is very similar for nitrogen and phosphorus (see Figure 1 and 2). As expected, the highest fertilizer use was observed in those states with the greatest amounts of cropland. These included states in the northern portions of the Midwest, many throughout the Central Plains, and several in the extreme western and northwestern portions of the U.S.
Nationally, nitrogen and phosphorus fertilizer use has increased markedly over the period 1945 to 1985 (see Figure 4 and 5). A nearly twenty-fold increase is observed in nitrogen fertilizer use during this period with peak use occurring in 1981. Nitrogen fertilizer use declined from 1981 to 1983, but increases in 1984 and 1985 brought total use to a level approaching that for 1981. Nationally, phosphorus fertilizer use increased approximately four fold from 1945 to 1985. Phosphorus fertilizer use peaked in 1978 and remained largely unchanged until 1983 when phosphorus fertlizer use declined approximately 25 percent.
State estimates of nitrogen and phosphorus fertilizer application rates were computed for 1982 as the ratio of state fertilizer use to state fertilized acreage. These application rates are displayed in Figure 6 and 7 and expressed as kilograms per acre. Estimated nitrogen fertilizer application rates ranged from a minimum of 20 kilograms per acre to a maximum of 90 kilograms per acre with a median of 40 kilograms per acre. The highest nitrogen application rates are found in states in the northern midwest, central plains, and far west. Estimated phosphorus fertilizer application rates ranged from a minimum of 4 kilograms per acre to a maxium of 24 kilograms per acre with a median of 9 kilograms per acre. The highest phosphorus fertilizer application rates are found largely in states in New England, the Mid-Atlantic, and the Great Lakes region.
U. S. Department of Agriculture, Statistical Reporting Service, Crop Reporting Board, Commercial Fertilizers: Consumption of Commercial Fertilizers, Primary Plant Nutrients, and Micronutrients (1965-1975), Statistical Bulletin No. 472, November, 1976, 64 p.
U. S. Department of Agriculture, Statistical Reporting Service, Crop Reporting Board, Commercial Fertilizers: Consumption for Year Ended June 30, 1977-1985 (Annual Data Released in November of Years 1977-1985).
U. S. Department of Commerce, Bureau of the Census, Census of Agriculture, Final County File, Technical Documentation, Washington, D. C., 1974, 1977, 1982.