Abundance

Agricultural crop residues are those left in the field or accumulated during sorting and cleaning of produce. Because of the discontinuity in growing seasons, the many crops that are grown, the differences between specific crops, variations in crop yields in different areas, the difficulty of acquiring reliable data, and long-term time effects, an inventory of the annual production of agricultural crop residues and their disposition might seem to be an impossible task. Fairly reliable data can be obtained, however, for small and large regions of a country. Somewhat detailed commentary is justified for one of these studies because it is a good example of how the task was addressed in a reasonably scientific manner for all 3069 counties of the continental United States (Stanford Research Institute, 1976). Although the assessment was done some time ago, the methods used still appear to be valid.

Hay and forage crops were excluded, since little residue accumulates from cultivation of these crops. Food processing wastes were excluded except for bagasse and sugarbeet pulp. With the exception of hay and forage crops, yields on a dry basis of the harvested crops and the areas harvested were tabulated for essentially all other cash crops, about 60, over a З-year period, 1971 to 1973, and averaged for each county by year and by quarter. At the same time, data were collected regarding what was done with the residue: returned to soil, sold, used as feed or fuel, and wasted. To estimate the quantity of residue generated, a residue factor for each cash crop was developed that, when multi­plied by the county yield total for that crop, gave the total mass of residue generated. The residue factors as used in the Stanford assessment were the ratio of field weight of residue per mass unit of crop yield and differed somewhat from those used by others in subsequent work (Table 5.4). It is important to

note these differences. Experimental measurements were made to determine the residue factors in the Stanford assessment by collecting field, packing shed, and mill residues immediately before or following crop harvest, determining fresh and oven-dry residue weights, converting the production figures from whatever the standard units were, such as bushels, to mass units, and calcu­lating the residue factors. For example, for field corn, the residue factor was 1.10 mass units of fresh residue per mass unit of corn yield. The weights of the residues were converted to dry weights later in the calculations. The residue factors as defined in the study were applied nationwide, except for two cases, the assumption being that geographical variation in residue generation in a given crop was accounted for by the geographical variation in the yield of that crop. The two exceptions were cotton gin trash and mint, wherein specific regional variations in residue production were considered, which led to devel­opment of separate regional residue factors. It is evident that this type of assessment is not an everyday event. Massive amounts of data are compiled, the easy manipulation of which requires computational methods.

A summary of the results of this assessment of residue generation from agricultural crops for the contiguous United States is presented here to provide an idea of the availability of agricultural residues for the contiguous United States in the mid-1970s. The data indicated a total of 292 million dry tonnes of residue were generated annually; about 252 million dry tonnes were judged to be collectible during normal operations. About 74% was returned to the soil, 19% was used as animal feed, 4% was sold, 3% was used as fuel, and a small amount was wasted. While there is controversy over what fraction of the residue returned to the soil could be utilized as waste biomass feedstock without adverse environmental impacts, it represented the largest portion of the collectible residue. Of the total available crop residues, 48% was estimated to be from small grains and grasses.

The top five states for agricultural residue production in millions of dry tonnes per year were Iowa, 25; Illinois, 24; Oregon, 23; California, 22; and Kansas, 21; the sum of these is 39% of total residue generation. Interest­ingly, only six counties in the continental United States averaged more than 350.2 dry t/km2-year (1,000 dry ton/mi.2-year). In units of dry t/km2-year, these were Lewis County, Idaho, 537; Delaware County, Indiana, 378; Lane County, Oregon, 363; Polk County, Oregon, 357; and Cook County, Georgia, 353. Also, only 55 counties averaged 2.24 dry t/ha-year (1.0 dry ton/ac-year) or more in available agricultural residue.