Coming to Terms with Terminology
Information supplied in this web site is based on published literature and the efforts of many researchers who have collected plant requirement information. Although care has been taken to collect accurate data and to compare data from different sources inaccuracies may occur. Problems also may occur in interpreting such data. For example, it was assumed that reported requirements had similar meanings. But, “absolute maximum temperature” could mean the temperature at which:  

• Cell division or cell elongation ceases and crop growth stops until conditions improve
• Tissue is destroyed, characterized by tissue turning yellow and eventually brown
• Harvestable yield is affected; fruit aborts 
• Plants die

The impact of the different definitions may be significant.  The number of crops included made it impractical to verify the definition of each requirement for each crop.  

Unique requirements associated with crop genetics and/or specific location effects exist. For example:

• Discrepancies exist for well-studied crops. The Natural Resources Conservation Service Plants Database states soybean requires from 32 to 50 inches of rainfall per year. The Food and Agriculture Organization of the United Nations Ecocrop web site suggests 24 to 60 inches. Although similar, the different ranges would indicate smaller or larger areas to be suitable for soybean. Estimates for more exotic and less researched crops would be expected to be less reliable. 
• For many crops, environmental requirements differ for varieties adapted to different geographical regions. Researchers reporting requirements for varieties adapted to their area may ignore requirements that describe the entire crop. Our intent was to include the range of requirements for widely adapted crops.  

Suitability versus Yield
Our intent was not to predict yield. The model relies on a relatively few climate and soil requirements to create overall suitability maps. Requirements were selected because of their importance and availability. Additional requirements likely would strengthen the suitability maps but undoubtedly would reduce the number of overall maps. Traditional crops would remain on the list of mapped crops while less studied potential alternative crops would not be included. 

Data used to compose the overall suitability maps are insufficient to predict yield. Although under certain circumstances suitability may be related to yield, computer models for traditional crops, such as corn, require many detailed inputs to estimate yield (e.g. daily weather information, planting date, variety, etc.).

Suitability versus Profitability
Suitability does not guarantee profit. A crop suited to an area may not be profitable due to other factors, such as insects, diseases, weeds, limited availability of a suitable variety, and market access. For example, although sorghum is well suited to Illinois, it may not be the best crop for many farmers because corn typically yields more than sorghum. Because sorghum is rarely planted, marketing may be a problem because grain elevators may not reserve bin space for it, resulting in additional transportation costs for growers.