Tall fescue can persist in soil drainage classes ranging from excessively drained to poorly drained. It tolerates long periods of flooding (24 to 35 d) when temperatures are below 27°C (USDA-NRCS, 2006b). It has been observed to survive flooding for up to 21 d at higher temperatures (35°C). A function depicting the growth response of tall fescue to variation in soil drainage levels based on NRCS forage suitability groupings (USDA-NRCS, 2006b) is shown in Fig. 3-4.

Fig. 3-4. Growth response of tall fescue to soil drainage: Y = 95.64*exp(-0.50*((x-4.46)/1.95)2); R2=0.97. (VPD = very poorly drained; PD = poorly drained; SPD = somewhat poorly drained; WD = well drained; SED = somewhat excessively drained; ED = excessively drained).

Since soil drainage classifications are ordinal, in-between values cannot be defined for the abscissa (x-axis). Thus, predicted Y values for percent relative growth were presented only for each of the categories. Seven drainage categories were assigned to each grid cell in the United States. Translation to the categories of well-suited, moderately suited, marginally suited, and unsuited was based on previously described limits (>75, 50-75, 25-50, and <25) (Table 3-1). For China, many of the drainage categories used were assigned a range of values that were found in a particular polygon. Since single values were required for each grid cell to accomplish the mapping, we selected the value based on the "worst-case" drainage category that matched the seven U.S. categories.

For tall fescue suitability classification, very poorly drained is considered unsuited, poorly drained and excessively drained are marginally suited, and somewhat poorly drained to well drained are categorized as well-suited (with values of relative growth >50%). Soil drainage suitability maps depicting these categories for the United States and China are provided in the online version of this chapter.