(1) There are certain restricted limits of time within which crops must be planted for best results, defined by the temperature conditions of the locality. The length of the time period within which the planting of a given crop may be accomplished decreases in general with increase in latitude.
(2) A definite amount of heat is required after planting to bring a crop to maturity; as a rough measure of this the accumulated day-degrees of temperature above the mean temperature at which planting is accomplished, may be taken. As thus computed there is very little difference in the amount of heat necessary to mature most staple spring planted crops, when the average variety of corn is considered. It is suggested that the mean temperature at which the planting of a given crop can be accomplished be used as a base, or starting point, for any method that may be employed for temperature summation, instead of a general base for all crops. If the frequently used 6° C. base be employed in the case of cotton, for example, we would begin the reckoning of effective temperature in the vicinity of Abilene, Tex., about three months before planting can begin, with a resulting indicated large accumulation of effective temperature before any growth is possible.
(3) Spring wheat seeding begin with a lower mean temperature than any other major spring crop. Seeding usually begins in the Dakotas and in Nebraska when the normal daily temperature rises to 37° (F.) and in Minnesota and Wisconsin when −10° (F.) is reached. Next in thermal order comes spring oats, the seeding of which usually begins when the normal daily temperature rises to −13° (F.) (corresponding to the advent of the vegetative period). Early potato planting begins as a rule when the normal daily temperature rises to 45° (F.), and corn when 55° (F.) is reached. The dates on which the later is spring. Cotton planting usually does not begin until the normal daily temperature rises about 62° (F.). The dates on which this temperature is reached correspond closely to the latest dates in spring on which killing frost as occurred.
(4) Cotton and corn are warm-weather crops and the areas in which successful production on a commercial scale can be accomplished are limited principally by both the general temperature conditions and the temperature at which planting may be accomplished. These limits are defined by an available thermal constant of about 1,600° (F.) for corn and about 2,000° (F.) cotton, computed from the normal temperature when planting usually begins. Its follows that if cotton could be planted with as low temperatures as corn planting is accomplished, the cotton area would be materially increased.
(5) Owing to the relatively large thermal requirements of corn and cotton, a comparatively warm spring is necessary for best results of germination and early growth. Thus there is a close relation between the spring temperatures and the conditions of these crops to certain dates in the early stages of growth.