Germination Temperatures (Degrees Farenheit)

Lowest  Highest  Best
Wheat  41      108      84
Barley 41      100      84
Corn   49      115      91

Germination occurs within the considerable range between the highest and lowest temperatures of this table, though the rapidity of germination decreases as the temperature recedes from the best.  This explains the early spring and late fall germination when the temperature is comparatively low.  If the temperature falls below the lowest required for germination, dry seeds are not injured, and even a temperature far below the freezing point of water will not affect seeds unfavorably if they are not too moist.  The warmth of the soil, essential to germination, cannot well be controlled by the farmer; and planting must, therefore, be done in seasons when, from past experience, it is probable that the temperature is and will remain in the neighborhood of the best degree for germination.  More heat is required to raise the temperature of wet soils; therefore, seeds will generally germinate more slowly in wet than in dry soils, as is illustrated in the rapid germination often observed in well-tilled dry-farm soils.  Consequently, it is safer at a low temperature to sow in dry soils than in wet ones.  Dark soils absorb heat more rapidly than lighter colored ones, and under the same conditions of temperature germination is therefore more likely to go on rapidly in dark colored soils.  Over the dry-farm territory the soils are generally light colored, which would tend to delay germination.  The incorporation of organic matter with the soil, which tends to darken the soil, has a slight though important bearing on germination as well as on the general fertility of the soil, and should be made an important dry-farm practice.  Meanwhile, the temperature of the soil depends almost wholly upon the prevailing temperature conditions in the district and is not to any material degree under the control of the farmer.

A sufficient supply of oxygen in the soil is indispensable to germination.  Oxygen, as is well known, forms about one fifth of the atmosphere and is the active principle in combustion and in tile changes in the animal body occasioned by respiration.  Oxygen should be present in the soil air in approximately the proportion in which it is found in the atmosphere.  Germination is hindered by a larger or smaller proportion than is found in the atmosphere.  The soil must be in such a condition that the air can easily enter or leave the upper soil layer; that is, the soil must be somewhat loose.  In order that the seeds may have access to the necessary oxygen, then, sowing should not be done in wet or packed soils, nor should the sowing implements be such as to press the soil too closely around the seeds.  Well-fallowed soil is in an ideal condition for admitting oxygen.