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.
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