The air surrounding seed can absorb only a certain amount of moisture,
and then drying ceases. When the air absorbs enough moisture for its RH to
be in equilibrium with seed moisture content, no further drying takes place.
To continue drying the seed, the drying air must be kept at an RH which
is dry enough to absorb moisture from the seed. Within specific limits, the
air can be kept dryer than the seed by three methods: