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Seeds equilibrated under conditions of constant temperature and relative humidity will approach a certain and repeatable value for temperature and moisture content.

Although many factors may be important in maintaining seed vitality during short and long-term storage, we limit this discussion to seed temperature and moisture content. Furthermore, we consider only constant temperature and constant seed moisture content during short or long-term storage.

At equilibrium, there will be no heat or moisture exchange between a seed and its environment. The temperature of the seed will equal the temperature of the environment and the moisture content of the seed will be in balance with the moisture content of the environment.

Equilibration data for yellow dent corn is shown in figure 2. The graph shows storage conditions that will maintain constant seed moisture content.

From figure 2, for seed at 15% moisture content, equivalent storage atmospheres are:

32 °F, 49% relative humidity;

70 °F, 62% relative humidity;

And 90 °F, 68% relative humidity.

At each of these storage atmosphere conditions the seed moisture content will remain constant at 15%. The seed vitality then becomes only a function of the storage temperature. We note that all of this seed storage information is directly available from the seed equilibration curves.

The equilibration curves were generated with the modified Henderson equation:

Where: Mc = moisture content dry weight %; rh = relative humidity %; T=temperature ºC (ºC = (ºF-32)/1.8)

And, the empirical constants for yellow dent corn are:

K=8.6541x10-5; N = 1.8634; C = 49.810.

The equilibration curve, figure 2, is actually a combination of the properties of the seed and the properties of atmosphere where the seed is stored.