If the temperature is right, germination begins by the forcible absorption of water by the seed from the surrounding soil.  The force of this absorption is very great, ranging from four hundred to five hundred pounds per square inch, and continues until the seed is completely saturated.  The great vigor with which water is thus absorbed from the soil explains how seeds are able to secure the necessary water from the thin water film surrounding the soil grains.  The following table, based upon numerous investigations conducted in Germany and in Utah, shows the maximum percentages of water contained by seeds when the absorption is complete.  These quantities are reached only when water is easily accessible:—­

Percentage of Water contained by Seeds at Saturation

German  Utah
Rye    58      —­
Wheat  57      52
Oats   58      43
Barley 56      44
Corn   44      57
Beans  95      88
Lucern 78      67

Germination itself does not go on freely until this maximum saturation has been reached.  Therefore, if the moisture in the soil is low, the absorption of water is made difficult and germination is retarded.  This shows itself in a decreased percentage of germination.  The effect upon germination of the percentage of water in the soil is well shown by some of the Utah experiments, as follows:—­

Effect of Varying Amounts of Water on Percentage of Germination

Percent water in soil 7.5 10 12.5 15 17.5 20 22.5 25
Wheat in sandy loam 0.0 98 94 86 82 82 82 6
Wheat in clay 30 48 84 94 84 82 86 58
Beans in sandy loam 0 0 20 46 66 18 8 9
Beans in clay 0 0 6 20 22 32 30 36
Lucern in Sandy loam 0 18 68 54 54 8 8 9
Lucern in clay 8 8 54 48 50 32 15 14

In a sandy soil a small percentage of water will cause better germination than in a clay soil.  While different seeds vary in their power to abstract water from soils, yet it seems that for the majority of plants, the best percentage of soil-water for germination purposes is that which is in the neighborhood of the maximum field capacity of soils for water, as explained in Chapter VII.  Bogdanoff has estimated that the best amount of water in the soil for germination purposes is about twice the maximum percentage of hygroscopic water.  This would not be far from the field-water capacity as described in the preceding chapter.