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. 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