Page:Popular Science Monthly Volume 62.djvu/147

Rh forms of bacterial life that are concerned with the transformations of the soil nitrogen it is necessary to consider separately some distinct phases of the nitrogen question. The plant tissues from which life had departed hold in them the nitrogen that had once moved in soluble form through the soil. The nitrogen in the dead plant tissue can not, however, again become a part of the food for other plants; not until it has again been changed into simpler soluble forms. This locking up of the nitrogen in forms slowly decaying, and therefore slowly available, is a wise provision, otherwise the nitrogen would soon be washed out of the soil. Thus we see that the soil nitrogen is contained in an insoluble form in the remains of former plants, and, no matter how much of it the soil contains it is inaccessible to the plant growing upon it until it has been first changed into the simpler forms. Now, as to the agents that produce this transformation. Bacteriology, in general, and soil-bacteriology, in particular, are subjects to which the attention of the scientific world has turned very recently. Of the many hundreds of different species of bacteria living in the soil, but few are known. Nevertheless, even at the present time, enough has been learned to enable us to form a conception, at least, of the changes that take place there. The nitrogen of organic substance, whether plant or animal, usually exists in the form of albuminoids, more frequently termed proteids. These proteid molecules are seized by the soil bacteria and are utilized by them for the formation of their own bodies. Being saprophytic by nature, that is, unable to build up organic substance from the simpler materials, as is done by higher plants, they must derive their energy from the tissues that chlorophyl-bearing plants had fashioned with the aid of sunlight. In availing themselves of this potential energy for their own purposes, they break down the complex molecules; to use a popular expression, they cause decay. Tn order to gain their end, that is, to secure the food contained in the proteid molecules, the bacteria must first change it into a readily diffusible, soluble form. For this purpose the chemical ferments known as enzymes are produced. With the aid of these enzymes, the albuminoid substances are 'peptonized.' In the laboratory such organisms are described as gelatin-liquefying bacteria. A part of the food thus made accessible is appropriated by the microorganisms and in their physiological processes is still further simplified. A part of the carbon is oxidized and escapes into the atmosphere as carbon dioxide, gaseous hydrogen or oxygen is set free, or the two are combined to form water. The nitrogen with which we are here concerned is subject to many changes. In the course of its migration it forms a part of the amid molecules; from these it is split off in the form of ammonia, and this again may be destroyed and gaseous nitrogen set free, or seized by another distinct class of organisms, and oxidized to nitrites