Germ Life In The Soil

No soil has exactly the same composition from year to year, or even from month to month. It is constantly receiving additions of new soil from the weathering of rocks, from the decay of plants, the deposits of winds and other sources. It is constantly losing by leaching, by erosion and by the demands of plant growth. It also has numerous activities within itself that exert a most potent influence on its fertility. Some of these activities are physical, some are chemical and some are due to germ life. A few are already known and understood, but only the merest beginning has been made in the study of soil life. Nitrogen-Fixing Germs. — One of the most interesting phases of soil life is the process called "nitrification," due to the activity of very minute germs or bacteria, and sometimes called the "nitric acid ferment." This is somewhat like the ferment that sours milk, and the bacteria in yeast that raise bread by their growth. Although the air contains vast amounts of nitrogen, this is not used by any plants, so far as is known, except to some extent by the "legumes," of which clovers, alfalfa and vetch are examples. (See Chapter XII.)

Most farm crops get their nitrogen, which they need in considerable quantities, solely from the soil. This nitrogen enters into their structure, and is returned to the soil when the plants decay, but not in the same form. It enters the plant as a salt of nitrogen — a nitrate; it returns to the soil in combination with many other substances, and is called by the chemist "organic nitrogen." The important point about this is that plants cannot use organic nitrogen, because it will not dissolve in water, and all the food that plants get from the soil must be taken in liquid form. It must first be separated from its partners in the compound, and then changed into a nitrate before the soil water can dissolve it, and the roots of plants absorb it. The work of transforming valueless organic nitrogen into valuable nitrates, which are plant food, is performed by our tiny helpers, the "nitrogen-fixing germs." They are found in all fertile soil in inconceivable numbers, busily engaged in making plant food out of all vegetation that is returned to the soil, provided the conditions are right. One essential condition is that they have plenty of food.

All these ferments may be considered very minute plants; they must have food like other plants. One food of the nitrogen fixing germs is phosphoric acid, which is also one of the most important foods of ordinary farm crops. If a soil has very little phosphoric acid in it, the transformation of humus into plant food is apt to take place very slowly. The principal food of the germs, however, is humus itself. This they can use only after the leaves, stems, or other vegetation has been thoroughly incorporated with the soil and is rotted. These minute plants need moisture and a medium temperature in order to thrive and do their work, as the yeast ferment needs moisture and a certain temperature in order to multiply and as a corn plant needs water and hot weather in order to bring forth its increase. The growth of these microscopic soil plants is checked in very dry weather as much as the growth of the larger plants above ground. Furthermore, they do not thrive in a very wet soil. The temperatures most favourable for their growth have been found to be 54° to 99° F. In the Southern States they grow the year around.

Another essential condition is a plenteous supply of oxygen, such as would be had if the soil were well drained and hence well ventilated. It will be seen, therefore, that the conditions that favour the growth of these useful workers are those that are most necessary for the growth of farm crops — a moist, well-drained soil and thorough tillage. Given these conditions, a multitude of the germs attack the rotten leaves, stems or stubble lying in the soil, or the clover, rye or cowpeas that have been plowed under, and soon change the useless organic nitrogen into a nitrate. In order to do this, however, the soil must contain a sufficient quantity of some "base," as lime, to combine with the nitrogen and so make it a nitrate. If the soil is at all acid, or sour, (see Chapter XIV), the germs cannot complete their work. Germs That Waste Nitrogen. — It is interesting to know that there are also at work in some soils bacteria that accomplish a result exactly opposite to that of the nitrogen-fixing germs. The process is sometimes spoken of as "de-nitrification," and the germs may be called "nitrogen wasting" germs. They feed upon the nitrates, and set free the nitrogen gas, which may then escape into the air and so be lost to the soil. These germs are abundant in wet soils ; under-draining benefits the soil in more ways than by merely removing water. Thus these two, the nitrogen-saving and the nitrogen-wasting bacteria, are pitted against each other; the one is a blessing to the soil, the other may be a detriment. It is wise farming to encourage the growth of the former by providing the conditions most favourable for them — thorough tillage and excellent drainage. Other Soil Bacteria. — These two kinds of bacteria are but a very small part of the germ life of the soil. Adametz has calculated that there are 50,000 germs of various kinds in a single gram of fertile soil. Many are beneficial, most of them are harmless, some are injurious. When the roots and stubble of a certain crop decay in the soil, a certain kind of "ferment," which is bacterial growth, is produced. If the crop is grown for several years on the same soil, after a while the soil may become crowded with the particular kind of ferment that the decay of the crop produces.

The result may be that eventually the soil will no longer produce satisfactory crops of this plant, but it will produce larger crops of some other. This is the explanation, in many cases, of "clover-sick" and "flax-sick" soils and other soils that fail to respond as they used to. The practice of inoculating soils with certain beneficial bacteria is discussed in Chapter XII , with particular reference to leguminous crops. The limits of the practical value of soil bacteriology can only be surmised at this time ; but it seems not improbable that the farmers of some future generation may be able to inoculate their soils with different beneficial bacteria and secure specific and valuable results, much as the butter maker of to-day secures certain flavours with certain cultures. The field of study opened before us by recent investigations in soil bacteriology is extremely interesting and it may yield extremely important results.