Bacterial life is suspended or may be de stroyed in very dry soils. Hence the number of bacteria in soils is low after drought and high after rain. Excessive moisture, however, produces conditions (reduced aeration, acidity, etc.) unfavorable to beneficial organisms but favorable to harmful organisms, such as deni trifying bacteria, which cause loss of nitrogen or its conversion into less assimilable forms, and protozoa and similar organisms, which feed and hence reduce the number of bene ficial organisms. Hence it is very important for bacteriological as well as other reasons that soils should he well drained. Bacterial action is more intense and long-continued in warm soils. Bacteria live but are not active in frozen soils. Like the higher plants they are dor mant in winter but awaken with renewed life and activity in the spring. The prompt ness and vigor with which they awake depends upon and in a measure determines whether a soil is or "late.* About 25° C. is considered the optimum temperature for the growth of the more important soil organ isms. Beneficial organisms do not as a rule thrive in acid soils, hence such soils should be limed to correct the acidity. Excessive alkali is also harmful. Soils must be well supplied with organic matter (humus) to furnish the food (energy) required by bacteria, and with a certain amount of phosphate, which is also a necessary food. The activity of soil organ isms is influenced not only by soil conditions but also by the character of the plants grown on the soil. For example, nitrification appears to be more active under corn than under tim othy and mixed grasses, and similar differences have been noted with other crops.
Among the more active and important classes of soil organisms are molds which fix nitrogen, produce ammonia and decompose cel lulose; mycorrhiza, which probably fix nitro gen; actinomyces, which decompose organic matter; alga', which manufacture food for azotobacter; protozoa, which prey, upon bene ficial organisms; bacteria, which oxidize carbon, fix and transform nitrogen and make mineral constituents of the soil available. Many or ganisms which cause plant diseases also live in the soil and play an important part in determin ing its productiveness. The destruction of these organisms as well as those that interfere with the growth of beneficial bacteria is of great practical importance from the standpoint of soil sanitation and productiveness. So called aexhausted,° or °sick° soils are very often simply those in which harmful or ganisms predominate and their productiveness is readily restored by partial sterilization by means of heat, as in the case of the steaming of greenhouse beds, or by the use of volatile antiseptics like carbon bisulphide, toluol and others.
Soil Toxins.—The theory has been advanced that soils are rendered unproductive by the presence of toxic substances excreted by the roots of plants, produced by bacteria or de rived from the decomposition of organic mat ter. According to Russell °there is no evidence of the presence of soluble toxins in normally aerated soils sufficiently supplied with plant food and with calcium carbonate, but toxins may oc cur on 'sour) soils badly aerated and lacking in calcium carbonate, or on other exhausted soils.
There is no evidence of any plant excretions conferring toxic properties on the soil, but . . . a growing plant may poison its neigh bour. . . . Soluble bacterio-toxins are not normal constituents of soils, but must repre sent unusual conditions wherever they occur. But the possibility of the existence of toxins insoluble in water still remains.° Schreiner in his exhaustive study of the organic matter of the soil has isolated a great variety of com pounds some of which have toxic properties.
Exhaustion, Improvement and Reclama tion of Soils.— Soils are said to be exhausted when they no longer yield profitable crops, but strictly speaking there is no such thing as abso lute exhaustion of soils. However, soils de cline in productiveness both as a result of loss of fertilizing constituents from the soil and of deterioration in its physical and biological con dition.
Among the principal causes of loss of soil fertility are (1) the growth and removal of crops without restoring the equivalent of the fertilizing constituents they contain; (2) sur face washing, and (3) leaching. All crops con tain a considerable amount of fertilizing mat ter drawn from the soil, and it is held that if these crops are grown continuously and sold away from the farm without return of an equivalent in manure or fertilizers the soil will in time show a decline in fertility. The harm ful effects of surface washing is a matter of common observation and needs no further dis cussion here. The loss of fertility in the drain age water is generally supposed to be very con siderable, and under certain circumstances this is true, depending upon the character of the soil and the treatment to which it is subjected and the fertilizers applied. °Leachya soils part very quickly with the fertilizing materials ap plied to them unless covered with crops which utilize the fertilizers promptly, and, as already shown, certain fertilizers have a tendency to set some of the soil constituents free and throw them into the drainage water.
While these are all possible sources of loss, it is probably safe to say that tinder ordinary conditions the chances of loss of any consider able amount of lime, potash or phosphoric acid in the drainage water of soils are very small. As regards the loss of the important and ex pensive fertilizing constituent, nitrogen, how ever, the case is very different. The soil ap pears to have very little affinity for one of the forms of this element most commonly used as a fertilizer, namely, nitrate of soda, and if it is not quickly taken up by the crop it is likely to pass into the drainage and be lost. Moreover, other less soluble forms of nitrogen are under favor able conditions rapidly converted into nitrates by the process of nitrification, and so may also be lost in the drainage. Deherain has reported experiments in which the loss of nitric nitrogen in the drainage from a bare soil in the course of a year was nearly 180 pounds per acre, while the loss from a soil which was kept covered by a crop was almost insignificant, although fully as large an amount of nitrates was formed in the latter case as in the former. Under unfa vorable soil conditions there may be consider able loss of nitrogen through denitrification.