When we consider the action of external conditions on the soil bacteria, the existence of a complex soil population and the interdependence of its members must be borne in mind. Changes in external conditions may affect the different components of the population in different ways or to different degrees, thus upsetting the equilibrium between the various groups. For example, the addition of a mild aromatic antiseptic to the soil apparently affects the protozoa in such a way as to disturb the bacteria-protozoa equilibrium in favour of the bacteria, while in some cases the aromatic compound affords a food supply to special bacteria, causing these to increase, upsetting the equilibrium between the different bacterial groups. When our knowledge of the effect of external factors on the soil population becomes sufficient, it will probably be found that in nearly all cases a change in the soil conditions produces some disturbance in the equilibrium between the components of the soil population, though at present there are only certain examples where this disturbance is a probable explanation of the facts.
Since bacteria are dependent on adequate supplies of energy and food, it is to be expected that additions of organic matter or of inorganic food materials will greatly benefit their activities. The effect of added farmyard manure in increasing bacterial activities has been much studied.[27] Some of the increased bacterial numbers and activities in this case may be due to the addition of bacteria with the manure, but it is thought that this factor is of less importance than the added energy and food supply which the general soil flora obtain from it. Nutritive salts such as phosphates and salts of potassium usually increase the bacterial activities.
The effect of alkali salts on soil bacteria has been especially studied in the Western United States, where the existence of alkali in the soil is a serious problem.[23] Soil bacteria are usually stimulated by small doses of alkali salts that are toxic in higher concentration. As a rule, chlorides are the most toxic salts, the electronegative ion playing an important part in the effect of the salt. Salts affect bacteria both owing to the changes in osmotic pressure which they produce, and through their specific action on the bacterial protoplasm.[26] When equal weights of various salts are added to soil, their toxic action on bacteria shows so little association with their respective osmotic pressures that we must conclude that this factor is the less important. There is reason to suppose that the toxic action of salts on bacteria is often connected with an effect of the specific ions on the permeability of the bacterial cell-wall. This conclusion is based on the changes in electrical conductivity of bacterial suspensions in the presence of various salts.[59]
A definite antagonism between various salts has been found to exist. It is possible that future work in this line may indicate what are the proportions of common electrolytes which will produce a properly “balanced” soil solution so that the harmful excess of one salt may be antagonised.
Certain salts, such as those of arsenic[24] and manganese, seem to exercise a stimulating action on bacterial activities; the causes of this action are not at present understood.
The acidity of the soil has an important effect on the bacterial processes. The acidity of soils may increase to such a point that the decomposition of plant tissues by bacteria is hindered, a peat layer being thus produced. The degree of acidity that is toxic varies very greatly with different soil bacteria, some of them, like Azotobacter and Nitrosomonas being very intolerant of acidity.
The conditions of aeration, water content, and temperature are inter-related in field soil. Ammonifying organisms are not greatly dependent on aeration, but this factor is sometimes a limiting one in the case of the very aerobic nitrifying bacteria. Hence efficient soil cultivation is beneficial to nitrification.
Many attempts have been made to correlate the temperature and moisture of field soils with the bacterial numbers and activities. These attempts have given very discordant results. It is generally agreed that a plentiful moisture supply is beneficial. Thus Greaves, in Utah, found the optimum water content for ammonia and nitrate production to be about 60 per cent. of the water-holding capacity. On the other hand, Prescott[56] found that the summer desiccation of soil in Egypt was followed by increased bacterial activities. Fabricius and Feilitzen,[18] using moor soil, found a direct relationship between soil temperature and bacterial numbers, showing that temperature can be a limiting factor under certain conditions. With normal arable soils, however, no such direct effect of temperature or moisture can be found[16] (see [Fig. 8]). It has even been found by Conn[11] that freezing of the soil may cause a marked increase in bacterial numbers. The erratic effects of temperature and moisture on the soil bacteria probably afford instances of a disturbance of the equilibrium between the bacteria and other components of the soil micro-population. Thus desiccation and freezing, though they harmfully affect the bacteria, may inhibit other micro-organisms to a greater degree, thus freeing the bacteria from competition. It is in the investigation of this equilibrium, and of the factors that can control it to our benefit, that the great advances in soil biology in the future are to be expected.
Fig. 8.—Effect of frost on the bacterial numbers in the soil. (After Conn.)