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.