There is another important respect in which soil bacteria influence plant growth. Their activities result in the release of inorganic salts, such as potash and phosphates, in a form available for the use of plants. The release of phosphorus and potassium compounds takes place in two ways. In the first place, organic matter containing phosphorus and potassium, in an insoluble form, is attacked by bacteria, resulting in these elements being set free as inorganic salts available to the higher plant. Secondly, much of the phosphorus supplied to the soil from rock minerals is present as insoluble phosphates, such as apatite and iron phosphate. Much of the potassium, too, is derived from insoluble silicate minerals. In both cases the conversion of the insoluble minerals into soluble phosphates and potassium compounds is brought about to a large extent by solution in water containing carbonic and other acids. These acids are largely produced by micro-organisms, which, in addition to carbonic acid, produce organic acids, and in specialised cases, sulphuric and nitrous acids. It has been found, for example, that in a compost of soil with sulphur and insoluble phosphate, sufficient sulphuric acid may be produced by the oxidation of the sulphur by bacteria to convert an appreciable amount of phosphate into a soluble form. When we consider the functions performed by soil bacteria, therefore, it is not surprising to find that high bacterial activity in the soil is associated, as a rule, with fertility. _E._ CHANGES IN BACTERIAL NUMBERS AND ACTIVITIES, AND THEIR RELATION TO EXTERNAL FACTORS. The object of soil bacteriologists is to discover means of favouring the activity of soil bacteria, especially those activities that are useful to the higher plant. Knowledge is therefore needed of the changes in numbers and activities of the soil bacteria, and of the influence of soil conditions on them. The necessity of studying these changes has required the development of a quantitative technique by which the numbers of bacteria in the soil and their activities can be estimated.