Another method, therefore, of studying the activity of soil micro-organisms is the obvious one of estimating the chemical changes that they produce in the soil itself. This method has obvious advantages over the unnatural methods developed from Remy’s, but it has a number of limitations that make its actual application difficult. In the first place, we cannot always tell whether changes found to occur in soil are due to the activity of micro-organisms, or are purely chemical reactions unassisted by biological agencies. Then, if we succeed in showing that the changes are due to micro-organisms, it is very difficult to determine which organisms are effecting them. This cannot be definitely tested by isolating suspected organisms and testing their activity in sterile soil, because in sterilising soil its nature and composition is altered. In spite of these difficulties, however, the study of the chemical changes that take place in the soil has produced valuable knowledge, when it has been combined with a study of the changes in the number and variety of the micro-organisms that accompany these reactions. This method of investigation is well illustrated by the work of Russell and Hutchinson on the effects of heat and volatile antiseptics on soil, where a study of the chemical changes such as ammonia production, that occurred in these treated soils, combined with a study of the changes in bacterial numbers, led to the realisation that the soil micro-population was a complex one, containing active protozoa. A great difficulty in applying quantitative methods to bacteria in the field is the great variation in the density of the bacterial population over a plot of field soil, which may be so great that a bacterial count from a single sample is quite valueless. For example, the distribution of bacterial numbers over a plot of arable soil near Northampton was studied by taking sixteen samples distributed over an area about 12 feet square. The result showed that in some cases the bacterial numbers in samples taken 6 inches apart differed by nearly 100 per cent. Fortunately, under favourable conditions, a remarkably uniform distribution of bacterial numbers over a plot of soil can be found.