The nitrate content of the soil was simultaneously determined by Mr. Page and found to vary from hour to hour, but the variations did not sharply correspond with the bacterial numbers; this, however, would not necessarily be expected. The production of nitrate involves various stages, and any lag would throw the nitrate and bacterial curves out of agreement. There is a suggestion of a lag, but more counts are necessary before it can be regarded as established. Examination of these and other nitrate curves obtained at Rothamsted has brought out another remarkable phenomenon. No crop is growing on these plots, and no rain fell during the eighty hours, yet nitrate is disappearing for a considerable part of the time. Where is it going to? At present the simplest explanation seems to be that it is taken up by micro-organisms. A similar conclusion had to be drawn from a study of the nitrogen exhaustion of the soil. The whole of the nitrate theoretically obtainable from the organic matter of the soil is not obtained in the course of hours or even days; in one of our experiments at Rothamsted nitrification is still going on, and is far from complete, even after a lapse of fifty-three years. The explanation at present offered is that part of the nitrate is constantly being absorbed by micro-organisms and regenerated later on. Now what organisms could be supposed to absorb nitrates from the soil? Certain bacteria and fungi are known to utilise nitrates, and one naturally thinks of algæ as possible agents also. Dr. Muriel Bristol was therefore invited to study the algæ of the soil. Her account is given in Chapter VI. She has found them not only on the surface, but scattered throughout the body of the soil, even in the darkness of 4 inches, 5 inches, or 6 inches depth, where no light can ever penetrate, and where photosynthesis as we understand it could not possibly take place. Some modification in their mode of life is clearly necessary, and it may well happen that they are living saprophytically. Dr. Bristol has not yet, however, been able to count the algæ in the soil with any certainty, although she has made some estimates of the numbers. The quantitative work on the soil population indicates other possibilities which are being investigated. There is not only a daily fluctuation in the numbers, but so far as measurements have gone, a seasonal one also. There seems to be some considerable uplift in numbers of bacteria, protozoa, and possibly algæ and fungi in the spring-time, followed by a fall in summer, a rise in autumn, and a fall again in winter. At present we are unable to account for the phenomenon, nor can we be sure that it is general until many more data are accumulated. In the cases of the protozoa and the algæ, there was a definite reason for seeking them in the soil.