The problem may seem superfluous in view of the difficulties just mentioned. Some aspects of it, however, are fairly clearly defined. In the first instance, some organisms appear to be wholly harmful to the plant; among them are parasitic eelworms and fungi, and bacteria causing disease. Control of these organisms can be brought about by partial sterilisation, and of all methods heat is the most effective, but it is costly, and attempts are now being made to replace it by chemical treatment. The results are promising, but the investigation is laborious; the organisms show specific relationships, and in finding a sufficiently potent and convenient poison it is necessary in each case to make an investigation into the relationship between chemical constitution and toxicity to the particular organism concerned. Formaldehyde is usually potent against fungi, and the cresols, and particularly their chlor- and chloronitro-derivatives, are potent against animals (eelworms, etc.). One group of organisms is wholly beneficial, those associated with leguminous plants. Attempts have been made to increase their activities by inoculating the soil with more vigorous strains. The practical difficulties still remain very considerable, but there is hope that they may be overcome. It is also possible to shift the balance of the soil population in certain directions. Special groups of soil organisms can be caused to multiply temporarily, if not permanently, by satisfying their particular requirements. Thus, when a soil has been heated above 100° C. it becomes specially suited to the growth of fungi, and quite unsuited to certain bacteria such as the nitrifying organisms and others; if this heated soil is infected with a normal soil population the fungi develop to a remarkable extent. The nodule organisms appear to be stimulated by addition of farmyard manure and of phosphates, and the phenol-destroying organisms by successive small additions of phenol. Finally, quite apart from the control of disease organisms, it is possible to alter the soil population considerably by partial sterilisation, using a temperature of only about 60° C., or a poison like toluene that favours few of the soil organisms. This problem has already been discussed in Chapter I. The control of the soil population is still only in its infancy, but it already promises useful developments. It cannot, however, be too strongly insisted that the only sure basis of control is knowledge, and we cannot hope to push control further till we have learned much more about the soil population than we know at present. AUTHOR INDEX. ADAMETZ, 118. Adams, 158. Aiyer, 113. Appel, 133. Artari, 107. Ashby, 42. BARTHEL, 24. Beijerinck, 6, 37, 41, 42, 46, 107. Berthelot, 5, 6, 41. Bewley, 47, 51, 132. Bezssonoff, 69. Boas, 138. Bokorny, 138. Bonazzi, 45. Boresch, 107. Boussingault, 3. Bredemann, 24. Bristol, 106. Brizi, 113.