Protozoa, inoculation into soil of, 85 _sqq._; isolation from soil, 69; classification of, 69 _sqq._; life histories of, 72 _sqq._; species of, in soil, 70 _sqq._; distribution of, in soil, 74 _sqq._; retention of, by soil, 78 (fig.); size of, 90; reproductive rates, 93; inverse relation with bacteria, 79 _sqq._; presence of trophic forms in soil, 9; numbers of, in soil, 90, 96, 97; fluctuations in numbers of, 10, 81 (fig.), 82; external conditions, effect on, 82; seasonal changes, effect on, 87 _sqq._; weight of, 90. _Pteridophyta_, 132. _Pythium_, 132. REACTION of soil, 17. Reaction of soil, effect on bacteria, 36, 37, 46, 48, 61; effect on protozoa, 93, 94 (see also hydrogen ion concentration). Relationships of Fungi, commensal, 132; mycorrhizal, 132; symbiotic, 132. _Rhizopoda_; classification of, 70, 71; species of, 70, 71. Rhythm, supposed in ammonification by fungi, 137. _Rhizoctonia_, 132. _Rhizopus_, 119, 120. Rice plant, aeration of roots, 113; physiological disease of, 113. Rock Phosphate as base for nitrifying organisms, 36. Rothamsted, Broadbalk plot 2 (Farmyard Manure) algæ, 109; fungi, 125, 127; Insects, 152. Rothamsted, Broadbalk plot 3 (Unmanured) algæ, 109; fungi, 120, 122, 127; Insects, 152. Rothamsted, Broadbalk Plots 10, 11, and 13; 122, 127. Rothamsted, Barnfield Plot 1-0 (Farmyard Manure), Protozoa, 80. Rothamsted, unmanured grass plot, 120. _Russula_, 132. Rusts, 119. _Saccharomyces_, 120. Saprophytes, facultative, 131. Saprophytism and algæ, 108, 110. _Scenedesmus_, 108. Seasonal fluctuations in numbers of soil organisms, 12, 87 _et seq._, 125. Selective media, use of, in isolation of soil bacteria, 21. Serological tests, separation of varieties of _B. radicicola_ by, 48. Slugs, see _Mollusca_. Smuts, 119. Snails, see _Mollusca_. Soil; comparison of, by volume, 17; effect of depth below surface on algæ, 101, 104, 109, 110, 113; effect of depth below surface on insects, 151; effect of depth below surface on fungi, 121, 126, 127; effect of various treatments on fungi, 126, 127, 132; environmental factors in, 16; inoculation of, for leguminous plants, 50; moisture (see Water supply); population, control of, 177 _sqq._; population, methods of investigation, 10, 15; sterilisation and fungi, 137, 138, 141 (see Partial Sterilisation); stored, survival of algæ in, 107; type and fungi, 121, 126, 127. Soil conditions, effect on bacteria, 33, 36, 37, 40, 46, 48, 50, 59 _sqq._; effect on protozoa, 82. Soil fertility, see Fertility of soil. _Spicaria_, 120. Spiders, see _Areinida_. _Spirochæta cytophaga_, 28, 43. Spore forming bacteria in soil, 23, 34. Spore, fungus, inhibition of formation, 123; presence in air of, 118. Standardisation of cultural methods for soil bacteria, 54 _sqq._ Starch, decomposition of, by fungi, 134. _Stichococcus_, 108.