Nitrogen, changes in amount in soil, 167; cycle in soil, 161; fixation by bacteria, 6, 40 _sqq._; fixation by fungi, 135, 136, 141; fixation of, in clover plant, 5; increase by protozoa of fixation of, 94, 95 (fig.); fixation sources of energy for, 43, 49; gain of, in soil, 174; in invertebrates, 162; loss of, by leaching, 112; loss of, from cultivated soils, 173; relationships of fungi, 135; relationships of algæ, 110-112; relationships of bacteria, 32 _sqq._, 40 _sqq._; relationships of insects, 162. _Nitrosococcus_, 35. _Nitrosomonas_, 35. Nodule Organism of the Leguminosæ, 6, 46 _sqq._ _Nostocaceæ_, 100, 101, 102, 107. _Oligochæta_, 149, 151, 153, 157. _Oospora_, 120. _Orcheomyces_, 132. Orchid cultivation and fungi, 132, 140. _Orthoptera_, 154. _Oscillatoriaceæ_, 100, 102. Osmotic pressure, influencing effect of salts on bacteria, 50. Oxalic acid, formation of, by fungi, 139. Oxidations effected by soil organisms; by bacteria, 26 _et seq._; by fungi, 139. Oxygen, absorption by soils, 4. PARTIAL sterilisation of soil, 8, 66 _sqq._, 96, 178; influence of organic antiseptics, 177; limiting factor in, 67, 68. Pectin, effect of, on fungi, 134. _Pedras negras_, 112. _Penicillia_, 134. Pentosans, effect of, on fungi, 134. Peptones, decomposition of, by fungi, 136, 138; source of nitrogen for algæ, 108. Periodicity, of protozoa in soil, 90 _sqq._ (fig.), 92 (fig.), 93. Phenol, decomposition of, by bacteria, 24, 25, 31. Phenylalanine, formation of, by algæ, 108. _Phoma_, 132. _Phormidium_, 106. Phosphates, availability of, influenced by bacteria, 52; by fungi, 139; effect on bacteria, 46, 51, 60. Photosynthesis, 99, 100, 107, 110, 113. Phycocyanin, 100. Physical conditions in soil, 16. Physiological criteria, of bacteria, 22; of fungi, 128. _Phycomycetes_, 119. _Phytophthora_, 132. Plant disease, and fungi, 139. Plant residues, decomposition of, in soil, 168; influence of soil reaction on, 165; relation to soil fertility, 1, 165. Plasticity of fungi, 119. _Plectonema_, 106. Potassium salts, effect on bacteria, 60; influence of bacteria on the availability of, 52. Protein, decomposition of, in soil, 169; decomposition by bacteria, 32; decomposition by fungi, 138, 140. _Protococcales_, 100. _Protoderma viride_, 105. Protonema of mosses, 100, 105, 106, 109. Protophyta, chlorophyll-bearing, 100.