“Hochmoor,” fungi in, 126. _Hormidium_, 104. Humus, the food of plants, 1; formation of, by fungi, 134, 141; formation of, in soil, 168; forest, 132; fungal hyphæ as constituent of forest humus, 132. Hydrogen ion concentration, in soil, 17; effect on fungi of, 124. _Hymenoptera_, 150, 154. _Insecta_, 150, 157. Insects, numbers present in soil, 154. Invertebrata, definition of, 147; method of investigating, 148; groups represented, 149; distribution in the soil, 151; dominant species and groups, 153; environmental factors of, 157; feeding habits, 156; relation to agriculture, 160; relation to nitrogen cycle, 161. Iron compounds, oxidation by fungi, 139. _Isopoda_, 150, 151. _Leguminosæ_, association with bacteria, 46 _sqq._; enrichment of ground by, 5. _Lepidoptera_, 150, 154. Life cycles, of bacteria, 22, 47; of protozoa, 72 _sqq._ Lime, effect on fungi in soil, 121, 126. _Lyngbya_, 112. MAGNESIUM compounds, effect on fungi, 139. Manganese compounds, effect on bacteria, 61. Manure, farmyard, effect on algæ, 109, 110; effect on numbers of bacteria, 60; effect on numbers of fungi, 126; effect on numbers of insects, 154, 155. Manure, Artificial, effect on fungi, 127. Manure, town stable, occurrence of disease organisms in, 132. _Mastigophora_, classification of, 71; species of, 71. Media, containing nitrates, chemical analysis of, 111; for counting soil bacteria, 54; for counting protozoa, 79; for counting fungi, 119, 123. _Melanconium_, 134. _Melolontha_, 150. Methane, oxidation of, by bacteria, 26, 27. Millipedes, see _Diplopoda_. Mites, see _Acarina_. _Mollusca_, 149, 157. _Moniliaceæ_, 136. _Mucor_, 120, 121, 136, 138. _Mucorales_, 121, 134. _Mucorineæ_, 118. _Mycetophilidæ_, 155. Mycorrhiza, 132, 135, 139, 140. _Myriapoda_, 150, 156. _Myxophyceæ_, 100 (see also _Cyanophyceæ_ and blue-green algæ). NAPHTHALENE, decomposition of, by bacteria, 31. _Naviculoideæ_, 100. _Nematoda_, 149, 151, 157. Nitrate, assimilation by algæ, 105, 108, 111; assimilation by bacteria, 33, 40, 44, 51; assimilation by fungi, 136, 138; removal from soil, 12, 112, 171; variations in amount in soil, 11. Nitre-beds, 1. Nitrification, and bacteria, 34; chemical changes in, 171; and fungi, 136; energy supply in, 35; mechanism of, 1, 3; and soil fertility, 1, 3. Nitrites and fungi, 136; formation by bacteria, 34. _Nitrobacter_, 35.