404 



SOIL CONDITIONS AND PLANT GROWTH 



Seed, effect of water supply, 44. 



— richer in phosphorus and magnesium 



than in calcium and potassium, 



75- 



— size and weight affected by potassium 



su.^ply, 70. 



slightly by nitrogen 



supply, 61. 



phosphate sup- 

 ply, 66. 



Seeds, viability after long period in soil, 



85 

 Sewage, absorption by soil, 152. 



— purification ; changes during, 214. 



effect of disinfecting filters, 281. 



nitrification during, 23, 190. 



nitrogen lost during, 192, 194. 



reactions during, 214. 



Shade crops, 229. 

 Shrinkage of soils, 106, 133, 219. 

 Sickness in soil, 248, 287, 294. 

 Silica, supposed production in plants, 8. 

 Silicates, beneficial effect on plant 

 growth, 15, 56, 



77- 



especially with lack 



of phosphate, 77. 



— in soil, 120, 121. 



Van Bemmelen's groups, 10 1, 



103. 



— Way's double, 153. 

 Silt, composition of, 102. 



— effect in soil, 107. 



— properties of, 107. 

 Silty clays, 312. 



Sodium carbonate, toxic effects, 72, 85. 



— nitrate, effect on soil tilth, 107, 116. 



— salts, effect on plant growth, 72. 

 supply of potassium salts, 



151. 

 Soil acidity, effect on flora, 240. 



micro-organisms, 242. 



hypotheses to account for : — 



Chemical: acid silicates, no. 



— aluminium salts, 112. 



— exchange of bases, 112. 



— hydrogen ion concentration, 113. 



— organic acids, no, 118. 

 Physical : preferential absorption, 



no, 118. 

 Soil acidity, summary of present posi- 

 tion, 117. 



— analysis, 234, 322 et seq., 347 et seq. 



— colloidal properties of, 149. 



— constitution of, 172 et seq. 

 American hypotheses, 172. 



— cultures contrasted with sand, 40, 41. 



— exhaustion, 319. 



— fertility, 319. 



Soil fertility, theories of, 27. 



— formation of, 92, 303. 



— in relation to plant growth, 301. 



— minute particles the " pabulum " of 



plants, 5. 



— sampling, 324, 347. 



— solution : concentration of, 125, 126. 



composition of, 93, 125, 127, 173. 



importance in soil fertility, 27. 



— — investigation in situ by freezing- 



point method, 125. 



— — methods of extraction : absorp- 



tion, 125. 



centrifugal, 124. 



displacement, 122. 



pressure, 124. 



osmotic pressure of, 125, 126. 



relation to climate, crop and 



season, 12 >, 127. 



plant growth, 173. 



soil mineral matter, 122, 



125. 

 supposed constancy of composi- 

 tion, 173. 



— surveys, 322, 345. 



based on mechanical analysis, 



343- 

 petrogeological analysis, 346. 



— type, characterisation of, 326. 



— — effect on plant, 45. 



— water, 122 et seq. 

 Soils, classification of, 94. 



— specially suited to certain crops or 



varieties, 316. 

 Sourness in soil, 118, 239 et seq., 315. 

 Specific gravity of soils, 220. 

 ■ — heat of soils, 229. 

 Spores, bacterial in soil, 216. 

 Stages of development of plant, effect 



of nitrates on, 61. 

 Statistical method of investigation, 18, 



29. 

 Steppe, 314 



Sterilisation of soil. See Partial steril- 

 isation. 

 not achieved without decom- 

 position, 250. 

 Stimulation of plant growth, possibility 



of, 87 et seq. 

 Stones, effect in soil, 109. 

 Storage of soils, change during, 169. 

 Straw and stubble, effect on soil, 213. 



— effect of nitrates, 61. 



— lodging of, 62. 



effect of potassic fertilisers, 



69. 



— relation to grain, 62. 



Strontium salts, effect on plant growth, 

 74. 



