TABLE OF CONTENTS 



XV 



CHAPTER XX 



/-"""ORGANISMS IN THE SOIL 421-442 



Macroorganisms. 



Rodents, 342 Worms, 343 Insects, 344 Large 

 fungi, 345 Plant root, 346. 



Microorganisms. 



Plant microorganisms, 347 Plant microorganisms 

 injurious to higher plants, 348 Plant microorganisms 

 not injurious to higher plants, 349 Bacteria, 350 

 Distributionof bacteria, 351 Numbers of bacteria, 352 

 Numbers as influenced by season, 353 Conditions 

 affecting growth, 354 Oxygen, 355 Moisture, 366 

 Temperature, 357 Organic matter, 358 Soil acidity, 

 359 Functions of soil bacteria, 360 Decomposition 

 of mineral matter, 361 Influence of certain bacteria 

 and molds on the solubility of phosphates, 362 Decom- 

 position of non-nitrogenous organic matter, 363 De- 

 composition of nitrogenous organic matter, 364. 



CHAPTER XXI 



; NITROGEN CYCLE 443-474 



Decay and putrefaction, 365 Ammonification, 366 

 Bacteria and substances concerned in ammonification, 

 367 Nitrification, 368 Effect of organic matter on 

 nitrification, 369 Effect of soil aeration on nitrification, 

 370 Effect of sod on nitrification, 371 Depths at 

 which nitrification takes place, 372 Loss of nitrates 

 from the soil, 373 Nitrate reduction, 374 Nitrate 

 assimilating organisms, 375 Denitrification, 376 Ni- 

 trogen fixation through symbiosis with higher plants, 

 377 Relation of bacteria to nodules on roots, 378 

 Transfer of nitrogen to the plant,' 379 Soil inoculation 

 for legumes, 380 Nitrogen fixation without symbiosis 

 with higher plants, 381 Nitrogen-fixing organisms, 382 

 Mixed cultures of nitrogen-fixing organisms, 383 

 Nitrogen fixation and denitrification antagonistic, 384. 



