JACOB G. LIPMAN 345 



inches, on the other, plowed, pulverized, disked, harrowed, and otherwise thoroughly- 

 stirred and mixed, would offer culture media markedly different in character. In 

 speeding up microbiological activities, tillage disturbs the balance between the ac- 

 cumulation and dissipation of organic matter. Whereas in forests, prairies, and 

 meadows the gains of organic matter are usually greater than the losses and the ac- 

 cumulation of carbon, nitrogen, and sulphur in the surface soil is the result, in tilled 

 soils the reverse is often true. It is only by establishing suitable crop rotations and by 

 the use of green manures and animal manures that the farmer is able to retard or pre- 

 vent the depletion of the energy reserves and plant-food resources in the soil. 



Crop rotations represent an orderly succession of different plantings. For in- 

 stance, the five-year crop rotation of corn, oats, and wheat for one year each and of 

 timothy and clover for two years is popular in the general farming sections of the 

 United States. A three-year rotation of corn, oats, and clover is quite common in some 

 localities in the Middle West. The possible number of rotations is almost unlimited. 

 From the standpoint of the farmer, crop rotations are important in that they allow 

 a better distribution of farm labor, lessen the risk of gluts and price declines, allow a 

 better control of insects and plant diseases and the more effective and economical 

 maintenance of soil fertility. From the standpoint of the microbiologist, crop rota- 

 tions are of interest principally for two reasons. While information on the subject is 

 still meager, there is reason for believing that there is distinct colonization of bacteria 

 and of other micro-organisms on the roots of plants. It may be suspected that there 

 are qualitative differences in the microflora and fauna colonized on the roots of differ- 

 ent crops. More definite experimental evidence should be forthcoming, before long, 

 on the direct influence which crops exert on soil micro-organisms. There is also an 

 indirect influence. Crops vary as to their food requirements, water consumption, soil- 

 reaction changes, and soil-texture and soil-structure modification. All of these factors 

 are also of significance in so far as they affect the nature of the soil as a culture 

 medium. Moreover, crops belonging to different botanical groups vary sufficiently 

 in composition to influence in a varying degree, by their stubble and roots, the quality 

 of the soil organic matter. 



The practice of liming is quite old. Chalk, calcareous marl, sand rich in shell 

 fragments, and wood ashes were recognized many centuries ago as having the ability 

 to ameliorate cultivated soils. The use of burned and slacked lime, and of pulverized 

 limestone, is of more recent origin. Lime has various functions to perform in the soil. 

 When properly used, it improves the texture of heavy, poorly aerated soils by flocculat- 

 ing the fine particles into larger aggregates. The circulation of air and water is 

 thereby affected. Lime enters into base exchange and other reactions and thus modi- 

 fies the soil solution and the supply of inorganic nutrients to crops. Finally, applica- 

 tions of lime modify the hydrogen-ion concentration of the soil solution, change the 

 reaction of the culture medium for higher plants as well as for micro-organisms, and 

 influence in a more or less specific way the decomposition of organic matter, the for- 

 mation of ammonia and nitrates, and the fixation of atmospheric nitrogen by sym- 

 biotic and non-symbiotic bacteria. 



When crops like rye, clover, vetch, cowpeas, etc., are plowed under in a green 

 State for the sake of adding organic matter to the soil they are cafled "green manures." 



