1 66 PHARMACEUTICAL BACTERIOLOGY 



maximum degree, the soil must constantly be supplied with air; in other 

 words, the soil must be aerated. This is done by stirring and turning 

 the soil. The stirring and turning operations must however be of such 

 nature as not to bring about any undue waste or loss of soil moisture. As 

 soon as the seeds of the crop plants have germinated and the young 

 plants are growing actively, any deep turning of the soil must cease as 

 this interferes with bacterial activity and furthermore causes undue loss 

 of moisture by evaporation. The ideal crop cultivator should keep the 

 soil (to a depth of from three to six inches) moderately fine, well stirred 

 and should carry new air into the soil. If properly done, the only hmit 

 to the number of times a crop plant may be cultivated, is of purely mechan- 

 ical nature. On an average one cultivation every two weeks during the 

 entire growing season, will perhaps induce an optimum bacterial develop- 

 ment, and the most desirable reduction in the rate of the upward circula- 

 tion of the capillary moisture. 



3. Soil Temperature and Bacterial Activity. — Not by any means do all 

 soU and plant bacteria develop to an optimum degree at the same temper- 

 ature. Some of the manure bacteria develop actively at a temperature 

 of 70° C. Some of the bacteria of the ocean water and of the polar regions 

 develop best at a temperature of from 10° C. to 5° C, and even at lower 

 temperatures. The great majority of soil bacteria in the temperate 

 regions develop most actively at a temperature ranging from 12° C. 

 to 25° C. It is interesting to know that while the soil owes its warmth 

 to the heat rays of the sun, the actinic rays of sunlight inhibit bacterial 

 development, especially in the surface soils, and this reduction in bacterial 

 growth in this soil stratum is further increased by the absence of capillary 

 moisture. The maximum bacterial development in soil takes place at 

 a depth ranging from three to six inches. Below that depth there is a 

 gradual decrease in microorganisms, few being found at depths beyond 

 four to eight feet. 



In countries having severe winters (November to April), the farmer 

 hastens the warming up of the soil air in the spring, preparatory to crop 

 planting, by turning the soil with a plough. This operation also causes a 

 better distribution of the soil bacteria and admits air. In semi-tropical 

 and tropical countries the preparatory turning of the soil for sun warming 

 purposes, is not so important, but proper soil aeration may nevertheless 

 not be neglected, for reasons already stated. 



Bacterial activity results in some warmth generation and deep ma- 

 nuring (hot beds) is practised to force young plants. The heat in sUo 

 fermentation may rise to 70° C. The curing of hay and of other vegetable 

 substances results in a rise in temperature. The bacterial activities in 

 the soil do not produce any definitely measureable rise in temperature. 



