376 . Agricultural Bactenoiogy. 



numbers. The rise in temperature has more effect at 

 this time of the year than later, since the supply of avail- 

 able food is likely to be greater than during the summer 

 an.'l fall, due to the fact that the remains of the previous 

 crop are added to the soil. 



The moisture content also exerts an influence. Bac- 

 teria demand a considerable amount of water for their 

 growth. If the soil is very dry, cell development is slow 

 or ceases. Again, if the soil is water logged as in 

 marshes and low lands, the growth of certain essential 

 kinds of bacteria is impossible, due to the fact that they 

 cannot get a supply of oxygen. Such wet soils warm up 

 very slowty and this acts as a restraining factor in bac- 

 terial growth. If the low places are drained, the ex- 

 cess of water is removed and air is allowed to penetrate 

 the soil and bacterial action is more rapid, both because 

 of increased aeration and higher temperature. The 

 effect of an abundant supply of oxygen is to increase the 

 growth of the most important classes of soil bacteria as 

 is noted in soils thoroughly cultivated. Permanent pas- 

 ture and woodland soils are poorly aerated and in them 

 certain kinds of bacteria grow poorly. As a result these 

 lands never produce such large amounts of plant growth 

 each year as does the same soil under cultivation. 



The bacteria as a rule grow best in an alkaline medium, 

 and this is especially true of many soil bacteria. The 

 soils of marshes and lowlands in general is apt to be acid 

 in reaction. Soils that have been cultivated for long* 

 periods without having had returned to them organic 

 matter, barnyard or green manures, tend to become acid. 

 In such soils the bacteria cannot grow well. If the acid- 

 ity is neutralized by the addition of lime, the number of 

 bacteria will increase. In an acid soil, the addition of 



