NUMBERS OF BACTERIA IN THE SOIL 119 



kinds of bacteria. The number present in it varies very 

 much, being dependent on a great many factors, the 

 chief of which are the kind of soil, the depth at which 

 the sample is taken, the amount of moisture in it, its 

 physical condition, the time of year at which the 

 examination is made, and the kind of crop grown upon 

 the land and the manuring it has received. 



Remy found in one gram of arable soil in fair con- 

 dition from 6 to 10 millions of bacteria and half a million 

 moulds, while in a poor sandy soil there were only 200 

 to 450 thousand bacteria and from 45 to 168 thousand 

 moulds. 



Other investigations have shown that from I to 5 or 

 6 millions may be expected in most ordinary soils. 



The bacteria are chiefly met with near the surface of 

 the ground, i.e. within the first 9 to 1 2 inches, where 

 good aeration and a suitable degree of moisture and 

 temperature prevail. Below this the conditions favour- 

 able for bacterial life are absent, and the numbers 

 diminish rapidly with increasing depth until a point is 

 reached where the soil is sterile. 



Houston found about 1,688,000 in a gram of surface 

 soil, at a depth of I foot 1,100,000, at 3 feet deep 

 174,000, and at 6 feet below the surface only 410. 

 Other examinations have shown that in some soils few or 

 no living micro-organisms were met with below about 3 

 or 4 feet. Where the soil is of open texture and contains 

 large amounts of organic compounds the number in- 

 creases, and more bacteria are able to penetrate and live 

 in the deeper layers than where the soil consists of poor 

 compact clay. 



The number of bacteria appears to go up and down 

 with variations in temperature at different seasons of the 





