152 BACTERIA IN SOIL 



may be desired as to the change its composition undergoes by 

 a bacterial action, the result of which may be an increase 

 in fertility and thus in economic value. Under this head may 

 be grouped inquiries relating to the bacteria which convert 

 ammonia and its salts into nitrates and nitrites, and to the 

 organisms concerned in the fixation of the free nitrogen of the 

 air. The discussion of the questions involved in such inquiries 

 is outside the scope of the present chapter, which is more con- 

 cerned with the 'relation of the bacteriology of the soil to questions 

 of public health. So far as this narrower view is concerned, soil 

 bacteria are chiefly of importance in so far as they can be washed 

 out of the soils into potable water supplies. An important aspect 

 of this question thus is as to the significance of certain bacterio- 

 logical appearances in a water in relation to the soil from which 

 it has come or over which it has flowed. In this country these 

 questions have been chiefly investigated by Houston, and it is 

 from his papers that the following account is largely taken. 



Methods of Examination. For examination of soil on surface or not 

 far from surface, Houston recommends tin troughs 10 in. by 3 in., and 

 pointed at one extremity, to be wrapped in layers of paper and sterilised 

 by dry heat. If several of these be provided, then the soil can be well 

 rubbed up and a sample secured and placed in a sterile test-tube for 

 examination as soon as convenient after collection. If samples are to 

 be taken at some depth beneath the surface, then a special instrument 

 of which many varieties have been devised must be used. The general 

 form of these is that of a gigantic gimlet stoutly made of steel. Just 

 above the point of the instrument the shaft has in it a hollow chamber, 

 and a sliding lateral door in this can be opened and shut by a mechanism 

 controlled at the handle. The chamber being sterilised and closed, the 

 instrument is bored to the required depth, the door is slid back, and by 

 varying devices it is effected that the chamber is filled with earth ; the 

 door is reclosed and the instrument withdrawn. 



In any soil the two important lines of inquiry are first, as to the total 

 number of organisms (usually reckoned per gramme of the fresh sample) ; 

 and secondly, as to the varieties of organisms present. The number of 

 organisms present in a soil is often, however, so enormous that it is con- 

 venient to submit only a fraction of a gramme to examination. The 

 method employed is to weigh the tube containing the soil, shake out an 

 amount of about the size of a bean into a litre of distilled water, and 

 revveigh the tube. The amount placed in the water is distributed as 

 thoroughly as possible by shaking, and, if necessary, by rubbing down 

 with a sterile glass rod, and small quantities .measured from a graduated 

 pipette are used for the investigation. For estimating the total number of 

 organisms present in the portion of soil used, small quantities, say "1 c.c. 

 and 1 c.c., of the fluid are added to melted tubes of ordinary alkaline 

 peptone gelatin ; after being shaken, the gelatin is plated, incubated at 

 22 C., and the colonies are counted as late as the liquefaction, which 

 always occurs round some of them, will allow. From these numbers the 

 total number of organisms, which grow in gelatin, in a given amount of 

 soil can be calculated. 



