BACTERIA IN SOIL 153 



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. 



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 door is reclosed and the instrument withdrawn.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 grin, 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 

 convenient to submit only a fraction of a grm. 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 

 reweigh 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. 



In certain cases it may be necessary to investigate the anaerobic 

 organisms of the soil. The inquiry is necessarily of a qualitative 

 character and the methods to be adopted are those already described 

 (p. 63). Sometimes information can be acquired by the injection of 

 small portions of the soil into animals (guinea-pigs, mice). 



The numbers of bacteria in the soil vary very much. Accord- 

 ing to Houston's results, fewest occur in uncultivated sandy soils, 

 these containing on an average 100,000 per gramme. Peaty soils, 

 though rich in organic matter, also give low results, it being 

 possible that the acidity of such soils inhibits free bacterial 

 growth. Garden soils yield usually about 1,500,000 bacteria 

 per gramme, but the greatest numbers are found in soils which 

 have been polluted by sewage, when the_figures may rise to. 



