BACTERIOLOGICAL EXAMINATION OF SOIL. 129 



investigated by Houston, and it is from his papers that the 

 following account is chiefly 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 door is closed and the instrument withdrawn.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 convenient 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 re-weigh the tube. The amount placed 

 in the water is distributed as thoroughly as possible by shaking, and, if neces- 

 sary, 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 i 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 present in the amount of soil originally present can be calculated. 



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 115,000,000. In addition to the enumeration of the numbers 

 of bacteria present, it is a question whether something may not 

 be gained from a knowledge of the number of spores present 

 in a soil relative to the total number of bacteria. This is a 



