124 LIVING ORGANISMS OF THE SOIL [chap. 



like diluted urine or liquid manure or sewage is allowed 

 to drip very slowly on to the top of the chalk, so slowly 

 that the liquid takes several days to work its way from 

 piece to piece of the chalk before it finds its way out at 

 the exit. When the tube has grown into good working 

 order the ammoniacal liquid will undergo pretty 

 complete nitrification as it soaks down the chalk, and 

 only nitrates will be found leaving the tube, but the 

 introduction of a little chloroform into the tube will 

 soon put a stop to the action. 



Though it was the first to be investigated, nitrifica- 

 tion is only one of the many changes going on in the 

 soil that are due to bacteria, and some of these changes 

 are of the utmost moment to the nutrition of the plant 

 and to the maintenance of the fertility of the soil. As 

 regards changes in the nitrogenous compounds, another 

 experiment can be performed which will illustrate the 

 main features of the cycle that is perpetually being 

 worked through. A flask is made up as before with 

 lOO C.C. of nutrient solution, in which, however, the 

 sulphate of ammonia is replaced by an equal weight of 

 peptone, this being a complex carbon compound of 

 nitrogen closely related to flesh, albumen, etc., but more 

 convenient in this connection because it is soluble. Half 

 a gramme of carbonate of lime is placed in the flask, 

 which is then sterilised as before in the steamer ; when 

 cold, about i gramme of soil is added. The flask is put 

 away in the dark as before, and at the end of a week it 

 is examined. It will be found that tlie first change to 

 take place has been putrefaction ; the liquid possesses a 

 strong and offensive smell and is turbid with swarms of 

 organisms it contains, some of which are easily visible if 

 a little of the liquid is placed under the higher powers 

 of the microscope. The liquid is then restored to the 

 dark for another fortnight or so, when the putrefactive 



