134 



Partial Sterilisation of Soil 



§ 27. The production of ammonia from urea. The experiments with 

 urea were on the same lines as the preceding and lead to the same 

 conclusions. 



Table 10. 



Decomposition has been most rapid in the "toluene evaporated" 

 soil but it also goes on in presence of toluene. It is not due in the 

 latter case to catalytic action of the soil since it does not take place 

 in the heated soils, but is most probably brought about by an enzyme. 



§ 28. There is a fundamental difference between the decomposition 

 of peptone and of urea. Peptooe acts as a nutrient, urea does not, but 

 is decomposed by a purely fermentative change. The aqueous extract 

 of the soils had little or no action on urea solution till peptone was 

 added and then decomposition took place : the amount of decomposition 

 increased with the amount of peptone added. 



Table 11. Effect of nitrogenous food supply on the rate of urea 

 hydrolysis (25 grams imtreated soil and 15 ex. 1 °/^ urea solution 

 and varying amounts of nitrogen as peptone). 



Nitrogen added as peptone, mgrams 



Mgrams of ammonia, expressed as nitrogen, 

 produced after 44 hours 



2 08 

 59-0 



Under the same conditions toluened soil produced 53'8 mgms. of 

 nitrogen as ammonia; a striking proof of its superior decomposing 

 powers. 



§ 29. Nitrogen fixation. 5 grams of soil were inoculated into 

 50 c.c. of a 2 per cent, mannite solution containing potassium phosphate 

 (Beyerinck's solution) and the whole was allowed to stand at 30° in 

 an Erlenmeyer flask plugged with cotton-wool for 21 days. As no 

 nitrogen compound was supplied those organisms alone could develope 

 that take their nitrogen direct from the air. The toluened soil fixed 

 less than the untreated, whilst the heated soil fixed practically none. 



