Apr. 22. 1918 Biological Activities and Concentration of Soil Solution 221 



obtained when calcium chlorid is added to clay loam ; and in fact all the 

 data from clay loam indicate the operation of some influence as yet 

 not understood. 



That the soil used has an important bearing on the results to be 

 expected is made further evident by a comparison of the amounts of the 

 different salts needed to bring about changes in the osmotic pressure of 

 the soil solution and also in the depression of ammonification. A 

 summary of results showing these points is given in Table V. 



Table V. — Percentage of salts and the resulting osmotic pressures at which depression 

 0/ ammonification and increcuse in osmotic pressure of the soil solution are observed. 



These data show that four times as much of the salt is required to pro- 

 duce a noticeable increase in the osmotic pressure of the soil solution 

 in clay loam as in sandy loam when calcium chlorid and potassium 

 chlorid are added. In the case of calcium nitrate only twice as much is 

 required, while with magnesium sulphate 19 times as much is necessary. 

 It is also noticeable that in sandy loam depression of ammonification 

 occurs with the same amount of each salt except where calcium nitrate 

 is used, when a greater amount is also needed to increase the osmotic 

 pressure of the soil solution. 



There is considerable variation in the osmotic pressure at which 

 ammonification is depressed in sandy loam. The osmotic pressure of 

 the soil solution in clay loam when ammonification is decreased by 

 potassium chlorid is much greater than the osmotic pressure in sandy 

 loam when depression occurs as the result of additions of any salt. 



It appears from these data that the decrease in ammonification 

 resulting from the addition of the salts used is not due to a change in 

 osmotic pressure and that the soil itself is a very potent factor in deter- 

 mining the effect. 



In order to judge more accurately the part played by osmotic pressure 

 in modifying the action of ammonifying organisms a series was run 

 with potassium chlorid in sand. The results are given in Table VI. 



