166 BOTANICAL GAZETTE [MARCH 
- fication and nitrification point to the conclusion that toxicity is 
due in a large measure to osmotic influences. This conclusion is 
based on the assumption that the salts on passing into solution 
behave as they would in pure water, an assumption which is not 
warranted, for it is a well known fact that the addition of a salt to 
a soil causes an exchange of ions. This may either increase or 
decrease the total number of soluble particles in the soil solution, 
and hence correspondingly change the osmotic pressure of the soil 
solution. The logical procedure, therefore, is to determine the 
osmotic pressure (14) of the soil at the concentrations at which the 
various salts become toxic to the different classes of microorganisms. 
The results for such tests for the ammonifying series are given in 
table IV 
TABLE IV 
OSMOTIC PRESSURE OF SALT-TREATED SOIL TOXIC TO AMMONIFYING ORGANISMS 
Chloride Nitrate Sulphate Carbonate Average 
POM: 6s oie eva siew nce 1.62 aaa, 1.96 8.41* 1.70 
Potassium... ..5...'.., 183 1.89 1.78 9.05* 1.73 
Calthite: foo. 33 oaks 1.43 118% 2.45 1.61 yf 
Magnestuny. oo. ois c) 1.62 1.68 1.88 1.80 1.75 
acc ces ee vea be ties 1.56 S053" 1.78 2.04 1.79 
ee rN 2.34 291° 1.84 1.84 2 
Averages.) 0 Ge. 1.68 1.70 1.94 BoA le ees 
Sixteen out of the twenty-four salts become toxic when the 
osmotic pressure ranges between 1.43 and 1.96 atmospheres. The 
average for the cation shows a variation of from 1.72 to 2.00 
atmospheres, and in all except the iron ranges between 1.72 and 
1.79, a difference of only 0.05 of an atmosphere. The anion shows 
a variation from 1.68 to 1.94. The small variation in osmotic 
pressure among the various salts at which they become toxic 
indicates that the osmotic pressure is an important factor in 
determining toxicity. 
We find that when the ammonia produced had been reduced by 
about ro per cent, the average osmotic pressure of the soil solution 
was 2.55 atmospheres; when the ammonia was reduced 25 per cent, 
the osmotic pressure was 5.49 atmospheres; and when ammonia 
