1922] GREAVES—SOIL 163 
The soil used was a calcareous loam (8), rich in potassium, 
phosphorus, and all the essential elements except nitrogen, which 
was low, as was also the organic matter of the soil. The results 
as reported are the average of a great number of determinations, 
and represent rather accurately the toxic point of the various 
salts in this specific soil, hence the results represent the relative 
toxicity for several salts in this specific medium. This may or 
may not vary in a different medium. The results, therefore, are 
not to be taken as absolute but as relative, which can only justly 
be comparable with other results obtained under like conditions. 
It is quite evident from these results that the toxicity of a 
compound is governed by both anion and cation. Without 
exception the chlorides are more toxic than the corresponding 
nitrates. The sulphate varies, depending upon the cation with 
which it is combined, whereas in every instance the carbonate is 
less toxic than any of the other salts. The relative toxicity for 
the anions, therefore, can be written in the order Cl>NO,>SO,> 
CO,, thus indicating that the monovalent anions are more toxic 
than the divalent anions. It must be borne in mind, however, 
that there would be more anions in unit volume of the monovalent 
than of the divalent, where a divalent cation is combined with the 
monovalent anion, as these salts were tested on the basis of mole 
concentrations. When re-examined with equivalent ionic concen- 
tration, the difference, although small, still maintained the same 
order. 
Excluding the difficultly soluble salts, and averaging the results 
for the cation, we obtain the series Mn >Mg>Fe>Ca>Na>K, 
or the divalent cation is found to be more toxic than the monovalent. 
Here also the number of ions would enter, and if the toxicity be 
due either to an osmotic or precipitant reaction the order is what 
would be expected. 
Examining the reais obtained with nitrification (13) on the 
same basis as has been done with ammonification, we obtain as the 
points of toxicity the results given in table IT. 
The most important conclusion to be drawn from these results 
as compared with the preceding is the greater sensitiveness of the 
nitrifying organisms to the different salts as compared with the 
