28 
the absorptive power of soils, but they also show considerable selective 
power in the absorption of mineral constituents. Soils high in 
humus have a high fixing power, due to the ability of humus to 
combine chemically with minerals, as well as its power of absorption, 
and therefore the effect of heat upon highly organic soils should tend 
to increase the solubility of the minerals. An example of this is given 
in the cases of soils Nos. 74 and 428. Another factor is that of 
precipitation following extraction, being the more marked in the 
acid extract due to a more complete extraction. 
In passing from 250° C. to ignition the effects are apparently of a 
specific rather than general nature, as has been already indicated. 
Among these effects are the volatilization of certain sulphur com- 
pounds, conversion of bicarbonates into normal carbonates, de- 
hydration of silicates, etc., replacing of potash by lime, and other 
chemical transformations. In addition there is produced a greater 
aggregation of the soil particles, resulting in a decrease in surface 
area exposed to the solvent and an accompanying change in the 
fixing and absorbing powers of the soil. It is possible, by application 
of these conceptions, to explain the majority of changes, both increase 
and decrease in solubility, resulting from ignition. 
THE EFFECTS OF HEAT ON SOIL NITROGEN. 
INTRODUCTION. 
The data presented in the preceding pages indicate the existence 
of colloidal films surrounding the soil particles. These films are 
probably both organic and inorganic in nature and undergo altera- 
tion under the influence of heat. By such alteration new surfaces 
become exposed to the action of solvents, thereby making possible 
the solution of materials otherwise effectively protected from the 
solvents used. There is considerable evidence in the data, however, 
that other changes were also produced by the heating. Some oxida- 
tions must have taken place, and probably decompositions of other 
types. Changes in the organic matter were produced at the higher 
temperatures, as shown by the color of the water extracts. 
Theeffectof heat on soil organic matter has been the subjectof some 
previous investigation. It has been observed, for example, that 
water extracts from heated soils are usually darker in color and con- 
tain greater amounts of organic matter than similar extracts from 
unheated soUs. 
Darbishire and Russell l found that plants absorb more nitrogen 
from soils that have been previously heated to 95° and 120° C. than 
from unheated soils. They concluded that the heating brought about 
some decompositions in the organic matter and also caused a modi- 
fication in the bacterial flora. 
i Jour. Agr. Sci., 2 (1907), p. 305. 
