158 
ARE SOILS ENRICHED, IMPOVERISHED, [Part III. 
The food of 
plants is ex¬ 
tracted from the 
soil. But if 
the plants are 
returned to the 
soil, no impo¬ 
verishment 
takes place. 
PART III. 
ARE SOILS ENRICHED, IMPOVERISHED, OR POI¬ 
SONED BY VEGETABLE GROWTH ? THESE QUES¬ 
TIONS INCLUDE EXCRETION FROM ROOTS; SO¬ 
CIABILITY OF PLANTS; ACCUMULATION OF SOIL 
IN WOODS; GENERAL DENUDATION OF SOIL FROM 
WASH OF RAIN. 
I think that the food of plants is absorbed from 
the soil, not from the atmosphere; but that, if 
the remains of dead plants are restored to 'the 
soil from which they grew, owing to vegetable 
chemistry, independently of disintegration of rock, 
soils would become enriched, not impoverished. 
The two great causes of impoverishment of soils 
are, abstraction of vegetable crops by man or 
animals, and aqueous denudation, that is, the 
wash of rain. The food of plants is of two sorts, 
the organic or combustible , that part which can 
be consumed in burning; and the inorganic or 
incombustible , that part which remains as ashes 
after burning. Both parts are, in my opinion, 
absorbed by the roots from the soil; at least, 
what is absorbed as food from the atmosphere 
may be reckoned as nothing in comparison to 
what is absorbed by the roots from the soil. In 
reference to the combustible constituents of the 
food of plants, Liebig tells us that the presence 
