n 
ELEMENTS OF AGRICULTURE. 
these soils, being generally too compact, and consequently 
too hard, require the application of substances tending to 
lighten them; and sand, as we know, possesses this prop¬ 
erty in a high degree, when it enters in a sensible propor¬ 
tion into the composition of a soiJ. But to produce this 
effect, it must be combined with the other elements of the 
soil; for where it is merely mixed with the soil, it does not 
loosen it. 
48. In fact, the experiments that have been made with 
sand prove that it has a constant tendency to descend 
through the arable surface, and that it reaches the sub-soil 
without having acted. It does not enter into the mole¬ 
cules of clay to form but a single body with them ; it does 
not then prevent these molecules from remaining agglom¬ 
erated among themselves ; and consequently it loosens very 
little, if any, stiff clay soils. 
49. But suppose even that the mixture of sand and clay 
was advantageous, this means of amelioration would be 
impracticable, on account of the immense quantity of sand 
required, and that it would be necessary often to renew, 
from its tendency to sink to the sub-soil. 
50. It is only where a clay soil is based upon a sandy 
sub-soil, that it would be advisable to seek to mix the last 
with the first, by means of deep ploughing ; and this 
should be done gradually, by setting the plough deeper 
each time the field is worked. The increase in depth 
should be gradual, in order to give the mineral parts of 
the soil and sub-soil time to adhere and combine, as it 
were, among themselves. 
51. Another means of ameliorating clay soils is to sub¬ 
mit the superficial crust of the arable layer to the action 
of fire. Particles of the clay are hardened by the heat, 
and produce the same effect as gravel and sand. But this 
method of loosening the land is liable to the same objec¬ 
tion as that by means of sand ; the calcined particles being 
