Causes of Fertility or Barrenness of Soils. 183 
the particles, and also to the adhesion of the soil to the imple- 
ment, which differs in different soils — clays of course most — 27 lbs. 
of pure ( hay adherin"^ to one square foot of iron ; 29 lbs. to the 
same surface of wood. This quality of coliesiveness in clay soils is 
of great importance, for although chemical forces may have much 
to clo with fertility, yet, independently of such sources, it is owing 
to the cohesive powers and state of minute division of particles 
that we justly consider clay as by far the most important substance 
found in soils. As has been shown, these properties may be in 
excess, rendering the mass so compact and wet that neither air 
nor warmth can penetrate ; healthy vegetation is then impossible, 
for if the germinating power be not suspended, growth is checked 
by starvation. But even these disadvantages are preferable to 
the opposite qualities of excessive porousness and looseness, inas- 
much as we can remedy the one by artificial means, such as 
drainage and cultivation, but cannot improve the other without 
the addition of foreign matters, always a most expensive process. 
As all plants feed by roots as well as leaves, and can only receive 
food in solution, the presence of a certain amount of moisture is 
indispensable. When, therefore, we find clay united with such 
substances as have an opposite tendency, and which prevent the 
excessive influence of those qualities, we should naturally expect 
fertile soils ; and such is pre-eminently the case from the union of 
clay and sand, or clay and lime. I'he distinctive character of 
sand being its porosity and inability either to absorb or retain 
moisture, when mixed with clay it tends to qualify the latter, and 
without injuring its useful powers, keeps the soil sufficiently open, 
allowing the water gradually to pass off, and thus, instead of the 
root being constantly surrounded by stagnant water and perishing 
from cold, it is supplied with a refreshing draught constantly re- 
newed, equalizing the temperature, &c. ; the soil retains its natural 
heat, which would otherwise be absorbed by evaporation, and the 
atmosphere is enabled to circulate and the sun's ravs to penetrate, 
revivifying the soil and causing those wonderful changes which 
are as necessary to the health of the plant as pure air is to an 
animal. Lime acts much in tlie same way as sand, being of a 
very porous character ; but possessing considerable absorptive 
powers, does not so thoroughly correct the retentive character of 
the clay. The cohesive property is of the first importance in 
estimating the fertility of soils by physical character. The porous 
property is also highly beneficial when acting as a check or 
diluent upon the former quality, but injurious when alone ; its 
action is rather negative than positive. Soils, porous without 
being retentive or absorbent, are barren, or liable to become so 
from drought. They possess warmth and admit the air very 
freely — important qualities ; but we might as reasonably expect 
