7-5 
stituents available for nutrition. The substances which are thus made 
available for the plant are retained by fertile soils,! and thus their solu- 
tion by rain, &c. ensured for at least a sufficient time for the whole of 
the soil to become completely saturated by them. It is, therefore, 
essential to good agriculture that the crop of the alkaline plant 
should be followed by one of another kind, say a siliceous plant; and . 
thus a well-chosen succession of different crops will remove from the 
‘soil one substance after another. This vofatzon must be continued 
until the substances first withdrawn are restored in such proportion and 
distribution that the same series of crops may again be grown. In 
former times the influence of the atmosphere on the soil was utilised 
by cultivating only a portion of the*soil, the rest being left untilled or 
fallow; but it is now acknowledged that this mode of agriculture is not 
economical. In those districts where it pays to cultivate large tracts of 
land, no considerable portion is now left fallow; but the object is, by 
a good rotation of crops, to use up as equally as possible all the con- 
stituents of the soil. Even under the best system of agriculture, how- 
ever, the soil must become gradually exhausted, and this exhaustion 
must be artificially counteracted. This purpose is effected by a 
scientific system of manuring, by which those substances are supplied 
to the soil of which it has most need. According to circumstances the 
most various organic and inorganic substances may be employed as_ 
manures. If, for example, there is a deficiency of nitrogenous sub- 
stances, an addition of nitrates or guano is made. We cannot here 
enter further on the physical changes in the constitution of the soil, 
often so advantageous or even indispensable to vegetation, caused by — 
the rotation of crops and by manuring. In Sicily and Asia Minor, 
once ‘the granary of Rome,’ in Campania and Spain, at one time so 
fruitful, we have the most sad and instructive examples of the exhaus- — 
tion of the soil. Care must on the other hand be taken not to impart 
to the soil such substances as are injurious to vegetation. Manuring 
with chlorides such as sodium chloride (common salt), for example, 
while increasing the weight of root-crops, has a decidedly injurious 
effect on their quality. 
i The Life of thé Plant. , 
The nutrient substances taken up by plants are, how- 
ever, by no means adapted, in the raw state, to take part in 
the construction of any vegetable structures ; they must, on _ 
: It has not yet been ascertained whether this is brought about by 
attraction, or by decomposition and transformation into other com- 
pounds, 
