48 
that the plant gets all it requires from the soil. We could, how- 
ever, set the matter at rest by making different kinds of artificial 
soil—as has been done by various experimenters—and trying to 
grow plants in them. We might take pure quartz sand, wash it 
in strong boiling acid, and, having obtained it perfectly clean and 
dry, mix it in different ways with the substances shown in the 
above list ; in one pot, for instance, we might put all the above 
substances, in another we might put all except the humus, in 
another all except the nitrogen, in another all except the oxide of 
iron, and so on all through the list. By such an experiment we 
should discover that a plant obtained from the soil all its foods 
excepé one ; that one exception is the carbon. This substance 
the plant obtains from the air. We burn fuel, and the carbon of 
the fuel goes into the air in the form of gas. Men and beasts, 
when they breathe, exhale the carbon of their bodies in the 
form of gas into the air. Plants absorb this carbon from the 
air; it goes to form wood, and grass, and grain, thus being 
converted into the fuel and food of man, from which the carbon 
again passes into the air, and so again forms the food of 
plants, and thus again and perpetually passes through the same 
cycle. With this solitary exception, however, the plant gets all 
its food from the soil. The greatest bulk of these foods consists, 
as we said, of water ; 72 per cent. of water, removable by drying, 
and 142 per cent. of the elements of water, removable by burning, 
making a total of over 86 per cent. ef water. Considering that 
a plant consists mostly of water, and that this has to be 
obtained from the soil, it will be readily understood how the mag- 
nitude of a crop is strictly limited by the rainfall or by the 
amount of water used in irrigating. Water is the chief food of 
plants, and no amount of manuring can ever make up for lack 
of water. 4 
But the water being provided for and the carbon obtained, as 
we see, from the atmosphere, let us turn our attention to those 
other constituents of the plant which are found on burning to 
remain behind as ash ; and let us consider at the same time the 
nitrogen which disappears on burning. The list is rather a long 
one :—Nitrogen, oxide of iron, lime, magnesia, potash, soda, 
phosphorie acid, sulphuric acid, silica. These things the plant 
obtains from the soil. Now, supposing a soil does not contain all 
these things, or contains some of them only in Very small quantity, 
shall we have to put them into the soil in the| form of manure? 
That depends upon whether the plant needs them all or not. It 
does not follow that, because a plant takes substances into it, those 
substances are essential to the well-being of the\plant ; they may 
be neither helpful nor hurtful to it. We might carry out a 
series of experiments with artificial soils made \by mixing sub- 
stances with pure quartz sand in the manner 
