FOOD OF PLANTS, AND HOW THEY TAKE IT. 165 
holding in solution ammonia, potash, phosphoric and silicic 
acids, be brought in contact with the sod, these substances 
disappear almost immediately from the solution; the soil 
draws them from the water. Only such substances are com¬ 
pletely withdrawn by the soil as are indispensable articles of 
food for plants; all others remain wholly or in part in solu¬ 
tion. If a funnel be filled with soil, and a dilute solution of 
silicate of potash be poured upon it, there will not be found 
in the filtered water a trace of potash, and, only under cer¬ 
tain circumstances, silicic acid. If freshly precipitated phos¬ 
phate of lime or phosphate of magnesia be dissolved in water 
saturated with carbonic acid, and filtered in like manner 
through soil, there will not be found a trace of phosphoric 
acid in the filtered water. A solution of phosphate of lime 
in dilute sulphuric acid, or of phosphate of magnesia and 
ammonia in carbonic acid water, composts itself in the same 
manner. The phosphoric acid in each case remains in the 
soil. Charcoal reacts in a similar manner with many soluble 
salts ; in this case, it is a chemical attraction acting from its 
surface, but in the soil its constituents take part in the 
action.” This remarkable property is not found to exist when 
solutions of substances are used which do not furnish food to 
plants ; it is in their case especially that this wonderful pro¬ 
perty exists in the soil. 
The value of these interesting! facts is immense. As a 
purifying agent, there is nothing better nor so cheap as 
clay, which possesses this power in a high degree. In the 
collection of liquid manure in tanks, the decomposition and 
escape of ammoniacal vapours may be wholly arrested by 
using clay as an absorbent; and, until the soil becomes 
saturated with these substances, the liquid that passes 
through is found to be devoid of smell; all the rich plant- 
food has been left in the compost. When one change of 
soil has become saturated, another should be introduced. 
Liquid manures, especially when largely diluted with 
water, are found to be highly serviceable; certain elements 
of the food of plants are thus carried to the soil; other 
matters equally needed by vegetable organizations are not 
soluble, and cannot be thus conveyed. The impression 
which has prevailed, that all plant-food must be reduced to 
a liquid form to enter the little root-mouths ( spongioles) is 
perhaps erroneous. Water is needed as food, and is absorbed 
as water; other substances, soluble in this menstruum, are 
conveyed into the organism with it; but Liebig contends 
that the roots have the power of taking in these matters 
without their previous solution. He says that “We have 
