THE FOOD OF PLANTS. 
65 
warrants us in giving it a first and foremost place. Agriculture 
has long recognised the special value of phosphorus in the 
form of phosphates. This element is present but in small 
quantity in most soils, even the richest ; 0*3 per cent, of total 
phosphates being rather a high percentage, though some soils 
of exceptionally phosphatic origin may contain as much as 
three parts of these compounds in 100 parts of soil. Phos- 
phorus, like the elements previously discussed, is probably 
only assimilated in its most highly oxygenised form, that of 
phosphoric acid or a phosphate. The phosphates existing in a 
soil are those of aluminium, iron, and calcium ; all very nearly 
insoluble in water, one part of the last-named salt, tricalcic 
phosphate, requiring no less than 100,000 parts of pure water 
for solution, though it is soluble to a considerably greater ex- 
tent in the carbonated water existing in a fertile soil. The 
special need for extra supplies of phosphates under our system 
of agriculture is clearly explained, when Ave consider not only 
the comparatively small proportion of these compounds usually 
present in soils, but the steady drain upon these supplies in the 
enormous quantities of phosphates taken off the land in the 
shape of animals reared and sold for food, in milk, and in corn. 
We cannot here discuss how far these losses are replaced in the 
course of legitimate farming, but the temporary removal from 
the soil of such large quantities of a compound therein present 
in but small proportion — a proportion, so far as it is available, 
verging close upon the necessary minimum — this removal is, 
we say, a matter of great interest. This interest in part rests 
upon the distribution of phosphoric acid in the different parts 
of plants and animals, and its remarkable migrations within 
them. Though we cannot here enter upon this subject, we may 
cite one fact concerning the special relation of phosphoric acid 
to the constitution of the seed. For example, we may take the 
wheat plant ; here the ash of the straw contains only 2*75 per 
cent, of phosphoric anhydride, P 20 ^, while the ash of the grain 
contains 46*79 per cent. But if you suppress at an early stage 
the development of the seed, the straw is proportionately 
enriched with this compound. So also with perennial plants ; 
suppression, from accident or influence of season, of the formation 
of seed or fruit, often causes the separation in the tissues of a 
tree of phosphatic crystalline deposits, which have been re- 
cognised in teak and other woods. Other causes may indeed 
conduce to this result, but the reason we have named affords 
generally sufficient explanation. • 
Silicon occurs in the ash as silica and silicates. It is probably 
assimilated in the form of silicic acid, which in some of its 
states is soluble to an adequate extent in water, and is present 
in all soil. The ash of wheat straw contains nearly 62 per cent. 
YOL. VII. — NO. XXVI. F 
