148 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 
would be generally advantageous, because the assimilable portions only 
represent about half the total quantity, itself already small : — 
Phosphoric Acid Phosphoric Acid 
Soils. soluble in soluble in 
aqua regia. acetic acid. 
Maurepas Peat- mould . . . 0-G4 per 1,000 0-82 per 1,000 
Rambouillet Leaf-mould . . 1*16 „ 0-40 ,, 
Maurepas Leaf-mould . . . 1-30 „ O'lO ,, 
Ghent Leaf-mould . . . 1*60 „ O-oO 
Trap Loam .... 0-62 „ 0-36 
Hotbed Soil . . . > . 4-84 „ 3-28 
I consider that a soil is sufficiently well provided with phosphoric 
acid when it contains at least O'GOO gr. per kilo, of phosphoric acid soluble 
in acetic acid. 
Sulphuric Acid. — Sulphur is an aliment absolutely indispensable to 
plant life : its importance is much greater than it is usually considered to 
be. Frequently the beneficial action of the superphosphates is erroneously 
attributed to the phosphoric acid, whilst often it is the free sulphuric 
acid and the sulphate of lime of the superphosphates which act as useful 
complementary aliments. Sulphur exists in the soil principally in the 
form of sulphates, sulphate of lime, sulphate of potash, and sometimes of 
magnesia. In some cases an excess of sulphuric acid is met with 
which may sometimes even become dangerous : this acid is formed by 
the oxidisation of sulphuret of iron in the form of pyrites brought to the 
surface of the soil by deep working. Exposed to the air this pyrite 
becomes oxidised, and gives rise to sulphate of iron, which, if very 
abundant, may become harmful. 
Sulphur is also found in the soil in the form of organic compounds 
of an altogether special character. In a soil analysed by M. Berthelot 
which contained 2-356 gr. of nitrogen per kilo., thus approaching the 
nature of horticultural soil, he found a total of 0*352 gr. of sulphur, 
including 0-169 gr. of sulphur attributable to sulphates. The relation 
between the organic combinations of sulphur and its mineral combina- 
tions is therefore very near to equality. In another case analysed the 
soil was four times as rich in organic sulphur as in sulphur derived 
from sulphates. M. Berthelot has remarked that this question is one of 
the most important from the physiological point of view. The albu- 
minoid principles contained sulphur as well as nitrogen. The proportion 
between the weights of these two elements, which is of the weight of 
nitrogen for the albumen, is for the horny matters. It is curious to 
remark that in the last case cited by M. Berthelot the increase in the 
sulphur contents is considerable. It is a question in this case of a forma- 
tion of sulphur compounds of an altogether special character, perhaps 
comparable to acids, conjoined with sulphur (acides sulfoconjugucs). This 
quite recent discovery will probably lead to a great advance in the know- 
ledge of the physiological properties of sulphur. It has been remarked that 
the difference between the quantities of sulphur which are found under 
the two forms diminishes as we pass from common to hotbed soil, and 
thence to the plant itself. 
Silica. — Silica, of all the mineral compounds, is that which is found 
most abundantly distributed in the soil, either in the form of pure silica 
or in that of silicates. In the tissues of plants there are found quantities 
