THE FOOD OF PLANTS. 
67 
compounds, the application of these salts usually causes a very 
moderate addition, if any, to the produce of the land. A good 
fertile soil has been known to contain only *09 per cent, of 
potassic oxide, and yet to receive no benefit from further sup- 
plies of potassic compounds. But that this particular soil was 
becoming poorer and poorer in potassic oxide was seen by the 
analysis of the subsoil at a depth of 18 inches, which was found 
to contain *29 per cent, of this compound. All good soils have 
however a singular power of retaining potassium compounds — a 
power which they do not possess to an equal extent with regard 
to the corresponding sodium salts, which are required in much 
smaller proportion by most plants. This retentive power for 
potassic compounds is shown by the relatively greater propor- 
tion of sodic salts carried off from cultivated land in the drain- 
age water, even where the soil contains less of this sodic salts 
than of the potassic. Cultivated plants have been observed to 
contain more potassium than the same species when wild. The 
ash of wild asparagus, for instance, contains only 18*8 per cent, 
of potassic oxide, but 50*5 per cent, is found in the ash of the 
same plant when in a high state of cultivation and develop- 
ment. The preference which most plants have for potassium 
over sodium is illustrated by a study of the ash analyses of 
plants grown in soil almost destitute of the former metal and 
containing abundance of the latter; the potassium in these 
plants, spite of its small proportion in such soils, greatly out- 
weighs the sodium. Even the sea, which contains 30 times as 
much sodium as potassium, furnishes to some of the algje 
drawing all their mineral matter from it equal quantities of 
these two metals, and to many others half as much potassium 
as sodium. Experiments on land plants have shown that sodium 
can replace potassium only in some cases, and then usually only 
to a very limited extent. 
Calcium occurs in the soil as carbonate, sulphate, and phos- 
phate. Calcic carbonate and phosphate are very slightly soluble 
in pure water, but the carbonic acid water of the soil dissolves 
them both very readily. Calcic sulphate only requires about 
430 parts of water for solution. Magnesium occurs in similar 
salts to those of calcium, and both metals are also sometimes 
present as silicates. Calcium occurs in small quantity in seeds, 
in large quantity in many stems and straws. Ash of wheat 
grain contains but T15 per cent, of lime, w^heat straw 7*42 per 
cent.; ash of peas contains 4*78 per cent, of lime, the ash of 
pea straw, 37*17 per cent. Magnesia is present to the extent 
of 13*39 per cent, in the ash of wheat grain. 
Iron and manganese may be considered together. Both metals 
are probably present in the majority of plants, and when there 
is abundance of iron then the manganese generally exists also 
