414 
Clover as a Prcparatonj Crop for Wlieat. 
The calculation is not given in proof of a conclusion which 
would be manifestly absurd, but simply as an illustration of the 
enormous quantity, in an acre of soil 6 inches deep, of a con- 
stituent forming the smaller proportions of the whole weight of 
an acre of soil of that limited depth. It shows the existence 
of a practically unlimited amount of the most important mineral 
constituents of plants, and clearly points out the propriety of 
rendering available to plants the natural resources of the soil in 
plant-food ; to draw, in fact, up the mineral wealth of the soil 
by thoroughly working the land, and not leaving it unutilised as 
so much dead capital. 
The exact determination of phosphoric acid in a soil, it may- 
be observed in passing, is attended with no difficulty, if certain 
precautions, which it is feared are sometimes neglected by 
chemists, be taken. I will, therefore, give a brief outline of the 
plan — commonly known to chemists as the Molybdic acid plan 
of determining phosphoric acid — which yields accurate results. 
Not less than 100 grains, or better 200 grains, of the dried 
and finely-powdered soil are digested for an hour or thereabouts 
with 3 or 4 ounces of moderately strong nitric acid. The acid 
solution is then passed through a filter, and together with the 
washings from the insoluble portion of the soil left on the filter 
is evaporated to a small bulk ; thus getting rid of the greater 
part of the acid employed for effecting the solution. During 
evaporation a large excess of molybdate of ammonia is added 
to the solution, care being taken to keep it strongly acid. 
If there be much phosphoric acid in the soil, a bright yellow 
precipitate, consisting of molybdic and phosphoric acid, makes 
its appearance at once ; if traces only be present, the yellow 
precipitate appears only on the concentration of the liquid, after 
the great excess of nitric acid has been expelled by evapo- 
ration. The yellow precipitate containing the whole of the 
phosphoric acid present in the soil, molybdic acid, together with 
a little silica, and frequently some oxide of iron, is thrown on a 
filter and washed with a solution of molybdate of ammonia ren- 
dered strongly acid by nitric acid, until a drop of the washings 
passing through the filter ceases to show a reaction of iron with 
yellow prussiate of potash solution. It is then dissolved on the 
filter in an excess of ammonia, and the ammoniacal liquid preci- 
pitated with an ammoniacal solution of sulphate of magnesia, 
which throws down the phosphoric acid as phosphate of mag- 
nesia and ammonia. After standing at rest for about 12 hours, 
the magnesia precipitate is collected on a small filter and washed 
clean with strong ammonia-water. Together with the phosphoric 
acid, traces of silica, and generally also traces of oxide of iron, 
are thrown down with the magnesia precipitate. In order to 
separate these impurities the precipitate is dissolved in a few 
