282 Mineral Manures and Manuring. 
ammoni<a, but find both close together. By the use of bone-meal 
we can give to the plant more absorbable phosphate than with 
guano, lor in guano the entire nitrogen-compounds come into 
play during the first crop ; but in bone-meal the decomposition 
of the gelatine and the resulting solution of the phosphates 
proceeds more slowly. Consequently, bone-meal operates upon 
the after-crop, especially in light soils, so that the total yield 
obtained by its use may even prove greater than the primarily 
larger crop yielded from the guano. 
When, however, the decomposition of the gelatine in bones 
is completed, the phosphates which remain undecomposed are 
less absorbable by the plants than are precipitated phosphates. 
Consequently the partial solution of steamed bone-meal with 
half the quantity of sulphuric acid generally employed, or its 
admixture with superphosphate, is far more rational than its 
application alone. This is especially true with regard to 
turnips, and accounts for the rapidly spreading custom of 
applying dissolved guano to them. In heavy clay soils rich in 
potash, superphosphates free from nitrogen, such as may be 
made from Baker Island guano or pure mineral phosphates, fre- 
quently give a better yield of turnips than guano and bone-meal. 
The considerable proportion of potash in these soils facilitates 
the action of the phosphates. The influence of the superphos- 
phate is not due to the absorption by the plant of a dilute 
solution of superphosphate, but to the changes which take place 
in the soil. 
The action of the soil upon superphosphate and the changes 
which the latter undergoes therein were first shown by Voelcker 
in 1863, and his exhaustive experiments practically brought the 
question to a conclusion. Like all his experiments, though 
essentially scientific, they show his thorough comprehension of 
their practical bearing, and thus enable not merely theoretical 
but practical results to be drawn from them. 
Simultaneously with the actual solution of the superphosphate 
in the water of the soil, its phosphoric acid becomes again pre- 
cipitated upon the particles of lime, alumina, and oxide of iron 
which surround it. Thus it becomes infinitely divided in the 
form of an impalpable powder, compared with which the finest 
powder obtainable by mechanical means is like a coarse meal, 
Ever}'^ kind of soil, without exception, is capable of precipi- 
tating the phosphoric acid from a solution of superphosphate 
within 24 hours. 
This was proved by Dr. Voelcker by the following experi- 
ments : — 
To 12 oz. of soil were added 109*24 grains of superphosphate 
