Fliosphatic Manures for Root-Crops. 
57 
manency, however, is no recommendation whatever ; for the 
primary use of all manures is to enable us to grow, not scanty, 
but heavy crops — not to deposit on the land fertilisers which 
may last for three or four years, but by prompt, efficacious 
action to render a quickly remunerative return from a moderate 
outlay. 
The advantage of bone-dust over superphosphate, as far as the 
phosphates in both are concerned, is only apparent, not real ; 
for the soluble phosphates in the latter, as has been shown, are 
retained in the soil in an insoluble condition, in which they 
cannot be washed out so readily as is commonly believed. 
The question may be asked : If soluble phosphate is rendered 
insoluble in coming into contact with the soil, which is really 
the case, why incur all the expense and trouble of treating bone- 
dust and similar phosphatic materials with oil of vitriol? In 
answering this question, the following explanation may be given. 
The treatment of bone-dust, bone-ash, and other phosphatic 
materials with oil of vitriol results, in the first place, in the 
more or less complete disintegration of their structure, and in the 
next, in the production of soluble or acid phosphate, which, 
when neutralised, as we have seen, by contact either with lime 
or with oxides of iron or alumina in the soil, in either case is 
reconverted into an insoluble phosphate in a highly divided 
condition ; and as this change takes place in the soil itself, a 
most intimate and uniform incorporation of the phosphates 
with the soil is effected. However finely ground bone-dust 
may be, the division, being effected by purely mechanical 
means, necessarily must leave the phosphates in a state which 
may be called extremely coarse in comparison with that resulting 
from their solution in acid. 
Precipitated phosphates not only are greatly more bulky than 
the finest powder obtained by mechanical means, but are also 
more soluble in water than merely powdered phosphate of lime. 
Though identical in composition, tiae fine state of subdivision 
of the particles which constitute precipitated phosphate of lime 
imbues the latter with properties that do not belong to bone- 
powder. Thus, for instance, weak vinegar readily redissolves 
precipitated bone-phosphates, but has hardly any effect upon 
even fine bone-dust. 
The whole secret of the energetic action of superphosphate 
thus depends upon the production of most minutely subdivided 
or precipitated insoluble phosphates within the soil itself, not, 
as is erroneously supposed, on the direct absorption of soluble 
phosphates by plants ; and it is not desirable to efiect the pre- 
cipitation before the manure is put on the land, for by so doing 
