54 
Absorption of Phosphate of Lime ; and 
asrent which produces this change. The reconversion of soluble 
into insoluble phosphate perhaps mav appear undesirable, but 
in reality it is not onlv beneficial but absolutely necessary to the 
healthy and luxuriant development both of turnips and of 
all other crops to which superphosphate is applied. Xo acid 
combination, as such, can enter into plants Avithout doing them 
serious damage ; even free vegetable acids, such as humic and 
ulmic acids, are injurious to all crops cultivated for food for the 
use of man or beast ; and unless these acids, which are alwavs 
present in what practical men call sour humus, are neutralized 
by lime, or marl, or earth, none but the roughest and most innu- 
tritions herbage can be grown. Earth of almost any description 
is a universal neutralizer of acids ; and for this reason anv kind 
of earth mav be used with more or less advantage for improving 
peaty land. If the earth or soil is rich in lime, or contains an 
appreciable quantity of it, so much the better ; but even should 
it be destitute of lime, or nearly so, still any earth, except a pure 
white sand, will be of considerable use in improving peat-land 
of any description by neutralizing injurious acids, apart from its 
mechanical tendency to consolidate loose or spongv soil. 
Free mineral acids are, I believe, still more injurious to all 
farm-crops, and perhaps to all plants, than the free organic acids 
which are frequently found in humus. A very dilute solution 
of sulphuric acid — say 1 part in 1000 of water — may be used 
with advantage for killing {jrass in gravel-walks made with 
flint or quartzose sand ; after one or two applications, the weeds 
will be destroved, and vriW not reappear lor a long time. But 
if the walks are made with limestone gravel, the application of a 
much stronger acid has little or no effect on the grass or weeds ; 
after some time the latter indeed seem to grow all the better for 
having had a taste of dilute sulphuric acid. In reality, however, 
no acid enters the plant, but on coming in contact with the 
limestone gravel unites with the lime to form that useful fertiliser 
sulphate of lime or gypsum. Flints and pure quartz-sand, on 
the other hand, contain hardly anything else but silica, which, 
in a chemical point of view, is an acid, and therefore cannot 
neutralise another acid. Dilute sulphuric acid, therefore, re- 
mains in a free state in tlie flint or quartz-gravel ; and in the 
measure in which it is absorbed bv the roots of the grass or 
other weeds destroys their vitality. These examples thus prove 
unmistakeably that a soil which contains free acids in ever so 
small a quantity is unfit to maintain a healthy growth. We 
liave, therefore, strong presumptive evidence that soluble phos- 
phate, a combination which has a stronglv acid character, does not, 
as such, enter the roots of plants, for we know that its application 
