368 
Supplement to the "Tropical Agriculturist." Nov. 1, 1899. 
the plant. By other similar experiments he showed 
that the fcUowiug quantities of mineral substances 
•were necessary for the production of 100 parts of 
oat-plant : — 
Phosphoric acid ... ... "50 parts 
Potash ... ... -80 „ 
Lime ... ... '25 „ 
Magnesia ... ... '20 „ 
Sulphuric ... ... -20 „ 
1"96 parts 
A total of 1*95 parts of mineral substances is 
therefore necessary in the case of the cat-plant. 
However, there is no oat-plant in nature whicJi 
contains so little as 1"85 per cent. The minimum 
is 3 per cent- The difference, 1'05 per cent, is the 
measure of the "mineral-hunger"' of the plant, 
and represents the mineral substances which does 
not perform any special function. This excess of 
mineral substance may be supplied in the form of 
some indifferent substance, such as silica. The 
observation is of considerable interest to the 
farmer, for it shows that it is not economical lo 
manure crops with pure substances. 
Having ascertained in general what substances 
are necessary as plant-food, the agricultural 
chemist has next to apply this general information 
to the manuring of soils which are more or less 
deficient in certain ingredients. It has been found, 
unfortunately, that the chemical analysis of a soil 
is of little use as a guide unless accompanied by 
^hat may be termed a " mechanical analysis," 
by which he meant chiefly a determination of the 
amount of finely-divided constituents present in 
the soil. It is only the finally divided earlli which 
presents a sufficiently large surface for the exer- 
cise of the solvent aclioii of the water and its 
dissolved carbonic acid. There is one case, however, 
iii which chemical analysis alone is of the greatest 
importance, viz , when only truces of some neces- 
sary element are present in a soil, Here there is 
no qustion of tlie need for a manure containing 
t]p.s substance. 
'■'If, on the other hand, large quantities of an 
element are preserit, it does r.ot follow that there 
is a sufficiency in the soil even when the latter is 
in a satisfactory state of division, for the sub- 
stance in question may be present in an insoluble 
or refractory form. This is commonly the case 
•with nitrogen, which exists in the soil chiefly 
in the form of a mixture of indefinite nitrogenous 
substances known as humus or mould. These 
substances sometimes easily give up their nitrogen 
to plants, but in other cases are very refractory. 
The uncertainty as to their action is indeed so 
great that certain peaty soils are known which 
consist almost entirely of humus, but contain, 
nevertheless, an insufliciency of available nitrogen. 
■ Phosplioric acid affords another illustration. 
The soluble phosplioric acid of tlie monureis ab- 
sorbed by the soil as dicalcic pliosphate, which is 
comparatively easily soluble in the soil-water. 
With time, however, it may change in the soil to the 
insoluble tricalcium phosphate, or even to iron or 
aluminium phosphates, which are still less soluble. 
In the case of calcium, chemical analysis has 
been found to lie of considerable .service in deter- 
mining what manuring is required, since calcium 
is chiefly valuable in the form of carbonate or 
humate, and these are easily estimated in the soil. 
Since then the direct method of soil-analysis is 
an insufficient guide to manuring, it is fortunate 
that chemists have been able to develop success- 
fully an indirect method. This is the cultivation 
metliod, by which plants are allowed to grow in 
the soil under examination, after taking care to 
provide a suflicieucy of all plant-food stuffs 
except the one, e.r/., phosphoric acid, whose 
presence in available form is being tested. The 
plants are tlien analj-sed, and the results compared 
with the analyses of the same plants grown on 
soils provided with all the necessary plunt-food 
stuffs. As an inportant result of the method it 
has been found that different plants take up very 
different quantities of the same mineral substan- 
ces. On this is largely based tlie system of 
rotation of crops, where the second crop is 60 
chosen that it chiefly removes the ingredients of 
the soil which have been left by the preceding 
crop. 
Witli the aid of the cultivation method ii has 
also been possible to draw up the following table 
which represents the relative values of the 
different nitrogen compounds for plant food. 
Nitrogen of Saltpetre ... 100 
„ „ Ammonia ... ... 85-90 
,, „ Albumen ... ... 60 
Tliis table may be made use of in determining^ the 
nitrogen value of a manure. 
The cultivation method may be used for testing 
the value of manures of all kinds. Thus it was by 
a few cultivation experiments that Wagner in 
1) irmstadt first showed the very great value for 
agricultural purposes of tlie "Thomas" Slag, 
produced as a bye-product iu the manufacture of 
iron by the basic process of Thomas Gilchrist. 
The million t«.ns of phosphate meal annually 
produced in Germany is now wholly utilised by 
the agriculturist, and its preparation for the 
farmer has become an important offshoot of the 
industry. 
Similarly the demonstration by the cultiva- 
tion method of the value of potash salts in 
manures has given an enormous impetus to the 
potash industry. 
Speaking generally, the method gives us 
complete control over the fertility of a soil in so 
far as this depends on manuring. One consequence 
of this has been that our views as to the value of 
agricultural land have completely changed, for 
whereas formerly sandy soils were generally con- 
sidered poor, they are now by means of a system of 
intelligently directed manuring, made to give yields 
which are scarcely inferior to those of the best 
soils. The beet-sugar industry, which formerly 
could only be conducted in the best soils, has now 
boen extended wit h marked success to sandy soils. 
AGRICULTURAL EDUCATION IN FRANCE. 
The .system of agricultural education in France 
is reported to be most perfect, and just at thia 
time when a Commission is sitting with reference 
to the question cf establishing a Department or 
Board of Agriculture for the island, an account of 
the Frer.ch system of agricultural education should 
prove both interesting and instructive reading. 
