Oct. 1, 1898.] 
Supplement to ike '* Tropical AgricUUurist.^'' 
297 
in earlier days produced no effect, now siifliced to 
treble the yield of wliesit. On analysis by menus 
of ncetic ncid he found that tlie^e phosphate- 
exhnustedscil.s now yielded only insignificant traces 
of phosphoric ncid, and, although formerly, when 
they were independent of phospliatic manure, 
they yielded ns much ns -03 per cent of phosphoric 
acid joluble in acetic ncid. 
A. Vogel in 1882 made a .similar .«uggeftion with 
regard to ncetic acid, and in 189i Dugast pnblisho 1 
the results of an investigation into the compositio-u 
of three soils of different degrees of f.;rtility, on 
which he investigated the solvent action of ncetic 
ncid, ammonium o.xalate, ammonium citrate, dis- 
tilled water, and water saturated with c.irbonic 
ncid. In the .same year G. Lechartier suggo.-ted 
the use of a 2 per cent solution of ammonium 
oxalate ns a means of gauging the available 
mineral contents of the soil. 
Ill 1889 Eggertz and Xilson extracted a .soil with 
a 2 per cent solution of hydrochloric acid, and 
after wa.shing it, found that the available plant- 
food had been so far removed from it that barh y 
would not grow in it, while, when the exhaustion 
was carried out with acid of double this strength, 
it was sterile to oats. 
Wiklund in 189:J published some work in the 
iime direction, ciiiicising the work of the last men- 
tioned authors, but dealing chiefly with tlie dis- 
tinction between inorganic and organic phosphorus 
and sulphur in the soil. 
Various papers by Berlhelot and Andre appeared 
in the " Comptes Kendus" on the subject of tlie 
condition of the mineral ingredients of tlie soil, 
without, however, suggesting any satisfactory 
conclusion as to the best mode of distinguishing 
between available and non-available plant-food. 
Petermann had long used alkaline ammonium 
citrate tor the purpose, and Schloosing had sug- 
gested the use of dilute mineral acids only just 
strong enough to leave an acid reaction alter 
neutralising the bases of the soil. 
From this brief summary, which, though imper- 
fect, illustrates sufficiently the general position, 
it will be seen that the difficulty which was early 
recognised in soil analysis has still persisted, 
although most chemists seem to have recognised 
that some very much weaker solvent than strong 
mineral acid must be used if the analyses were to 
indicate th.e pi-oportion of actually availabie plant- 
food, The difficulty felt throughout, however, was 
that of arriving at any definite standard of weak- 
ness or strength in the solvent to be used. All 
that was clearly recognised was that it should be 
Tery much weaker than the acids used for deter- 
mining the total ingredients of the soil. 
The attention of agricultural analysts had been 
for some years engaged in a matter of more 
immediately pressing interest — namely, the deter- 
mination of the degree of availability of the con- 
stituents of manures. It was no doubt interesting 
to determine how much available phosphoric acid 
might exist in a soil ; hut the rent of land is not 
lixod by the proportion of phosphoric acid that it 
contains, while tlie price ot phosphatic manure is. 
It was vecogni.sed ns essentially important for the 
farmer to know whether the whole — or at any rate 
ft fair jiroiiortion — of the phosphates which he 
might purchase in a manure existed in a condition 
iu wliicl' lie might rousouubly e.xpect them to be 
available for early utilisation by plants. In th<^ 
case of manures in which tlie phosphates were 
soluble in water, no serious difficultly arose. The 
difficult cases were those in which tlie phosphate? 
were not actually soluble in water, althougli 
ex])evience showed that, to a larger or smaller 
extent, as the case might be, they were manu- 
rinlly available. Various processes were devised, 
and nre still in vogue — chiefly in the various 
Continental countries and in America — for deter- 
mining what is called "available" phosphate in 
manures. jMost of these processes take tlie form 
of ascertaining the proportion of phosphate 
soluble, under certain arbitrary conditions, iti 
solutions of ammonium cicrateof various arbitrary 
strengths, containing in some cases an excess of 
free ammonia, and in other cases an excess of free 
citric acid. The varying results obtained by the 
different processes are, it may be observed, constant 
sources of annoyance to manufacturers, dealers, 
and consumers, especially in international trade. 
In 188i Professor Stutzer of Bonn protested 
emphatically, but lam sorry to say, vninly, against 
the u-e of ammonium citrate, although granting 
its utility within certain limitations, lleendorsed 
an ealier sugge=!tion of Tollens to u'e dilute citric 
acid as a means of gauging the availability of the 
phosphates in fertilisers, ai;d fixed the strength of 
the solvent at a solution containing 1 per cent, of 
citric acid. Both Stutzeraiid A. Tiiomson showed 
that the results obtained by acting on various 
fertilisers with a one per cent solution of citric acid 
were in fair correspondence with the comparative 
efficacy accorded to the fertilisers by practical 
experience. 
The strength ot the solution— namely 1 per 
cent — appears to have been arrived at more or 
le~s arbitrarily; but as it appeared to give fairly 
rational results when applied to fertilizers, it 
struck me forcibly tliat the same or some similar 
solution might very probably give useful results 
when applied to soils. A large number of ex- 
peiiments had been made in my laboratory on 
the solvent action on soils of water, carbonic- 
acid-saturated water, and ammonium citrate solu- 
tions of very various strengths, without leading 
to any definite conclusions. It was then decided 
to try the effect of dilute citric ncid solution, 
in strength varying from 0'12o per cent up to 
5 per cent on the same soil. It was found 
that when the proportion of solvent to soil was 
ample, there was not much difference as regards 
phosphoric ncid at any rate, between the quanti- 
ties extracted by the weaker solutions of citric 
ncid employed, up to a strength of 1 per cent ; 
but when the strength was increased to 2A per 
cent the proportion dissolved was ve<ry much 
higher, nnd certainly indicated far more phos- 
phoric acid than could be accounted readily 
available. There was, in fact, nothing to indi- 
cate what particular strength was preferable, and 
the problem seemed to me not much more nearer 
solution tlian before- 
The 1 per cent solution of Tollens and Stutzer 
was very temjiting, but, ns apjilicd to the soil, 
Its use lacked justilication, whether regarded 
form a practical or a scientific point of" view. 
8 ■ i 'J' that what was reiniired was a means of 
di : :Mgui.-hii)g between matter that a plant could 
take up aud ma'ter that a plaut could uot tuka 
