7i 
THE TROPICAL 
AGPJCULTUrJST. [Aug. 1, 1901. 
are injurious to all crops cultivated for food for the 
use of man or beast : and unless these acids, which 
are always present in what practical men call sour 
hnmus, are neutralised by lime, or marl, or earth, 
none but the roughest and most innutritious herbage 
can be grown. 
" Free mineral acids are, I believe, still more in- 
jurious to all farm crops, and perhaps to all plants, 
than the free organic acids that are found in humus. 
" A very dilute solution of sulphuric acid— say 
1 part in 1,000 of water— may be used with advan- 
tage for killing grass in gravel walks made with 
flint or quartz sand ; after one or two applications 
fhe weeds will be destroyed and will not reappear 
tor a long time. 
" But if the walks are made with limestone gravel, 
the application of a much stronger acid has little 
or no efiect 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 these plants, 
but on coming into contact with the limestone gravel 
unites with the lime to form that useful fertilizer 
sulphate of lime or gypsum." 
The above paragraphs express the opinion of one 
who was rightly regarded as an authority upon the 
properties and application of artiiioial manures. 
In 1875 the Aberdeenshire experiments, conducted 
by Professor Jamieson, were instituted, and continued 
for some years. 
The published results excited much interest, for 
they demonstrated by actual field experiments that 
insoluble, or more properly termed undissolved, phos- 
phates, if applied in a finely ground state and iu 
sufficient quantity, were of very considerable value 
as a manure, whereas according to the previously 
held theory they were supposed to possess no prac- 
tical value at all. 
Further, these experiments proved that on certain 
soils ordinary soluble phosphate, that is to say, 
phosphate of lime rendered soluble by acid, was not 
superior in its action to undissolved phosphate to 
anything like the extent that had hitherto been 
generally supposed. 
Not unnaturally, these results, being opposed to 
the theory hitherto held, excited a considerable 
amount of hostile criticism, which, however, time 
and more extended enquiry has proved to have 
been unreasonable. 
The experiments were carried out at five stations 
situated in different parts of the country, and the 
soils are described as being black mould resting 
upon a granite subsoil, and the analyses show that 
in every case they were specially deficient in lime. 
The figures for lime at these five stations being 
respectively 0-08. 0-17, 0-21, O'SS, and 0-38 per 100 
parts of the dry soil. 
Such soils were, in fact, exactly those upon which 
soluble phosphate, as supplied by superphosphate, 
would not be likely to exert its full benefit, while 
the vegetable acids present in the black mould 
would be certain to dissolve the finely-ground 
mineral phosphate to a very considerable extent. 
In short, the conditions were most favourable to 
the action of dissolved phosphates. 
About the year 1883 the now well-knowu Thomas 
phosphate powder, prepared from basic slag, was 
introduced to the agricultural world as an entirely 
new manure. 
At first, agricultural chemists of high repute were 
disinclined to place any agricultural value upon this 
material, which was really ouly the refuse in the 
manufacture of iron from ores which, by reason of 
containing much phosphorous and silica, required 
additional lime to be specially added to remove 
the same, 
Little by iiltle, however, farmers were induced to 
take email quantities for trial, chiefly on their 
old grass lands, and the practical results were so 
good on certain soils, rich iu vegetable remains but 
poor in lime, that scientific authorities were soon 
compelled to recognise the value of this manure 
when applied to suitable soils. 
In this country the importance of fine grinding 
has been fully recognised as a test of the probable 
manurial value ; but in Germany Prof. Paul Wagner 
of the agricultural station of Darmstadt has in- 
sisted upon the solubility in citric acid as a farther 
test of probable manurial value, and it is to be hoped 
that the attention recently directed to this test by 
Mr. F.J. Lloyd will accentuate the importance of 
adopting some such additional test, rather than to 
confine the examination to a statement of the tot»l 
phosphate of lime and the fineness of the grinding. 
The author, however, had already adopted this 
citric acid method of testing the probable manurial 
value ; but instead of employing the 2 per cent, cold 
solution of citric acid used by Wagner, or the ] per 
cent, cold solution recommended by Dr. Bernard 
Dyer in his paper " On the Determination of probably 
available Mineral Plant Pood in Soils " fJournal 
of the Chemic»l Society, 1894), he prefers to use a 
solution very much weaker, and in his opinion more 
nearly resembling the acidity of soil water. 
The solution employed is 1 in 1,000, namely 1 grm. 
ff citric acid to 1 litre of cold distilled water, taking 
1 grm. of the slag or ground phosphate, and allowing 
the same to be exhausted with occasional stirring 
for 24 hours. 
On referring to Table I. the relative solubility of 
five different kinds of ground phosphate is compared 
with that of basic slag. 
The Peace Biver, with a fineness of 93'61, shows the 
highest figures for phosphoric acid dissolved out, 
namely 9'90 as against 8 70 in the basic slag ; but as 
regards lime the latter shows 22'17 as against 15'23 
in the former. 
The French phosphate, specially rich in carbonate 
of lime, upon which the citric acid first acts, shows 
16"34 of lime and only 2 85 phosphoric acid. 
Table I. 
Soluliliti/ in a Weak Solution of Citric J.cid 
1 grm, phosphate powder- 
1 grm. citric acid 
1,000 c.c. cold distilled water. 
Allowed to stand 24 hours, with occasional stirring • 
then filtered, the insoluble portion weighed, and' 
the solution analysed. ' 
^ ^ O S c3 V 
r" r2 r- « ^ 7i 
■< H Hi m 
^ to ?£> 
Containing phosphate 
of lime per cent. SO'St 55-99 7S'26 79-57 61-23 38-97 
Fine powder passed 
through 100 -hole 
siere. .. 76-21 67-69 72-37 91*63 93-61 83-8 
^ Portion soluble in 
citric acid solution .30' 00 30 00 22-60 22-80 31-40 38-80 
* Containing— 
lime ]6-.-'* 13-66 11-87 11-64 15-23 22-17 
Phosphoric acid. 2-85 ,6-35 8-25 8-40 9-90 8-7Q 
Equall to phos- 
phate of lime. 6-22 13-86 18-01 18-34 21-61 18-99 
These results certainly go to prove that much 
of the good effects of basic slag must be ascribed 
to the ready supply of lime rather than to the 
supply of phosphoric acid. Further, that the solubility 
in citric acid solution depends upon the character 
of the phosphate, the fineness of the grinding, the 
strength and quantity of the solvent, and the time 
allowed for the treatment. 
It has been stated that ordinary basic slag contains 
as much as 20 per cent, of fre« caustic lime, but 
this is not the |oase, and obviously so, because any 
such excess of lime would indicate a wasteful method 
