Oct. i, 1894 ] THE TROPICAL 
slender pole to a height of 8 feet or more." He 
adds: -'I was informed by residents on Indian Key 
that t'lis premature blossoming of a young plant or 
sucker while yet attached to the parent root is not of 
uncommon occurrence." (Dodge, I.e. p. 18.) 
Kew, 16th August 1893. D. M. 
[Note added.— The plant cultivated in Mauritius, 
and yielding what is known as Mauritius Hemp is 
the Green or Foetid Aloe (Furcrwa gigantea). The 
value of the exports are about 50,Ou(W. annually. 
This p'ant is similar to an Agove, both in appear- 
ance and habit, and it also produces numerous 
bulbils or pole-plants after flowering. The experience 
in Mauritius with regard to poling of the Furcrcea 
has a striking resemblance to what has been observed 
in regard to the Agave in Yucatan. M. de Chazal, 
who has written an account of ''Li fibre d' Aloes" 
(Uaur'tius, 1832) states (p. 21) "that the plants 
generally pole at the age of seven or ei*ht years ; 
they can, therefore, be tut four or five times before 
poling and before it is necessary to replace them," 
On the same subject Mr. John Horne, f.l.s., late 
Director of Ga dens and Forests in Mauritius, 
writes under date of 4th .-eptember 1893, as follows: 
"The life of Furcrcea gigantea in Mauritius is from 
seven to 10 years as many of the plants flower 
three to four years earlier than others, the leaves of 
the seedlings (or pole-plants) from these are tit for 
cutting whn the late flowering plants are drying 
out, so cutting once begun on a plantation may be 
said to be continuous Supplying amongst od 
plants ahould le done in time so that as the old 
ones die out cutting from the young ones should 
begin. Over-cutting the leaves is common in Mauritius. 
This is generally held to be injurious to the plants, 
weakening their growth and causing them to flower 
and die prematurely. Peop e in Mauritius say that 
by cutting only the mature leaves the growth of 
the plants is not weakened, «nd thus large fine leaves 
are oDtained, yielding long fibre of the finest quality. 
But I have never heard the idea expressed that such 
a manner of cutting prolonged the life of the plant 
beyond what I might call the natural limits. It is 
said in Mauritius that over-cutting weakens the plant 
and Ciuses them to flower aud did prematurely, so 
it may be also said in Yucatan that cutting only 
the mature leaves prolongs the life of the plants 
to its natural limits. It conies to this, that what is 
said not to s-horteu the 1 fe of the plants in Mauri- 
tius is said to lengthen it in Yucatan — a case of 
arriving at the same place from opposite directioLS."] 
— Kew Bulletin. 
* 
PLANT FOOD IN SOILS. 
A very nseful contribution to our knowledge upon 
this important subject has recently been contributed 
by Dr. Bernard Dyer, to the Journal of the Chemical 
Society, and of which we give a summary. 
The chemical analysis of soils, which, in the early 
days of agricultural chemistry was looked upon as 
likely to be of very great practical use in agriculture, 
was soon found to be, as ordinarily practised, of 
very limited value. Determinations of the total 
quantities of the most important mineral elements 
of plaut food in the soil have been long recognised 
as affording useful information only in exceptional 
cases. Thus, the fact that a soil contains much less 
phosphoric acid than is contained in average soils is 
a "probable indication" only that it is in need of 
phoaphitrtic manure ; ai.d the fact that another soil is 
much poorer in potash than average soils i3 legarded 
aa a " probable indication " only that it needs 
potassic manure. 
Xbe reason is, that an analysis of soil, as ordinarily 
made, shows the total percentage of its cai stituents 
dissolved by strong mineral acids, without reference 
to the fact that only a very small proportion of this 
total may bo available for plant use. 
For example, it is very usual to find about 0 - 15 
§er cent of phosphorio acid in an oidinary average 
English agricultural soil. An average loamy soil, 
9 inches deep, in its dry state, may be said to 
weigh 1,200 to 1,500 tons per acre. Such a soil, cou- 
AGR1CULTUEIST. 229 
taiuing 015 per cent, of phosphoric acid, would 
accordingly contain about 2 tons of phosphoric acid 
to the acre, disregarding the subsoil altogether. 
Such a soil contains as much phosphoric acid per 
acre as would be contained in about 17 tons of 
superphosphate, or in nearly 10 tons of bone-dust; 
and yet on sucfi a soil the addition of a few hun- 
dredweights of phosphatic manure may make the 
difference between a full crop of Turnipa and a bad 
one. And similar statements would apply to other 
constituents of the soil. 
The obvious explanation of such anomalies is that 
it is not the total proportion of plant-food that rules 
a soil's fertility, but the proportion of each constituent 
that is preset t in an immediately available condition. 
The end that the author has in view, therefore, 
is to distinguish between matter that a plant can 
or cannot take up and assimilate as food, and the 
chief solvent agent for soil ruim rala he finds is the 
root-s-.p of the plants. 
Acidity of Root sap. 
It has long teen accepted as a fact that plants 
help themselves to a part of l heir mineral food by 
means of the solvent actiou of their acid root-sap 
on the particles of soil with which ti e rootlets come 
into contact. In order, therefore, to obtain some 
information on (his point, the author made deter- 
minations of the degree of acidity of root-sap in 
100 plants, comprising ordinary agricultural and 
horticultural plants from twenty different natural 
orders — annuals, biennials, and perennials — all being 
taken, a< far as was practicable, during active growth. 
The results showed that the variations were wide 
even in plants of the same order, and even of the 
same species. The average '.' sap acidity " of the 
roots of the 100 plants is 091 per cent, reckoned as 
citric acid, which represents very nearly the averages 
found in each case for the plants included °in 
Riinunculacese, Cruciferae, Caryophyllaceae Legumi- 
nosse, Onagraceaj, Araiiacese, and Boraginaceas, which 
averages vary between the limits of 0'81 per cent 
and 1 - 12 per cent. 
Of the remainii g orders, we have Tropeeolaceaa, 
Primulacete, Umbelliferaa, Composites, Campanulaceas, 
Chenopodiaceae. and Gramineaa, ranging from O'oH 
per cent to 0'68 per cent Dipsaceaj and Solanacera 
(single species only) fall much below the average 
(0-44 per cent and 0'34 per cent), and so do the 
Liliaceae (3-36 per cent), though one of the only two 
species comprising the four Liliaceous plants ex- 
amined gave higher results. On the other hand, the 
plants examined in Rosaceas and Plumbaginese gave 
exceedingly high results, checked in each case by 
operating on two distinct plants of each speciea 
chosen. 
These results appear to be sufficient to indicate that 
the ratio of the soiuble free acid in the roots of 
plants to the moisture contained in them — whicu is 
here called sap acidily — probably generally fall within 
1 per cent, calculated as crystallised citric acid. 
Citric acid is chosen to express the acidity partly 
on account of its being an organic acid, and in that 
sense kindred to other root-sap acids. 
Experiments on Rothajisted Barley Soils. 
The remainder of the research deals with the 
question of soil analysis. By permission of Sir John 
Dawes aud Sir Henry Gilbert, the author was ena- 
b'ed to draw a complete set of samples of soil from 
the world-famed Hoosfield at Rothamsted, on which 
Barley has been grown for forty years in succession 
and on which each plot has been year after year 
subjected to some om unvarying bind of manurial 
treatment. 
A precise record having been preserved, not only 
of tno mauures applied to each plot, but also of its 
yield of gram and straw year by year, a study of 
a summary of the field's history is sufficient to show 
which plots are languishing for phosphorio acid, 
which for potash, which for nitrogen, and which in 
varying degrees, for ail. 
From a careful consideration of the whole of tho 
results obtained, the author thinks it would not bo 
unreasonable to suggest that, wh«j a soil is found 
o contain aa little as about 0'01 per cent of phov 
