CORRESPONDENCE. 239 
EcoLoGicAL METHODS OF SoIL ANALYSIS. 
Referring to Mr. J. W. H. Johnson’s notes in The Naturalist for April 
(pp. 169-171), I must point out that my paper on ‘ The Water Content of 
Acidic Peats ’ is only an abstract and obviously a very condensed abstract, 
for it not only summarises in a couple of pages over 70 analyses, but 
introduces a new method of assessing the humidity of a soil. 
When Mr. Johnson complains that my figures expressing the water- 
content of peats are ‘ incomprehensible ’ and the ‘ character of the whole 
paper enigmatical,’ I may hope to enlighten him, But I cannot unravel 
the tangle in the main paragraph on page17o. Simply I deny the truth of 
every conclusion at which he arrives. I propose to confine myself mainly 
to two objections, viz. :— 
(x) ‘the unusual methods of stating the results obtained,’ and 
(2) ‘the erroneous results obtained by confounding ratios with 
percentage composition.’ 
In reality they resolve themselves into one charge, viz., that I express 
my analyses (including the water-content of the wet peat) in terms of 
the air-dry peat. As far back as 1865 Sachs found that ‘a young plant 
began to wither when the soil still contained water equivalent TOMmL222 
per cent of its dry weight.’1 In his standard work on The Soil, A. D. Hall? 
gives a table of the water capacity of soils expressed in three ways. The 
first method states the water absorbed by 100 of dry soil and gives, in 
the case of peat, such ‘ incomprehensible figures ’ as 155 parts*of water per 
100 of dry soil. Is there anything but a verbal difference between ‘ per 
100 of dry soil’ and ‘ per cent of dry soil’? .So again, Briggs and Shantz* 
of the U. S. Department of Agriculture, gies the wilting co-efficient as 
the mositure content of the soil expressed as a percentage of the dry weight 
when the plant wilts, and they give 1,300 analyses on that basis. 
It is true I base my own analy ses on the air-dry weight, but that is 
only a refinement that eliminates a small proportion of water of no service 
to the plants. It in no wise affects the principle involved, and agr icultural 
analyses of soils are regularly expressed in terms of the air- dry weight. 
I am quite aware of the difficulty encountered in the case of peats by 
those unfamiliar with the process, and have anticipated it in a paper 
just published in the New Phytologist.* I there state that ‘ partly to 
conform to agricultural practice and partly because the wet soil represents 
no fixed standard,’ the results are expressed ‘in terms of 100 parts of 
aty-dvy soil, z.e. of soil dried at 15°C. The water-content, otherwise the 
loss at 15°C., then becomes an addition to the Ioo parts. 
But in the case of peats the statement of the water-content is paradoxical 
unless the exact expression receives careful attention, e.g., ‘the water 
lost at 15°C. is 135 per cent. of the air-dry peat’ means that there are 
135 parts of water lost on every 100 parts of the air-dry peat that remains 
after drying.’ 
According to Mr. Johnson, such values as 135 per cent of the air-dry 
peat ‘on closer examination appear to be ratios and have nothing what- 
ever to do with percentage composition-amounts, ’ and ‘ by confounding 
ratios with percentage composition,’’ I have arrived at ‘erroneous re- 
sults.’ I venture to affirm that my abstract published in The Naturalist 
contains no erroneous results, though Mr. Johnson arrives at several, 
as when he discovers that a peat w hich I state to contain 80 per cent. of 
humus, ‘ has a humus content of 8? or 40.’ Nor is there any confusion of 
percentage amounts and ratios, for all percentages are ratios. The very 
water content 
number (57 that Mr. hnson deduces > ratio ————_— 
(575) John deduces for the ratio 100 ise ae aa 
Warming, Oecology of Plants, p. 49. 
Edition 2, p. 66. 
The Wilting Co-effictent for Different Plants, 1912. 
The Co-efficient of Humidity: a new method of expressing the 
Soil Moisture. 
[a  ~ d 
1913 June fr. 
