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Darwin Correspondence Project

To Henry Johnson   2 May [1872]1

Down, | Beckenham, Kent.

May 2d.

My dear Johnson

I am very much obliged to you for your notes of experiments, for I want to learn all that I can about the castings. The amount of Ammonia surprises me & shows that the castings must serve as a good top-dressing.2

I am doubtful about the percentage of 11.1 of organic matter, because in speaking lately to David Forbes on the subject, he positively asserted that almost all published analyses of the amount of carbon in vegetable mould were valueless, as the water of composition had not been allowed for. This is not of course expelled by any ordinary drying, but is expelled at the same time that the carbonaceous matters are burnt.—3 Your notes will in all probability come in & be of use some day.— I have not been observing much on subject of late; but I have heard from N. America & Calcutta that the worms threw up castings there as with us; & at Calcutta apparently on a greater scale even than here.—4 Some time this autumn I hope to put my notes together & see what results turn up.—

With many thanks | Yours sincerely | Ch. Darwin

I am beginning to suspect that worms perhaps search for animacules or other food, & do not blindly swallow earth for the sake of extracting the commingled organic matter.— I feel certain that a good deal of matter is swallowed, simply for sake of burrowing.—5


The year is established by the reference to CD’s meeting with David Forbes (see n. 3, below).
In Earthworms, p. 242, CD wrote that Johnson had found 0.018 per cent of ammonia in worm castings. Johnson’s notes have not been found in the Darwin Archive–CUL.
CD visited Forbes on 1 March 1872 (letter from David Forbes, 1 March 1872). Water of composition: water that is chemically (rather than physically) bound. See also Earthworms, p. 238.
See letter from Asa Gray, 2 February 1872, and letter from John Scott, 22 March 1872.
See Earthworms, chapter 2, for CD’s conclusions on worms’ feeding.


Thanks for notes on worm-castings. Amount of ammonia surprises CD. David Forbes asserts that published analysis of carbon in vegetable matter valueless. Suspects that worms search for food and do not blindly swallow earth.

Letter details

Letter no.
Charles Robert Darwin
Henry Johnson
Sent from
Source of text
Torquay Museum Society (AR 470)
Physical description

Please cite as

Darwin Correspondence Project, “Letter no. 8306,” accessed on 27 April 2017,

Also published in The Correspondence of Charles Darwin, vol. 20