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Darwin Correspondence Project

From Francis Darwin to J. H. Gilbert   8 June 1876

Down, | Beckenham, Kent. | Railway Station | Orpington. S.E.R.

June 8 /76

My Dear Sir

I succeeded in burning the soil according to your instructions1 & having done so, my father thought it would be more satisfactory to know the actual amounts of the various constituents. Dr Frankland most kindly undertook to have the soil analysed in his laboratory: & the following is the result obtained.2 The “natural soil of which the analysis is also given, is from a heap close to my fathers house; we thought that as nothing seemed to grow on this soil it might serve our purpose another year & save all trouble of burning &c.

Will you be so very kind as to tell whether you would consider these soils agriculturally very poor in potassium phosphorus magnesia lime? Dr Frankland says “you must not conclude from these results that the potash soda phosphoric acid are at present in such a form as to be capable of assimilation by plants. All that can be said with certainty is that they may be assimilated”3

Natural soil Natural soil
Moisture expelled at 100oC 2·22 ·50
Loss on ignition4 4·26 1·20
Silica  77·72 89·09
Oxide of Iron  5·52 3·88
Alumina  8·79 3·41
Lime ·48 ·84
Magnesia ·51 ·49
Potash ·33 ·15
Soda ·26 ·04
Phosphoric acid ·04 ·09
Sulphuric acid traces none
100·13 99·69

If my letter finds you especially busy, pray do not think of answering it until you have convenient leisure. I am very sorry to trouble you again—5

With many thanks for your kind help— | Believe me | Yours sincerely | Francis Darwin


See letter to Edward Frankland, [before 6 June 1876], and letter from Edward Frankland, [before 6 June 1876]. CD wanted to be able to control the nutrients available to plants so that he could assess which nutrients had an effect on variation (see letter to J. H. Gilbert, 16 February 1876). His experiments were later abandoned (LL 3: 342).
Frankland must have made this remark in a missing part of his letter of [before 6 June 1876].
‘Loss on ignition’ is a test used in analytical chemistry in which a sample is heated and then weighed until the mass is stable; for organic matter the temperature is high enough to burn organic matter but not so high as to decompose carbonates.
No correspondence between Francis and Gilbert earlier in 1876 has been found, although CD had written to Gilbert twice (letters to J. H. Gilbert, 16 February 1876 and [before 9 March 1876]). Francis acted as CD’s amanuensis for these letters.


LL: The life and letters of Charles Darwin, including an autobiographical chapter. Edited by Francis Darwin. 3 vols. London: John Murray. 1887–8.


Asks about constituents of burnt soil.

Letter details

Letter no.
Francis Darwin
Joseph Henry Gilbert
Sent from
Source of text
Rothamsted Research (GIL13)
Physical description

Please cite as

Darwin Correspondence Project, “Letter no. 10535F,” accessed on 4 July 2020,

Also published in The Correspondence of Charles Darwin, vol. 24