Dear M^r. Darwin—

I am ashamed to see that your unanswered letter is dated Feby 5, and I fear that I can render you but little aid in the matter of your enquiries.¹

You ask whether vegetable mould has an acid reaction, and suppose some connection between this, if it exist, and the acidity of the contents or excretions of earth-worms, which you are disposed to attribute to the humus-acids.²

It happens that we have here scarcely any experimental evidence bearing directly on either point. In this laboratory M^r. Warington has frequently concentrated the watery extract of vegetable mould, and has found the product to have an acid reaction; but having quite other objects in view the nature of the acid has not been investigated, and it may or may not belong to the humus series.³

Some investigators of soils have noted whether they have an acid reaction or not—some having & some not having this character. Detmer (Versuchs-Stationen vol XIV. 248 et seq), discusses the humus bodies of the soil; and among other points refers to the ‘sourness’ of certain soils as having been assumed to be connected with their richness in humus.⁴ He states, however, that this by no means depends on the richness in humus alone, but on other coincident characters; that if the soil be rich in bases it will not have the acid reaction; but that wet humus soils, or such soils deficient in bases have such reaction, especially sandy soils rich in humus.

For further information on the humus-acids I may refer you to the earlier results of Mulder (The Chemistry of Animal & Vegetable Physiology—about 1845);⁵ to a useful discussion of the then existing knowledge on the subject of the organic matter of the soil in “How Crops Feed” by S. W. Johnson;(1)⁶ and for later information to the Vols of the “Jahresbericht der Agricultur-Chemie”;⁷ and to the Landwirthschaftlichen Versuchs-Stationen, especially the paper of Detmer above referred to. You will be aware that it is stated that the fine root fibrils of some plants at any rate have an acid reaction. It seems probable that this acid may in some cases be malic acid, perhaps more frequently Oxalic acid, but it may be others. I think you will find reference to this in Sachs, but I have lent my volume so cannot refer to it.⁸ Our own observations lead us to conclude, however, that the mycelium of the fungi of Fairy rings is rich in oxalates.⁹

As to the acid reaction of the contents and castings of earthworms—is it not possibly due to some acid secreted by the animal itself—uric acid for example?—perhaps Michael Foster would give an answer on this point—or Ray Lankester?¹⁰

Much regretting that I cannot render you better service, I am, My Dear Sir | Yours sincerely | J. H. Gilbert

(1) Chapter II, Section 5, and Chapter V. Section 3.

I can lend you “How Crops Feed”—or the Versuchs-Stationen with Detmer’s papers if you have not got them.