To J. H. Gilbert   5 February 1881

My dear Sir

I trust to your kindess to forgive me for troubling you with a question. It is, whether vegetable mould, such as occurs close beneath the roots of the grasses on a pasture field, or in a Garden, has an acid reaction. I have tried in a few places & find no signs of acidity by neutral litmus paper, excepting on one damp & not very well drained place on my Lawn, where the mould was distinctly acid.— I ask because I have no experience in such work, & dare not fully trust my own observations.— I have often read of poor soil being “sour”, but imagined that this was only a form of expression for a poor soil.

I have long attended to the action of earth-worms, & I find that the contents of their intestines & their castings (or ejections) are acid, (except when contents were white with chalk) & this leads me to wish to know about mould.¹ I have read that the humus acids, (to which class I have some slight reason to believe that the intestinal acids of worms belong) are easily decomposed.—

I hope that it will not cause you much trouble to answer this question, & that you will kindly oblige me.—

My dear Sir | Yours very faithfully | Ch. Darwin