My Dear M^r. Darwin—

What will you have thought of me for delaying so long to thank you for your kind present of a copy of your remarkable book on the production of vegetable mould by earthworms?¹ I sincerely hope you have not thought so badly of me as I seemingly deserve!

The truth is, that the first hurried turning over of the leaves convinced me that there was so much of special interest & importance to us, that I was led to postpone, until I had more leisure, the careful reading of the book—and alas!—I postponed also writing to thank you, little thinking that I should leave an immediate duty so long unfulfilled. In fact, waiting for “a more convenient season” has had its usual result! I am, however truly sorry and ashamed, so hope you will forgive me? I did indeed commence a letter to you some weeks ago, but I see it is now out of date, so must begin again.

Allow me now to say a few words on the points of special interest to us in your work.

The evidence of long continued experiments, of various kinds, leads us to conclude that, at any rate in many cases, ordinary arable culture tends to a gradual reduction of the stores of nitrogen within the soil. In the case of permanent grass-land on the other hand, not only is more nitrogen yielded in the crops without artificial supply of it, than from arable land under like conditions, but the percentage of nitrogen in the surface soil remains at a considerably higher level; and in the case of newly laid down grass this level is rapidly approached.

Careful comparison of this gain with the amounts estimated to be supplied from external sources, has at any rate suggested the question—whether there be not some other, and not explained source? For example— how far may it be due to the roots of the perennial vegetation bringing up stores from lower depths?— how far to a greater condensation, or utilisation, of combined nitrogen from the atmosphere by the above ground growth the year round?— further, there is the improbable supposition that the free nitrogen of the air becomes a source of combined nitrogen, either by the intervention of the humic matter of the soil, or directly through the agency of vegetation itself?²

That the roots of some plants do bring up stores from below, and leave a residue near the surface seems indeed very probable. But admitting this, some further explanation of the facts would still seem to be wanting. How far have earth worms influenced the result? Of course, so far as their nutriment, or the soil they pass through their bodies, whether as food or otherwise, is derived from above ground growth, or within the range of our surface samples (9 inches), their action would not explain any gain or accumulation. But if the evidence were clear that they bring up much from below the depth of our samples of surface soil, the fact would be of considerable interest and importance.

From your description I gather that your soil and subsoil are very similar to those at Rothhamsted—that is, that you have below the surface soil some feet of stiff reddish yellow clay, resting upon chalk. Now, may I ask whether your observations would lead you to conclude that, under such circumstances, much of the matter of the castings would be derived from the clay-subsoil? Small as is their percentage, the actual quantity of nitrogen and carbon in such a subsoil is very large; but their chemical condition is not satisfactorily determined, and without some direct evidence on the point, it seems difficult to suppose that, excepting in the case of comparatively recent vegetable residue, they would exist in a condition to serve as food for worms? If so used, or if passed through the body for other purposes, and transported from below to the surface, practically the whole of the nitrogen would be voided, and so we should have a useful agency in the formation, and the maintenance, of permanent pasture, so far as the nitrogen is concerned. In the case of old grass land we of course only want time enough to accomplish the higher percentage of nitrogen in its surface soil than in that of arable land, whether with or without the aid of earthworms; but, in the case of newly laid down grass the proof of their agency would be of much interest.

On this point I may mention that several weeks ago, I had a portion of my lawn brushed quite free of worm-casts, and then, after 2 or 3 weeks, had those collected which had been accumulated over several square yards of surface; these were dried, and found to contain in that state 0^.35 of Nitrogen. This is from 2$\frac{1}{2}$ to 3 times as much as we find in our ordinary arable surface soil; more than in our ordinary pasture surface soil; but less than in rich Kitchen garden mould. Supposing a quantity equal to 10 tons in the dry state were annually deposited on an acre, this would represent a manuring of 78(1) lbs of Nitrogen per acre per annum. Obviously, so far as this nitrogen is derived from surface growth, or surface soil, it is not a gain to the latter; but so far as it is derived from below it is so.³ May I ask whether your observations enable you to explain its source?

Hoping you will pardon, not only my long delay in writing, but this long story at last, I am My Dear Sir, Yours very sincerely | J. H. Gilbert

(1) This is very much more than in the annual yield of hay per acre without nitrogenous manure.