Sir Albert Howard’s introduction to “The Formation of Vegetable Mould through the Action of Worms with Observations on their Habits” by Charles Darwin, John Murray, London, 1881; Faber and Faber, London, 1945 (Link to free online copy)Why do the roots of the potato always invade earthworm casts? The answer is to be found in the work of a number of investigators. In 1890 Wollny began a study of the place of earthworms in agriculture (Forschungen auf der Gebiet der Agrikultur-Physik, 13, 1890, s. 381). He found, as a result of five years’ work with cereals and legumes, that the mere addition of earthworms to soil led to a marked increase of grain (35 to 50 per cent) and of straw (40 per cent) above that of similar cultures without earthworms. Equally favourable results were obtained with flax, potatoes, and beetroots. In not a single instance did his cultures suffer any damage from earthworms. No support was, therefore, obtained for the common assertion that earthworms pull up young plants and carry them into the mouths of their burrows. Having seen for himself that soil containing earthworms was considerably more fertile than a soil free from these animals, Wollny set to work to ascertain the cause. He found that earthworms markedly improved the permeability of soils and led to better aeration. As regards the chemical composition of the soil inhabited by earthworms, he observed a considerable increase in the soluble nitrogen and in the available minerals as compared with similar worm-free soil. How far these results were due to the worm casts or to the dead earthworms was, however, not determined.
Wollny’s work was afterwards confirmed by a number of investigators. In 1910 Russell (Journal of Agricultural Science, 3, 1910, p. 246) showed that earthworms contain 1.5 to 2 per cent of nitrogen and decompose rapidly and completely, thus furnishing plant food to the soil in which they die. In 1942 H. J. M. Jacobson of the Connecticut Experiment Station compared the composition of the earthworm casts and uncontaminated soil from the farm of Christopher M. Gallup at North Stonington and obtained the following results:
||Lb. per Million of Soil
||Soil 0-6 in.
Curtis has summed up these interesting results in a press note issued by the Connecticut Station, which was reproduced in the Gardeners’ Chronicle of 17th July 1943. These analytical results amplify and re-state in terms of chemistry Darwin’s conclusion that ‘worms prepare the ground in an excellent manner for the growth of fibrous-rooted plants and for seedlings of all kinds’. They also direct attention to a sadly neglected branch of the chemistry of the soil — the part played by the waste products of the soil population in plant nutrition. A detailed account of these Connecticut investigations has just been published in Soil Science (58, 1944, pp. 367-75).”
“In the course of these green-manure experiments in Lincolnshire I spent some time in studying the earthworms on land similar to Mr. Caudwell’s farms but which were regularly dressed with farmyard manure. Here earthworms were abundant and in some of the old tunnels I frequently observed the reaction of the roots of the potato (King Edward) to fresh worm casts. The fine roots often followed these tunnels downwards, but whenever they passed the earthworm casts a fine network of roots was given off laterally which penetrated the casts in all directions. Obviously the potato was making full use of these accumulations.