“When humus remains constantly damp, without, however, being covered with water, it forms a very unpleasant smelling acid, which is more particularly, characterized by the property which it possesses of colouring blue litmus paper into red. This circumstance has long been known, and it is the reason that land and meadows which are not properly drained, and which exhibit these phenomena, are called sour. We have carefully examined these facts, and have endeavoured to discover the peculiar constitution of this acid. At first, we were inclined to regard it as being of a distinct nature, and having carbon for its base; but we have since become convinced that it is generally composed of acetic acid, and occasionally contains a portion of the phosphoric. This latter always adheres so firmly to the humus that it cannot be separated from it either by boiling or washing. The liquid in which the humus is boiled certainly acquires a slight acid flavour, but the greater part of the acid remains attached to the humus.
This acid or sour humus it not at all of a fertilizing nature; on the contrary, it is prejudicial to vegetation* Where it is very strong and pervades the whole of the humus, the soil only produces reeds, rushes, sedge, and other useless, unpalatable plants; and whenever these abound, it may be inferred that the soil contains a great deal of sour or acid humus… There are various means of getting rid of this baneful property, and rendering the humus fertile. It is well known that with the aid of alkalies, ashes, lime, and marl, humus may be deprived of its acidity, and rendered easily soluble… Heaths do not thrive where this humus does not exist, and when they have established themselves in one particular spot, they suffer few other plants to appear. This humus may be changed by a dressing composed of marl, lime, or ammonia; and where this has been mixed with the soil, the heaths, &c., speedily perish.”

— Albrecht Thaer

Source: The Principles of Agriculture, 1844, Section III: Agronomy, p. 343-4, as cited in Ruffin (1852, p. 85).

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Albrecht Thaer 34

German agronomist and an avid supporter of the humus theory… 1752–1828

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“In both the kinds of land we have been considering [i. e., classes of very fertile soil, rich in humus], we have supposed the humus to be mild, or exempt from acidity. J Sour or acid humus totally destroys the fertility of a soil; sometimes, however, the soil contains so very small a portion of acidity that its fertility is very slightly diminished, and only with regard to some few plants. Barley crops become more and more scanty in proportion as the acidity is increased; but oats do not appear to be at all affected by it. Rye grown on such land is peculiarly liable to rust, and is easily laid or lodged. The grains of all the oereals become larger, but contain less farina. Grass which grows on these spots is, both in species and taste, less agreeable, and less suitable for cattle, than any other, although it yields a very considerable produce in hay. In fact, in exact proportion with the increase of acidity, is the decrease of the value of the soil…”

Albrecht Thaer (1752–1828) German agronomist and an avid supporter of the humus theory for plant nutrition

p. 345 http://books.google.com/books?id=zAhJAAAAMAAJ&pg=PA345, as cited in Ruffin (1852, p. 85).
The Principles of Agriculture, 1844, Section III: Agronomy

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Source: Philosophical Papers (1979), P. 49.

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