Der heutige Stand unseres Wissens vom Humus und neue Wege zur ... Wittich
remarks that recent work in humus chemistry has given a clearer and as a whole.
REVIEWS Der heutige Stand unseres Wissens vom Humus und neue Wege zur Losung des RoKhumusproblems im Walde. By W. WrrricH. Schriftenreihe der forstlichen Fakultdt der Universitdt Gottingen, Band 4. J. D. Sauerlander's Verlag, Frankfurt am Main, 1952. Pp. 106. Mull and Mor Formation in Relation to Forest Soils. By W. R. C.
HANDLEY.
Forestry Commission Bulletin No. 23. Her Majesty's Stationery Office, London, 1954. P p . 115. Price 10/. Review by L. G. ROMELL
O TTJDY1NG Danish beechwood soils in the seventies and describing mull and mor as V j two contrasting types of an uppermost soil horizon, the seat of humus formation, P. E. Muller was well aware of the amount and importance of organic matter in mull and he emphasized the need of leaf-litter covering the soil, supporting the humus formation and keeping the mull from degrading and eventually changing into mor. Later, German workers led by Ramann forgot about the high totals of organic matter contained in mulls and took mor to be an accumulation of 'raw humus' bound to occur when the annual litter-fall exceeds a critical value. This view was quickly adopted by forest soils men and developed by some to the extreme point of stating: 'by far the best forest humus is the kind that is never produced' (H. Siichting, 1922). Meanwhile the chemical study of humus made little headway. In the thirties a leading authority took stock of the available evidence and, even though granting the importance of soil humus, discredited the very notion of 'humus' in the old sense of the word. Ramann interpreted mull and mor as expressing different overall rates of decomposition. This simple idea, excellent for teaching, survives in textbooks and papers to the present day although more than 20 years ago it was shown to be wrong. Lack of chemical knowledge has contributed to preserving it. Only recently has progress in humus chemistry again given more individuality to the true humus, thus preparing a return to older ideas as to the nature and formation of humus. The effect of this in the field of forest soils is strikingly illustrated by Professor Wittich's important memoir, most fittingly supplemented by Dr. Handler's equally important monograph. In both, the return to P. E. Muller is noteworthy and gratifying to Scandinavian workers. Wittich remarks that recent work in humus chemistry has given a clearer and as a whole a simpler picture of the humus proper. He rightly refers to the work of J. Hudig in Holland as fundamental. Reference could well have been made also to the experimental work of P. Th^nard in the years around i860 and to its correct interpretation by Dumas, and further to Sante Mattson's paper on proteinated complexes (Soil Science, 1931) whence came the idea of a humus complex comparable to tanned leather. Two main results stand out from this and more recent work: nitrogen has a central role in humus formation, and quinoid structures are essential in building up the humus complexes. The formation of such complexes is favoured by a high base status and by the action of worms. Opposing the teachings of Ramann and Suchting as well as of Erdmann, Wittich holds that large stores of humus should be maintained in order to keep forest joils productive.
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Having studied the liming experiments made in Ebnath, Adorf, Syke, Sauerlach, and elsewhere, he concludes that the important effect of lime has been to help producing and maintaining a large enough store of stable humus in the soiL It may be necessary to supply nitrogen along with the lime. This is shown by Hassenkamp's experiences in Syke. Wittich is not concerned with mull and mor. He distinguishes 'mild' humus from 'moderately acid' and 'strongly acid' and writes: 'The formation of unfavourable humus covers is always a consequence of too low a base content of the soil.' This does not agTee with evidence collected in Denmark and elsewhere. A higher nitrogen content is reported by Wittich as a further characteristic of 'mild' humus. The problem of mor formation, barely touched by Wittich, is discussed by Handley on the basis of an extensive review of literature, surveying the differences that have been taken to exist between mull-forming and mor-forming sites, climates, or plants, and scrutinizing available data on differences between mull and mor as to the course or rate of biological and chemical processes. The general result of this penetrating analysis is remarkably negative. Few if any of all the differences reported were found constant. Vegetation appears as the most important single complex of factors favouring the formation of mull in preference to mor, or the reverse. Yet a careful review of available facts seemed to show that none of the known characteristics of litter could serve to distinguish mor-forming plants from other plants. Looking farther for attributes of litter that could possibly be held likely to determine the formation either of mull or of mor, as the case may be, Handley discovered remarkable differences between kinds of leaf litter as to the order in which their tissues are consumed by decomposing micro-organisms. For instance, while in decomposing leaves of Ilex the mesophyll quickly disappeared leaving the vascular tissue apparently unaltered, the reverse was found in decomposing leaves of Calluna. An explanation of these unexpected facts was sought in extensive series of tests and experiments. From the results it appears that the mesophyll tissues resisting decomposition, as in leaves of heather, may do so by virtue of a tanning effect like the one suggested by Sante Mattson in 1931 for proteinated complexes resisting decomposition in the soil. The findings might furnish a clue to the formation and some of the characteristics of heather mor, 'astringent humus' as it was called by Th. Hartig in 184x1. For a possible wider application of the findings it is natural to think in the first place of hemlock mor and similar, very acid, greasy types of mor, where phenolic compounds contained in the litter may not be broken down, the pH being too low for soil white-rots to grow. One or more compounds thus persisting may exclude parts of the mull fauna and set going a circular process of mor formation, and possibly start at once a rapid podsolization, as seen under hemlock and, in experiments by C. Bloomfield, in soil columns under various kinds of litter. It is difficult to believe that the same mechanism is at work generally in mor formation. No attribute of plant debris can reasonably be taken to account for facts such as the increasing tendency to mor formation observed with increasing age of forest stands, as under old beech in Denmark. This points instead to a competitional factor in mor formation. Indeed there are good reasons for believing that no single clue to mor formation can be found. A complicated biological equilibrium such as that of a mull can be overthrown by a number of interferences and mull must be excluded under a variety of conditions not permitting the organisms active in mull to thrive and to do their work.
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