Theories, Humus

These observations later stimulated the general belief that prevailed through the earlier decades of the 19th century—namely, that humus is the only or the major soil product supplying nutrients to plants. The direct utilization of humus by plants was fully developed by Thaer (1808,1846), who stated that humus comprises a more or less considerable portion of soil fertility of the soil depends largely upon it since, besides water, humus is the only material that supplies nutrients to plants. This concept was referred to as the humus theory. 

Continental authors. H. Davy, although he gives more emphasis to the mineral contents of soil, thought the peculiar extractive matter formed in the decomposition of vegetables appears to constitute a prime cause of the fertility of soil . Karl Sprengel (see p. 310) rejected the humus theory in books of 1837 and 1839,2 but it was defended by Thaer (see p. 252). ... 

Jean Henri Hassenfratz (Paris 20 December 1755-26 February 1827), from 1795 professor of mineralogy in the Ecole des Mines and from 1797 professor of physics in the Fcole Polytechnique, published on respiration, affinity, metallic acids (tin and iron), the calcination of metals in pure air (oxygen), the distinction between inflammation and combustion, the amoimts of light produced by different combustibles, hydrometry, the oxides of iron, and papers on mineralogy, etc. He put forward the humus theory of vegetation, and published a large work on iron and steel. He and Adet devised a system of symbols in which, e.g., the metals are denoted by capital letters (C cuprum, P plumbum, S stannum, Fe ferrum, Sb stibium) enclosed in circles. ... 

The Humus Theory was popular because it combined, in an uncritical way, the views of chemists and biologists of the period. The chemists had isolated a group of humus components, the humic acids, which were soluble in alkalis but insoluble in water. It,therefore,seemed reasonable to suggest that alkalis and alkali earths present in the soil would dissolve the humus and make it available to plant roots. 

In spite of these experiments the idea that humus (a mixture of complex organic compounds resulting from the decomposition of biomass) is the dominant source of carbon, as well as other elements needed by plants, remained highly influential. The humus theory was favored by writings of Daniel Thaer (1752-1828), a Prussian professor of agriculture, and de Saussure himself continued to believe that humus has a key role in plant growth it was resolutely defeated only two generations later. 

Karl Sprengel (1787-1859) and Justus von Liebig (1803-1873) deserve most of the credit for disproving the humus theory. Liebig demonstrated beyond any doubt... 

For a review of the humus theory and its demise see Waksman, S. A. 1942. Liebig the humus theory and the role of humus in plant nutrition. In F. R. Moulton, ed., Liebig and after Liebig A Century of Progress in Agricultural Chemistry. Washington, D.C. American Association for the Advancement of Science, pp. 56-63. 

Field JA, Cervantes FJ (2005) Microbial redox reactions mediated by humus and structurally related quinones. In Perminova IV, Hatfield K, Hertkom N (eds) Use of humic substances to remediate polluted environments from theory to practice, vol 52. Springer, Dordrecht, pp 343-352... 

The experiments quoted by the physiologists were considered by Liebig to be valueless for the decision of any question. These experiments are considered by them as convincing proofs, whilst they are fitted only to awake pity. Liebig s ridicule killed the theory of humus as a nutrient. 

Figure 2.5. Schematic representation of the modified lignin theory of humus formation. Adapted with permission from Stevenson, F. J. (1994). Humus Chemistry Genesis, Composition, Reactions, 2nd ed. John Wiley and Sons, New York.

The view that lignin was the precursor of humic acids was further advanced by Hobson and Page (1932), Waksman (1936), and others. The theory became generally adopted and had a dominating influence on humus chemistry for several decades. Waksman (1936) postulated that proteins combined with modified lignins through the Schiff reaction, as follows ... 

Despite the current popularity of the polyphenol theory, a completely satisfactory scheme for the occurrence of humic and fulvic acids in diverse geologic environments has yet to be established. In practice, all pathways may be operative, but not to the same extent in all environments or in the same order of importance. A lignin pathway may-predominate in wet sediments, such as peats and swamps. The drastic conditions existing in soils under a harsh continental climate (e.g., some Mollisols) may favor humus synthesis by sugar-amine condensation. The disappearance of amino acids from buried sediments has been attributed to the formation of brown nitrogenous polyelectrolytes by reaction with reducing sugars (Stevenson, 1974). 

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