Some of the structure material—the lignin—contains aromatic ring groupings. Fischer and Schrader have shown that in alkaline conditions these ring substances absorb oxygen and form something very like humus. It is quite possible that humus formation also proceeds in the soil in this way. Whether the two products are chemically identical is not known.
The scheme can be represented thus:—
| Cell structure material | ||
| Aliphatic (Hemicelluloses, Pentosans, etc.) | Aromatic (Lignin, etc., in presence of oxygen and under aerobic conditions) | |
| Fatty acids | Furfuraldehyde or Hydroxymethylfurfuraldehyde (in presence of acid) | |
| Calcium carbonate. | Humus. | |
The disintegration of the cell and the first stages in the decomposition of the structure material are almost certainly brought about by micro-organisms. Whether they complete the process is not known: purely chemical agencies could easily account for part.
The decomposition of protein in the soil has not been studied in any detail. From what is known of the acid hydrolysis and the putrefactive decompositions, however, it is not difficult to draw up a scheme which, at any rate, accords with the facts at present known. It is probable that the protein gives rise to amino-acids, which then break down by one of the known general reactions.
Two types of non-nitrogenous products may be expected: The aliphatic amino-acids give rise to ammonia and fatty acids; these form calcium salts which break down to calcium carbonate. The aromatic amino-acids—tyrosin, phenylalanine, etc.—which would account for about 6 per cent. of the nitrogen of vegetable proteins, would be expected to give ammonia and phenolic substances. Now phenols are poisonous to plants and if no method existed for their removal the accumulation would ultimately render the soil sterile. Matters would be even worse on cultivated soils, since cows’ urine, which enters into the composition of farmyard manure and is the chief constituent of liquid manure, contains, according to Mooser, no less than 0·25 to 0·77 grams of p-cresol per litre,[I] a quantity three to ten times that present in human urine. Fortunately this contingency never arises, for the soil contains a remarkable set of organisms capable of decomposing the phenols and leaving the soil entirely suitable for plant growth. This affords an interesting case of an organism—in this case the plant—growing well in a medium in spite of some adverse condition, not because it is specially adapted to meet this condition, but because some wholly different agent removes it.
[I] Mooser, Zeitschrift physiol. Chem., 1909, lxiii., 176. No phenol was found. It is possible that the p-cresol is not entirely derived from the protein, but that some comes from the glucosides in the animals’ food.
Other ring compounds, e.g. pyrrol, arise in smaller quantity in the decomposition of protein, but their fate in the soil is not known.
We may summarise the probable changes of the protein as follows:—