Humification see also Chapter 5 Method

Apart from the overall humus content, the type of humus is important. Stable humus improves soil texture, and friable humus supplies nutritive substances to the plants. Balzer refers to Welte (1955), who used alkali and acid solutions to extract humic acids, which were classified into brown and grey humic acids. Paper electrophoresis gave two fractions for the brown and three for the grey humic acid. The optical densities at 472 (E4) and 664 nm (E6) are expressed as a ratio, termed a 'colour quotient' (Q 4/6), where Q 4/6 = E4/E6. Q 4/6 values for brown humic acids vary from 5.0 to 5.5, and grey humic acids from 2.2 to 2.8. The colour quotient of grey humic acid depends strongly on the nitrogen content, and will be displaced to lower values with increase in N-content. Further method details are in Schlichting and Blume (1961, pp. 126 and 136). There are others, however, who claim that 'There are no reliable or agreed scientific interpretations of the meanings of these ratio values' (Simpson et al., 1997). Balzer gives the optimum humus content of sandy soils as 3.5-4.5%, loam as 3.0-4.0%, clay as 2.5-3.5% and peat bog as >20%.

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