ABSTRACT
In soil literature the smooth monotonous nature of the electron absorption spectra of HA was long accepted as fact. Significant distortions of electron absorption spectra could also arise due to heating of solutions. These experiments demonstrate that one of the reasons responsible for differences in the optical properties of humic substances from various types of soils and their genetic horizons could be the effect of light. Humic acids contain comparatively stable and intensely coloured structural units of aromatic nature. These units are joined by aliphatic bridges whose double bonds are close to conjugate chains. A single system of conjugate double bonds of adequate length arises, which is accompanied by a sharp rise in the colour intensity. The absence of absorption maxima on the spectra of classical humic acids prompts the need to analyse their distribution and absolute intensity of energy absorption and their changes during chemical transformation of humic substances.
