1. Acton E.H. 1889. The assimilation of carbon by green plants from certain organic compounds. Proceedings of the Royal Society, Series B, 150–175.
2. Allison F.E. 1973. Soil organic matter and its role in crop production. Developments in Soil Science, Volume 3, Elsevier Press, New York.
3. Anderson H.A. and Russell J.D. 1976. Possible relationship between soil fulvic acid and polymaleic acid. Nature, London, 260, 597.
4. Andreyuk E.I., Gordienko S.A., Konoto I.N. and Martynenko V.A. 1973. Assimilation of humic acid nitrogen by micro-organisms. Mikrobiologichnyi Zhurnal, 35, 139–142.
5. Aso S. and Sakai I. 1963. Studies on the physiological effects of humic acid. 1. Uptake of humic acid by crop plants and its physiological effects. Soil Science and Plant Nutrition, 9, 85–91.