BuMines Rept. of Inv. It is hard to explain the reasons behind the differing optimum concentrations. Different humic substances also produced different effects on the growth of algae. But, the results were similar. The mixture of high and low molecular weight humic substances seemed to be the best for stimulation of algal growth.
More investigation is needed to explain the differing effects of different humic substances.
It is interesting that the effects of soil treatments were much less than the treatments with humic substances. This suggests that it might be possible, under natural conditions, to increase algae populations by the applications of humic extracts. Increases in populations of N-fixing algae in paddy fields could increase the N content of the soil. Direct stimulation effects of humic acids on rice plants also would be expected. On the other hand, humic substances from polluting sewage, manure, or from erosion of top soil, may stimulate undesirable algal growth in streams, lakes, and other bodies of water.