Biological activity of humus

The addition of organic manure not only maintains but even improves soil fertility and, from earliest times, humus has been recognized as one of the major natural resources for agricultural purpose. In the absence of animal dung the value of green manure was recognized by the ancient Chinese (Jiao, 1983), while the Romans systematically used animal dung on their crops (Parsons, 1985). Soil fertility depends on the organic matter content, which can vary from less than 1% in young or in overworked soils to over 95% in some deep peats. Nevertheless, there are varying opinions on the importance of humus in promoting plant growth ranging from its absolute necessity as expressed in Thaer's "Humus Theory" (Thaer, 1808) to complete insignificance according to Liebig's "Mineral Theory" (Liebig, 1841; 1856). The controversy between these two theories has continued into the 20th century. For example Lawes and Gilbert (1905) first demonstrated that soil fertility could be preserved for many years by applying only mineral fertilizers (see also Wild, 1988). At the same time Bottomley (1914a; 1914b, 1917; 1920) asserted that small quantities of humus substances favoured plant growth (Lemna minor). However Clark and Roller (1924), although working side by side with Bottomley, found that humic substances were not essential for plant growth. In recent years other researchers, have verified a positive effect of humic substances on plant growth (Niklewski, 1931; Flaig, 1953; Flaig, 1956; Kononova, 1956; Rerabek, 1960; Chaminade, 1958; Chaminade, 1966). Pagel (1960) demonstrated that in rye plants humic substances had a greater effect on root development than on leaf and stem development. These studies were later carried out with the humic substances labelled with 14C in order to establish whether the positive effect of these substances was only through an indirect effect i.e. superficial physical contact, or direct effect wherever the substances were absorbed and translocated into the plant. From these studies it was established that labelled humic substances, above all those with low MW (molecular weight), could be absorbed by the plant and be translocated into the plant itself. Over the last 20 years, as amply demonstrated by the important work of Vaughan and Malcolm (1985a), the studies of humus biological effects on plants and their metabolism, have rapidly progressed. Nowadays, the positive effect of humic substances on vegetable metabolism in general and especially regarding the yield increase is no longer doubted. The need to integrate inorganic fertilizers with lower energy organic inputs is becoming more and more evident not just for economical reasons but above all for environmental questions (Parsons, 1985). In fact the great interest surrounding the use of these substances derives from the necessity to reconsider the indiscriminate use of chemical resources in modern agriculture and to recycle organic wastes that can affect crop growth and productivity (Petruzzelli, 1989).