As Terra Roxa has a high clay content, and a lot of fissures occur in the surface soil under the no-tillage cropping in habitual drought terms, thus the root system of crops expands to the subsoil through the fissures. Under the tillage cropping, roots expand transversely among the granular structure of tilled soil layer. Roots can hardly penetrate through the underlying compact layer downward as there are only a few of finer fissures. For the non-tilled surface soil, fissures occur in the parts where hardening has been developed. Since the broken structure is preserved, the state of surface soil gets closer to that of tilled one by the accumulation of the structure as time goes by. An examination was conducted to make evident the process of nutrient supply to the subsoil which is brought about by falling of fertile surface soil into the fissures. Clods were collected from each soil horizon and fractionated samples were prepared ; i.e. the main bodies of clods and fine soil particles which adhered to the surfaces of clods and these were lightly scraped off with a metal brush from the surfaces. The contents of several nutrients were compared between the fine particle fraction and clod fraction at each horizon. Under the no-tillage cropping, generally, the nutrient content of fine particle fraction was higher than that of clod fraction, especially on K, Ca, and Mg. On the other hand, those differences were rather smaller under tillage cropping. Thus, under no-tillage cropping, falling of fertile surface soil was presumed but nutrient supply attributable to root decay was also suggested, because the content difference was found in the subsoil as well where the fissures became less and finer. Under the no-tillage cropping, improvements in the physical and chemical properties of subsoil are considered to be possible by such a mechanism mentioned above, with maintaining the surface soil in a fertile state.
View full abstract