Analysis of the shallow root system of maize grown by plowing upland fields converted from paddy fields: effects of soil hardness and fertilization

The root system of maize tends to be shallower with plowing tillage than with rotary tilling in upland fields converted from paddy fields. Soil hardness and fertilizer distribution differ between plowing tillage and rotary tilling; thus, we investigated the maize root system at different growth stages with or without fertilizer application in both of these tillage methods. We evaluated the effect of soil hardness on the root system by comparing plowing tillage and rotary tilling in unfertilized plots, and the effect of fertilization by comparing responses to fertilizer application with plowing tillage and rotary tilling since the effects of tillage and fertilization cannot be separated in each tillage method. Root depth index (RDI), which indicates average root depth, was about 20% smaller with plowing tillage than with rotary tilling after the 7th leaf growth stage (V7) in unfertilized plots. Although RDI in fertilized plots was similar or slightly smaller than that in unfertilized plots, the interaction between fertilization and tillage was not significant, except at the tassel formation stage in 2016. Analysis of root distribution indicated that root length density at soil depths of 0–5 cm tended to be higher with plowing tillage than with rotary tilling after V7, but the effect of the interaction between fertilization and tillage was not significant. These results suggest that the root system of maize becomes shallower after V7 with plowing tillage than with rotary tilling mainly due to higher soil penetration resistance in upland fields converted from paddy fields.