Growth and Low Yield Factors of Wheat in an Upland Field Converted from Paddy Field with Three-Crop Rotation of Rice, Wheat, and Soybean in Two Years

Abstract

To improve wheat productivity, we investigated the relationships between wheat yield and growth- or field-related factors, and outline of cultivation in 26 cultivated fields. The results indicated that yield was significantly correlated with growth index （plant length × number of stems × leaf color/10000） at flag leaf emergence and leaf color at heading. In addition, in high-yielding fields, a high growth index value （about 120） was obtained at flag leaf emergence, and high leaf color was maintained at the heading stage. However, no correlation was found between yield and outline of cultivation, including the amount of nitrogen fertilizer applied. With regard to field-related factors, yield was significantly correlated with water retention time in the soil layer, ground water level, bulk density, and porosity of the subsoil. Therefore, to improve yield, it is necessary to improve the physical properties of the dense subsoil, to reduce the time of water retention in the soil layer due to heavy rainfall, and to reduce the number of days on which the groundwater level rises to < 40 cm below the ground surface. There were also significant correlations between the physical properties of the subsoil layer, the number of days the groundwater level rose to < 40 cm, and water retention time of the soil layer, suggesting that the water retention time of the soil layer could be reduced by improving the physical properties of dense subsoil and suppression of groundwater level rise.