Effect of plowing depth on soil physical characteristics and spatial distribution of root system of flue-cured tobacco

In order to explore the effects of plowing depth on yield and output of tobacco (Nicotiana tabacum L.) leaf, a field experiment was conducted, and soil physical characteristics and water content, spatial distribution of flue-cured tobacco root system as well as yield of tobacco were investigated. A flue-cured tobacco variety of ‘K326’ was planted with plowing depths of 20 cm (GS20, conventional tillage depth, CK), 30 cm (GS30) and 40 cm (GS40) in 2015 growing season, respectively. Leaf area and soil water content of 060 cm were measured after 30 days (rosette stage), 45 days (early vigorous growing stage), 60 days (later vigorous growing stage) and 75 days (early maturing stage) of transplant. Soil bulk density, total porosity, capillary porosity and non-capillary porosity after 54 day of transplant (vigorous growing stage) were investigated. Special distribution of root fresh weight after 65 days of transplant (squaring stage) was investigated too. The results showed that deep plowing had positive effect on soil structure, soil water storage, spatial distribution of flue-cured tobacco root system finally enhanced the yield and output. The deep plowing significantly decreased soil bulk density, increased soil total porosity, and capillary porosity at 2040 cm subsurface soil layer. Compared to CK (G20), GS30 and GS40 decreased soil bulk density by 8.4% and 9.4%, increased soil total porosity by 15.6% and 13.1%, and increased soil capillary porosity by 25.8% and 24.8%, respectively. The deep plowing with depths of 30 cm and 40 cm significantly increased soil moisture content at 020 cm surface soil layer at rosette stage and at 2040 cm subsurface soil layer at vigorous growing and maturing stages compared to those under GS20. Furthermore, the deep plowing not only significantly increased flue-cured tobacco root weight but also promoted the root content and proportion in deep soil. Root fresh biomass weight under GS30 and GS40 was 31.2% and 89.2% higher than that under GS20, respectively. Root depth index under GS30 and GS40 was 7.6% and 4.5% higher than that under GS20, respectively. In addition, tobacco leaf yield increased by 7.0% and 27.3%, average price increased by 1.8% and 6.2%, the ratio of the first-class tobacco increased by 10.4% and 24.4%, and the output value increased by 9.0% and 35.1% under GS30 and GS40 than those under GS20, respectively. There was significant difference between GS40 and GS20. The results showed that the improved soil physical characteristics, e.g., bulk density, total porosity and capillary porosity under deep plowing could increase soil moisture content at plough layer which benefited tobacco seedling growth, and lead to an optimization of the horizontal and vertical distribution of tobacco root system. A strong root system under deep plowing directly strengthened the growth of over-ground part of tobacco. Finally the yield and output value of tobacco leaf were increased.