Objectives Deep tillage is a key measure to improve the soil structure of infertile paddy fields and enhance production capacity. We compared the effects of tillage depths on the yield of the first crop and ratoon rice, and the physical and chemical properties of infertile paddy soil in Jianghan Plain.

Methods Field experiments were conducted in the Jianghan Plain in 2022 and 2023. The plow layer of the paddy fields in this area was 13−22 cm deep. Three tillage depths of 15 cm, 20 cm, and 25 cm were set up in the experiment, with conventional rotary tillage (12 cm) as the control. At the maturity of the main crop and ratoon crop, the aboveground parts of the rice plant were collected and divided into stem, leaf, and spike samples, the rice biomass, nutrient accumulation in each part, yield and yield components were investigated. The 0−20 cm soil samples at the beginning, and the 0−10 cm, 10−20 cm, and 20−30 cm soil samples of each treatment at the end of the experiment were collected to analyze the physical and chemical properties.

Results Compared with the control, tillage 15 cm and 25 cm deep increased the ratoon rice yield by 19.01% and 25.15% respectively in 2022, and tillage depth had no significant yield effects on both the first crop and ratoon rice in 2023. Deep ploughing at a depth of 20−25 cm could increase the number of grains per spike of the first crop and the regrowth crop. The ploughing depth did not affect the number of effective spikes and the rate of fruiting. Compared with conventional tillage, the three deep ploughing treatments in 2022 had no significant effect on the nitrogen accumulation of the main crop rice grains, stems, and above-ground parts. In 2023, the nitrogen accumulation of the main crop rice grains was significantly reduced, while the nitrogen accumulation of the ratoon crop rice stems was increased. Moreover, the nitrogen accumulation of the stems in the 25 cm deep ploughing treatment was significantly higher than that in the 15 cm and 20 cm ploughing treatments. Except for the 25 cm ploughing depth in 2022, which significantly increased the phosphorus accumulation of the main crop rice grains, stems, and above-round parts, the two-year ploughing depths had no significant effect on the phosphorus accumulation of rice grains, stems, and above-ground parts of both the main crop and the ratoon crop. In 2022, the three ploughing depths significantly increased the potassium accumulation of the main crop rice stems, and in 2023, the 15 cm and 20 cm ploughing depths significantly increased the potassium accumulation of the ratoon crop rice stems. In most cases, there was no significant difference in the nitrogen, phosphorus, and potassium cumulation of the above-ground parts of the main crop and ratoon rice among the three ploughing depths. After two years of tillage 20 cm and 25 cm deep, the soil bulk density of 0−30 cm soil layers decreased by 1.80%−5.36%, and the total porosity increased by 2.62%−5.13%, with the most significant effect observed in the 0−10 cm soil layer. The treatments with tillage depths of 20 cm and 25 cm increased the capillary porosity in the 0−20 cm soil layer and enhanced the organic matter content in the 20−30 cm soil layer. The treatment of deep tillage at 25 cm reduced the available nitrogen and available phosphorus contents in the 0−10 cm soil layer, while the treatment with a tillage depth of 15 cm decreased the available phosphorus content in the 0−10 cm soil layer.

Conclusions For the infertile paddy fields formed by the alluvial deposits of the Yangtze River, a deep tillage of more than 20 cm every two years can increase the yields of the first crop rice and the ratoon rice, and achieve the effect of loosening the soil and enriching it.