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Agronomic practices increase sunflower yield in the rabi (dry) season in clay-textured, salt-affected soils of the coastal region of Bangladesh

Paul, Priya Lal Chandra (2020) Agronomic practices increase sunflower yield in the rabi (dry) season in clay-textured, salt-affected soils of the coastal region of Bangladesh. PhD thesis, Murdoch University.

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Agriculture in the coastal zone of Bangladesh is threatened by a range of abiotic stresses, including salinity, waterlogging and drought. Rice is generally grown in the wet (kharif) season, but soils lie fallow in the dry (rabi) season. This thesis was framed around the opportunity to increase the intensity of cropping in this region by shortening the rice-growing phase in the kharif (by planting short-season rice varieties) so that high-value adapted crops can be grown in the rabi season. Prior to this research, the best methods for the timely establishment and management of rabi crops on the poorly structured clay soils were unknown. Common soil constraints and requirements during the rabi season in the coastal zone of Bangladesh include the need to: rapidly decompose the mass of rice straw from the end of the kharif season, sow as early as possible to maximise growing season duration while avoiding extreme waterlogging effects at the end of the (rice) kharif season, and then maintain soil moisture and high solute potentials in a drying salinity-prone soil environment. A range of field experiments was conducted in the three rabi cropping seasons of 2016-17, 2017-18 and 2018-19. The research was in four theme areas with experiments conducted in three of these themes over two consecutive years. Using sunflower as a model crop, rabi cropping was possible provided: (a) soils received appropriate tillage (theme 1), (b) soils had application of surface mulches to maintain high soil solute potentials (theme 2), and (c) crops were sown early in the rabi season, but after waterlogging had abated (theme 3). In experiments in theme 4, it was shown that mulches decreased soil resistance and cracking, and improved root growth.

Experiments examined in Theme 1 showed that intensive soil disturbance such as bed planting, double pass shallow tillage, and single pass shallow tillage maintained higher soil water content and soil solute potential in the surface soil (0-15 cm depth) than less disturbance soil such as zero tillage, narrow strip tillage and wide strip tillage. The highest yields (19 % and 10 % improvements in 2016-17 and 2017-18, respectively) were associated with the tillage treatments involving greatest soil disturbance, the bed planting and double pass treatments in 2016-17, and the single pass shallow tillage treatment in 2017-18. The benefits of intensive soil disturbance were mostly due to increases in soil water content and increases in solute potential in surface soil layers, leading to higher water uptake by plants.

Experiments in Theme 2 found that rice straw mulches and irrigation increased soil water content, reduced soil salinity, and increased solute potentials. The substantially higher solute potential of the soil solution with the rice straw mulch at 0-7 and 7-15 cm (-644 and -588 kPa in 2017, and -649 and -558 kPa in 2018) than with no-mulch (-925 and -728 kPa in 2017, and -801 and -641 kPa in 2018) was associated with increased sunflower yield (26 % and 16 % in 2017 and 2018, respectively). The rice straw mulch also increased grain and biomass water productivity by 26-32 % in the first season and 16 % in the second season relative to the no-mulch treatment.

Experiments in Theme 3 showed that early sowing before 15 December was associated with larger heads, more seeds per head, heavier seed, and higher grain yields (3.5 – 4 t ha-1) except in the second year when sowing on 25 November was affected by waterlogging because of heavy rainfall. Data collected across both seasons showed that early sowing before 15 December tended to have higher soil water content, lower soil salinity, and, therefore higher solute potential than later sowings. For late sowing after 15 December, the lower yield was also associated with higher temperatures. However, the main driver of yield determination was EC1:5 and, to a lesser extent, to temperature in both years.

Experiments in Theme 4 demonstrated that rice straw mulch at 5 and 10 t ha-1 increased mean root dry weight, total root length, and root length density at 0-20 cm soil depth, but there was higher total root length at 60-80 cm with the no-mulch treatment. Rice straw mulch significantly reduced crack volume, cross-sectional area, crack length density, depth and width by 84-91 %, 63-69 %, 57-70 %, 42-52 %, and 42 %, respectively, relative to the no-mulch. With increased soil water content at 0-30 cm under the straw mulch, there was decreased average soil resistance by 77 %, 49 % and 28 % at 0-7, 7-15 and 15-30 cm depths, respectively, compared to the no-mulch. Overall, for wet-clay saline soil, soil and crop management treatments that enabled early sowing also enhanced the establishment and yield of sunflower due to increased soil water availability in the upper root zone, especially by increasing soil water solute potential. Increased soil water, in turn, decreased soil resistance and cracking. I conclude that early establishment leads to higher yield by enabling sunflower to escape soil surface dryness and salinity and heat stress in the later part of the growing season. Although sunflower is a promising rabi crop for Southern Bangladesh, only few farmers grow it due to a lack of proper agronomic practices to manage soil constraints. The current findings, therefore, would help to expand sunflower cultivation across the salt-affected coastal region of Bangladesh.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): Agricultural Sciences
United Nations SDGs: Goal 2: Zero Hunger
Supervisor(s): Bell, Richard, Barrett-Lennard, Ed and Kabir, Enamul
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