Optimal magnesium management for better seed yield and quality of rapeseed based on native soil magnesium supply

Soil magnesium (Mg) deficiency is a common problem in many crop plants including rapeseed (Brassica napus L.), resulting in significant impairments in seed yield and quality. However, precise application approaches and technologies used for Mg fertilization are still not well established for rapeseed plants grown under field conditions. A better understanding and characterization of the relationship between soil Mg supply and yield response of plants is of great importance for appropriate Mg fertilization management. This study used large-scale experiments to (i) establish a criterion for soil extractable Mg (ex-Mg) in rapeseed-cultivated soils, (ii) evaluate the role of Mg in improving rapeseed yield and quality under contrasting soil ex-Mg status and (iii) optimize Mg fertilization programs based on native soil Mg supply. A large-scale field study program comprising of 56 rapeseed field trials, each with five Mg application levels (0, 9, 18, 27, 36 kg Mg ha⁻¹), was conducted in 13 provinces of China to investigate the effects of soil Mg status on rapeseed yield and quality. A soil ex-Mg criteria system was established to optimize Mg fertilization of rapeseed plants based on yield responses and soil ex-Mg levels. The critical soil ex-Mg for rapeseed was found to be 200 mg kg⁻¹, and seed yield was increased by more than 5 % after adding Mg fertilizers in over 85 % of the experimental trials. In case of severe Mg deficiency (ex-Mg < 70 mg kg⁻¹), Mg fertilization improved seed yield by about 40 % and this increase was closely related to the number of pods per plant and seeds per pod. When soil ex-Mg concentration was above 70 mg kg⁻¹, the increments of seed yield by Mg fertilization were diminished. Magnesium fertilization also improved seed oil and oleic acid concentrations of plants grown on severe Mg-depleted soils. The average recommended Mg rate was 23.2 kg Mg ha⁻¹ under severe soil Mg deficiency conditions, which then showed decreases as the soil ex-Mg increases. Besides soil ex-Mg, soil pH, the soil Ca²⁺ and K⁺ concentrations as well as the yield capacity of plants were further factors affecting the agronomic effectiveness of Mg fertilization on crop yield and Mg application rates. For example, seed yield significantly increased in response to Mg fertilization under high soil K/Mg (>2) and Ca/Mg (>20) ratios. The results collected from 56 field experiments indicate that soil ex-Mg level is a critical factor in determining Mg application rates in rapeseed-cultivated soils, and represents an important factor involved in varied response of rapeseed plants in respect to seed yield and quality. The study highlights the significance of soil ex-Mg levels and optimization of Mg recommendations in achieving improvements in seed yield and quality of rapeseed plants grown under diverse native ex-Mg levels.