Alginate-enriched diammonium phosphate improves flowering traits, yield, and phosphorus use efficiency in oilseed rape

This study aimed to determine the optimal application rate of alginate-enhanced phosphate fertilizer (alginate diammonium phosphate, SA-DAP) for optimizing both flowering performance and seed yield in oilseed rape ( Brassica napus L.), in compared with conventional diammonium phosphate (DAP). A field experiment was conducted in 2022 in Xinji Village, Xinyang City, Henan Province, China, using the oilseed rape cultivar ‘Dadi 199’. Five phosphorus application rates were tested: 0, 45, 90, 135, and 180 kg P 2 O 5 ha −1 . The results showed that phosphorus application significantly advanced and extended the flowering duration. Compared with DAP, SA-DAP extended the flowering period by 1 day and, even at low phosphorus levels, maintained a favorable ornamental period while substantially increasing flower number by 6.9–15.9% across 45–180 kg P 2 O 5 ha −1 , as well as enhancing petal size. The highest yield under SA-DAP was 3,001.6 kg ha −1 , achieved at an application rate of 90 kg P 2 O 5 ha −1 , which was significantly greater than the maximum yield of DAP of 2,932 kg ha −1 , obtained at 135 kg P 2 O 5 ha −1 . Model analysis indicated that at an application rate of 90 to 95 kg P 2 O 5 ha −1 , SA-DAP not only reduced phosphorus input below the optimal rate of 110.4 kg P 2 O 5 ha −1 for conventional DAP, but also simultaneously optimized flower number to a maximum of 587 per plant and seed yield to approximately 2,952 kg ha −1 , making it an efficient choice that balances ornamental value and high yield.