Best management practice of potassium for sunflower (Helianthus annuus, L.) in Northwest China

The effect of potassium (K) fertilization at various water regimes on oil sunflower yield and quality was studied during 2015 to 2017 in Dingxi County of Gansu Province. The experiment was a randomized complete block designed with three replicates. There were three irrigation regimes: 1) rain-fed without plastic mulch (R); 2) double ridges of plastic-mulched furrows added at sowing (PM); and 3) PM plus irrigation (PMI). Four rates of potassium chloride (KCl) were applied at 0, 48, 84, and 120 kg K₂O/ha. In the PMI water regime, sunflower was irrigated at budding, early flowering, and later flowering with 60 to 108 mm water each time. The average precipitation during the growing stage from April to September in three years was 270 mm. The sunflower variety was Longkuiza No. 3 and the plot area was 33 m². All the plots received the same amount of recommended N at 150 kg/ha and P₂O₅ at 90 kg/ha. All N, P, and K fertilizers were applied basally.

The results from the three years of the study indicated that the water regime significantly influenced total dry matter, achene yield, and thousand-achene weight of oil sunflower. These parameters were further improved with better water availability. Compared with R, the PM and PMI water regimes increased total dry matter by 72% and 98%, increased achene yield by 63% and 104%, increased thousand-achene weight by 24% and 39% respectively, and improved fertilizer K use efficiency. Water regimes also influenced sunflower grain quality. The grain oil content resulting from PM and PMI was 2% more than that of R.

Compared with zero-K control plots, K fertilization increased achene yield by 4 to 9% with a mean of 6% with the R water regime, increased achene yield by 9 to 20% with average of 15% with the PM water regime, and increased achene yield by 12 to 20% with mean of 16% with the PMI water regime. Thus, the effect of K fertilization on achene yield was increased by the improvement of water conditions. Achene yield increased with increasing K application rates, and the highest achene yield was observed at 120 K₂O/ha under the three water regimes. Potassium fertilization also improved grain quality of oil sunflower. The average oil content in sunflower grains by K fertilization was 1.2, 2.5, and 3% over that of the K control treatment at R, PM, and PMI water regimes respectively, and this effect increased with K rate increase. Potassium fertilization also decreased saturated fatty acid content and improved the unsaturated fatty acid content of the grains, increased the oleic acid and linolenic acid content, all of which also increased with improved of water conditions. Based on these results, applying KCl at a rate of 120 K₂O/ha under PMI water management was the best management practice in oil sunflower production in Gansu.