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

This project was initiated in 2014, and the objectives of this study were to: 1) determine the critical level of soil potassium (K) for sunflower production; 2) assess the right source and timing of K application; and 3) determine the appropriate combination of K rate and water regime. A total of 60 sunflower fields in Gansu and Inner Mongolia Autonomous Region (IMAR) were selected to study the relationship between soil available K and relative yield during 2014 to 2016 using two treatments (i.e., NPK and NP). The experiment on source and timing of K application had 7 treatments, with potassium chloride (KCl) and potassium sulfate (K₂SO₄) as K sources, and application time of full basal, and two or three split application. Interaction between water regimes and KCl rates were assessed by split plots that had three irrigation regimes and four K levels.

More than 75% of the trials showed a positive response to KCl application, with an average increase of 4.0 kg and 3.0 kg seed per kg of K₂O for oil and edible sunflower, respectively. The KCl application increased seed quality, such as 1000-seed weight, kernel rate and oil content, and improved unsaturated fatty acids, including oleic acid, linoleic acid, and linolenic acid. The kernels, however, removed a small amount of K from soil, and straw return is important in maintaining soil K fertility in the long-term. For producing 100 kg sunflower seeds, an average of 4.5 kg N, 3.2 kg P₂O₅, 13.0 kg K₂O were required in the ratio of 1:0.7:2.9 for oil sunflower, and 3.8 kg N, 1.6 kg P₂O₅ and 9.1 kg K₂O, in the ratio of 1:0.4:2.4, were required for edible sunflower. The critical level of soil exchangeable K for 90% relative yield of sunflower was 142 mg/kg. The effect of KCl and K₂SO₄ on yield and quality was similar at the same K₂O rate, but KCl resulted in similar or more income than K₂SO₄ at the same K rate. The basal application of K produced similar or more benefit than split application for both oil and edible sunflower. However, KCl resulted in a positive Cl^- balance for straw return, causing a Cl^- accumulation in soil. For oil sunflower in Gansu, application of KCl at 120 K₂O/ha under plastic mulching with supplemental irrigation was recommended. For edible sunflower of IMAR, application of KCl at 84 kg K₂O/ha under rain-fed condition, and 120 kg K₂O/ha under drip irrigation under plastic mulching were recommended.