Effects of potassium foliage supplementation on Coix lacryma-jobi L. yield formation and source-sink relationship compared with those of soil supplementation

The medicinal value of Coix lacryma-jobi L. has been recognized worldwide, but yields are unstable. Potassium (K) is one of the three most required and widely used nutrients, but the global soil K storage and K utilization rates are low. Based on a strategy of K fertilizer separate application, 20% K fertilizer (i.e., 30 kg K2O ha−1) was applied in three ways (K soil application–SK30, K foliar spray–FK30, and no K application–K0) at the flowering stage, to compare the effects of K application methods on coix seed filling process, source–sink balance, and economic yield. Results showed that K application increased coix seed yield and lipid yield by 25.7% and 39.6%, respectively, and increased the 100-seed weight and seed setting rate by 10.7% and 7.4%, respectively. The increase in lipid yield caused by FK30 was due to the increase in seed yield, since there was no significant difference in oil content between SK30 and FK30. Coix has the characteristics of a large source and small sink, and FK30 has a strong promoting effect on sink organs. Potassium foliar spray significantly increased the rate of seed filling and accelerated seed maturity by 1.2–2.8 days compared to soil application. In summary, K foliar spray based on K fertilizer separate application is a feasible strategy for improving coix seed and lipid yields, promoting seed maturity, and reducing the leaf area per sink capacity. This strategy could effectively alleviate the adverse effects of K deficiency on coix economic yield during the seed filling stage.