Sodium (Na) Stimulates Barley Growth in Potassium (K)-Deficient Soils by Improved K Uptake at Low Na Supply or by Substitution of K at Moderate Na Supply

Sodium (Na) can alleviate potassium (K) limitations by either increasing K uptake or by substitution of K functions in plants, but there is limited information about the levels of soil Na and K at which these separate Na effects operate in cereals. Barley was grown in two sandy soils to assess plant growth and nutrient uptake responses with varying soil Na (5–95 mg kg −1 ) and K (30, 90 mg kg −1 ) treatments. Compared with very low Na (5 mg kg −1 ), low Na (35, 65 mg kg −1 ) enhanced barley tillering in low K soil (30 mg kg −1 ) up to 6 weeks after sowing (WAS). Low K with 95 mg Na kg −1 or adequate K (90 mg kg −1 ) with 65 mg Na kg −1 also produced more tillers at 6–7 WAS. Shoot dry weight of low K plants was significantly improved by low Na at 7 and 9 WAS and even by 95 mg Na kg −1 at 9 WAS. While low Na increased shoot K concentration and the K/Na ratio in low K plants, 95 mg Na kg −1 mainly increased shoot Na concentration at 7 and 9 WAS. By comparison, soil Na had less effect on shoot K concentration and dry weight of adequate K plants. Barley growth at low soil K was stimulated by increasing K uptake at low Na (35–65 mg Na kg −1 ), but mainly by Na substitution of K at 95 mg Na kg −1 , i.e. the beneficial effect of Na on response of low K plants was attributed to two distinct mechanisms that operated at different soil Na levels.