Sodium-Potassium Ratios in Soil Solution and Plant Response under Saline Conditions

The objective of this study was to examine the soil and plant factors that might affect the uptake and distribution of K by wheat plants (Triticum aestivum L. ‘Cajeme 71’) irrigated with high-Na, low-K water synthesized to represent cooling tower water from an electrical power generating plant. Lysimeters 122 cm in diam and 150 cm deep filled with either Holtville clay loam (clayey over loamy, montmorillonitic [calcareous], hyperthermic Typic Torrifluvents) or San Emigdio sandy loam (coarse-loamy, mixed [calcareous], thermic Typic Xerofluvents) were irrigated with three concentrations of simulated cooling tower water at electrical conductivities of 2.1, 4.2, or 7.1 mmho/cm. Sufficient water was added to each lysimeter to slightly exceed calculated evapotranspiration. The crop studied was the third wheat crop in a wheat-sorghum double-cropping rotation. Root length density distributions with depth were determined at several times during the growing season. Concentrations of K and Na in saturation extracts from these same samples were also determined. Xylem leaf water potentials were measured at intervals throughout the growing season with a pressure bomb. Leaf and stem samples were also taken throughout the growing season and at harvest for elemental analysis. Increasing the salinity of the irrigation water increased the concentration of both Na and K in the saturation extracts and greatly increased the accumulation of Na in leaves of wheat plants (r² = 0.990***). However, there was a significant inverse relationship between the K in plant leaves and the depth-weighted K in solution (r² = 0.821**) as well as between Na in leaves and K in leaves at harvest (r² = 0.968**). The best indicator of the K/Na ratio found in leaves at harvest was the product of K/Na in soil solution and the root length density, summed over each depth increment sampled (r² = 0.957***). An extremely good relationship was found between 1/(K/Na) in leaves at harvest and the average leaf xylem water potential measured at 1800 hours (r² = 0.968***). A much poorer correlation was found between depth-weighted electrical conductivity and average leaf xylem water potential (r² = 0.884**), suggesting that 1/(K/Na) was a better indicator than was total salinity. However, because several factors varied simultaneously in this trial, it is not possible to unequivocally establish cause-and-effect relationships between the K/Na ratio and xylem water potentials. Our results suggest that when wheat is irrigated with saline cooling tower waters high in Na and low in K, some attention should be given to the K/Na ratio to guarantee that no adverse nutrient imbalance develops in plant tissues that will lower yield.