Effect of lowering nitrate nitrogen and elevating potassium concentrations in the nutrient solution on the growth, yield, and NO3 content in spinach grown in hydroponics

Spinach (Spinacia oleracea I,. cv. Sunlight) seedlings grown under the NFT system were administered 6 or 12 me.liter(-1) K from the onset of the experiment to which nitrate-free solution was added 6 days prior to harvest (Experiment 1) or K solutions were added with the nitrate-free solution simultaneously 6 days before harvest (Experiment 2). At harvest the leaf nitrate, ascorbic acid, potassium concentrations, and the residual mineral concentration in the hydroponic solution were determined. No significant reduction in the yield was observed in Exp. 1 in which plants grown in the NFT system were administered 12 me.liter(-1) K from the onset of the trial and supplemented with nitrate-N free solution 6 days before harvest. The leaf-nitrate content decreased significantly in plants adiministered 6 and 12 me.liter(-1) K with nitrate-free solution 6 days before harvest. The residual nitrate-N in the nutrient solution decreased by approximately 32% at 12 me.liter(-1) K as compared to plants exposed to 6 me.liter(-1) K in the supplemental nutrient solution. In Experiment 2, in which plants were supplied with low nitrate concentration and 6 or 12 me.liter(-1) K in the same hydroponic solution 6 days before harvest, an economically satisfactory yield was obtained. The leaf-nitrate content decreased significantly by lowering nitrate-N concentration, but no significant difference in the leaf-nitrate content was noted between the N-free and N-plus treatments whether the K concentration was 6 or 12 me.lite(-1). However, the nitrate-N level in the nutrient solution decreased by 24% at 12 me.liter(-1) as compared with that of the 6me.liter(-1) K in the supplemental solution.