Dissolve the desired mass of salicylic acid in 1 L of water.
Salicylic acid (SA) inhibits the accumulation of Na⁺ and Cl⁻ in plants.
Most researchers have used concentrations ranging from 0.25 mmol/L to 180 mmol/L.
Assume that you want to use 1.00 mmol/L SA.
So you would dissolve 138 mg of SA in 1 L of distilled water.
SA is slightly soluble in water. You may wish to dissolve it first in about 2 drops of dimethyl sulfoxide. Then add the water.
Some researchers use distilled water containing Polysorbate 20 (200 mg/L).
Polysorbate is a surfactant. I presume that it facilitates absorption of SA by the plant.