Boron is injurious to plant when irrigational water contains . . . . . . . . concentration of boron.

Boron is injurious to plants when irrigational water contains > 3 ppm concentration of boron.
The range between deficiency and toxicity of boron for many crops is narrow. To sustain an adequate supply of boron to the plant at least 0.02 ppm of boron in the irrigation water may be required, the maximum value should be less than 3 ppm.
The B concentration in the irrigation water (0.1 and 0.5 mg/L) was chosen to avoid acute toxicity and to reveal the effects of B in physiologically healthy plants. These concentrations are below the B threshold values for irrigation water in most countries.