Boron has a narrow range from insufficiency to toxicity. In soil 1 to 2 ppm is usually the recommended and above that level toxicity will accelerate. I good crop as a bioindicator of deficiency in broccoli which at low levels will show hollow stalks. Tissue analysis can help pinpoint the issues with boron deficiency and foliar applications can be used to eliminate losses.For addressing these deficiency and toxicity issues the use of soil and foliar analysis are good complements. While deficiencies are ideally remediated in the soil foliar applications are useful also.

Fe - the concentration should be below 2000 mg/kg and above that concentration it is toxic.

Mn - the concentration should be below 200 mg/kg and 200 mg and

more than that concentration it is toxic.

Cu - the concentration should be below 25 mg/kg and above that concentration it is found to be toxic.

Zn - the concentration should be below 120 mg/kg and above that concentration it is toxic,

B - the concentration should be below 80 mg/kg and above that concentration it is toxic.

Mo - the concentration should be below 90 mg/kg and above that concentration it is toxic.

there is not a standard values for all soils...every soil has paticularities as organic matter content and microflora activity and these parameters are capables to neutralise micronutrients..

agree with @ J. C. Tarafdar

regards

Dear Wagas

please have a look to following documents. also to determine any level of the micro or macro and toxic level of the nutrient there should be done local tests:

Article UPPER CRITICAL LEVELS OF TOXIC ELEMENTS IN PLANTS

Article Upper critical levels of toxic elements in plants

https://www.agriculturejournals.cz/publicFiles/52993.pdf

https://pubs.usgs.gov/bul/1466/report.pdf

Interesting question. Agree with J. C. Tarafdar

regards