The N-Tester BT has only been calibrated in the UK for use on winter cereals - winter wheat and winter barley as well as winter oats and winter rye - On these crops can be used to determine the nitrogen status of the plant and from this, a variety-specific nitrogen recommendation can be calculated.
In theory, N-Tester BT could be used on any plant to measure the chlorophyll content of the leaf and as such could be a useful management tool, however, no other crop calibrations exist so these would need to be developed locally by the user and any recommendations would need to be confirmed by locally conducted trials.