Choosing A Ballast

The metal halide lamp is the best source of light for parent plants and cuttings/seedlings.

During the flowering period, the plants need more light of the yellow/red spectrum (The need for 'blue' light, however remains unaltered).

If you use one or two ballasts, the best choice for you is the AGRO lamp.
They produce 30% more light in the blue spectrum than the regular SON-T. In case of a growing area with multiple ballasts, it is advisable to use metal halide lamps in addition to high pressure sodium lamps.
In case of a ratio of one MH lamp to four/five
HPS lamps, your plants will thrive.

In such a case it is advisable to suspend the MH lamp somewhat higher so that it covers the whole growing area.