Re: N-P-K electronic test kit?
ISE electrodes do exist for NO3, NH4.
PO4 is a bit tougher, so is K+.
They do make colormetric kits, such as the Hanna monitors. These run about 175$ea and do not include reagents.
They make a multiparameter unit etc as well, for about 800$.
But, like the probes, you need to calibrate them to make sure they are accurate.
Regards,
Tom Barr