Are you using the method developed by Virginia Tech regarding filtration and dilution to prepare your leachate sample?

I was using pour through method to catch about 50ml leache.

I keep flushing my pot for about 10 times, finally the EC value drops to about 500us/CM.

That keeps me thinking that after making the fresh soil mix, maybe we should flush the mix really well with clean water until the EC value drops to a reasonable range. There is a possibility that for slight acid mix like mine, once you add lime or gypsum or even bone meal, there is a reaction to release the Ca+ ion. If you don't flush the mix well with water, the EC value going to be very high initially. The value is only going down after about 10 deep watering. Unfortunately most of the time when we transplant, we just water once until the water runs out from the drain, the shocked plant will stay in high EC value soil medium, the water intake is restricted, we water less often, the poor plant will stay in such soil for a very long time. That may explains that why so many new transplants get stunned growth for such a long time.

I am also thinking that the micro-nutrients and all other organic supplements should be mixed after the flush.

Thanks
Tao