Hi,
You should start using your bloom nutrients now when you start flowering but before you give them that you should flush all your plants, which is basically giving them lots of plain water which effectively cleans the soil. Slowly introduce the bloom to the plants with a weak concentration then gradually increase. Flush your plants once every 1 - 2 weeks to stop nutirent build up. Don't use your fert two weeks before harvest, thats when you want to stop and start using plain water otherwise your bud will have chemicals in it from the fertiliser..ew!!

hoppe this helps