It is advised that you stop using your cannabis flowering fertilizer 2 weeks before harvest time to ensure that all chemicals are out of the plant. If chemicals are in the plant when you harvest, the smoke will be very harsh. To prevent harshness, flush the plants heavily with fresh water 2 weeks prior to harvesting them. Ensure you flush at least 3 times the capacity of your pot. So a 4 gallon pot would be flushed with 16 gallons of fresh water. From that point water your plants with pure water, absolutely NO additives should be given to the plant. This is especially important if you have been using chemical fertilizers.
This is the time when the bulk of the final, "usable" part of the plant is produced. As you may well already know, there are over four hundred separate chemicals associated with cannabis and her effects. It is during the final bud-building stage that most of these chemicals are produced. Thus, it is very important to give the plant as much pure water as possible during this crucial period. I like to remember it as the "rinse" and "flush" cycle. Simply remember to give the plants only water for the last two to three weeks in order to rinse and flush them clean. This is to purge unwanted impurities from the plant.
Pot that has been fertilized right up to harvest is harsh to smoke, sometimes the joint will even sizzle and pop as unmetabolized fertilizer salts combust. Unflushed pot leaves black ash, is hard to keep lit and burns your throat. Pot which has been organically grown and properly flushed is more flavorful and fragrant, burns easily, leaves grey ash, is easier on the throat and is much more pleasurable to smoke.
SOURCE:http://www.marijuanafertilizers.com/harvesting-curing-drying-marijuana-flowers-buds-plants.html
