you only need to flush if you have been feeding heavily(organic or inorganic), towards the end, there is no reason to flush during the life time of a plant unless your using synthetics and going on beyond 3-4 months of growing it.
you should flush at the end though especially if you feed heavily and not what the plant needs and uses,which most of you do if you follow the instuctions on the bottles for the most part(some, very few, but some are labeled well)
here is a small list of some of chemical ferts when overly fed would ideally need flushed out, most inorganic fertilizer contain one or more of these, because they are cheaper than other chemical forms of nutrition.
Ammonium Sulphate
This fertilizer type comes in a white crystalline salt form, containing 20 to 21% ammonia cal nitrogen. It is easy to handle and it stores well under dry conditions. However, during the rainy season, it sometimes, forms lumps. (TIP: When these lumps do occur you should grind them down to a powered form before use.) Though this fertilizer type is soluble in water, its nitrogen is not readily lost in drainage, because the ammonium ion is retained by the soil particles. A note of caution: Ammonium sulphate may have an acid effect on garden soil. Over time, the long-continued use of this type of fertilizer will increase soil acidity and thus lower the yield. (TIP: It is advisable to use this fertilizer type together with bulky organic manures to safeguard against the ill effects of continued application of ammonium sulphate.)
Ammonium Nitrate
This fertilizer type also comes in white crystalline salts. Ammonium Nitrate salts contains 33 to 35% nitrogen, of which half is nitrate nitrogen and the other half in the ammonium form. As part of the ammonium form, this type of fertilizer cannot be easily leached from the soil. This fertilizer is quick-acting, but highly hygroscopic thus making it unfit for storage. (TIP: Coagulation and Granulation of this fertilizer can be combated with a light coating of the granules with oil.) On a note of caution: Ammonium Nitrate also has an acid effect on the soil, in addition this type of fertilizer can be explosive under certain conditions, and, should thus be handled with care.
'Nitro Chalk' is the trade name of a product formed by mixing ammonium nitrate with about 40% lime-stone or dolomite. This fertilizer is granulated, non-hazardous and less hygroscopic. The lime content of this fertilizer type makes it useful for application to acidic garden soils.
dressing, but it should not be applied along the seed.
Ammonium Chloride
This fertilizer type comes in a white crystalline compound, which contains a good physical condition and 26% ammoniac nitrogen. In general, Ammonium Chloride is similar to ammonium sulphate in action. (TIP: Do not use this type of fertilizer on crops such as tomatoes because the chorine may harm your crop.)
