I usually sidedress fertilizer at some point in the growing season. What and when depends on the crop. For example, sweet corn is pretty tolerant to more concentrated formulations of fertilizer, so I'll side dress it at the 6 leaf stage with a modest rate of straight urea (46-0-0). On more sensitive plants like tomatoes and peppers, I use a lower concentration formula that is less likely to cause salt injury on the plant. A couple examples would be the Osmocote multipurpose plant food (15-9-12) or that Miracle-Gro Shake 'n Feed (10-10-10). Some of those specialty vegetable fertilizers are also encapsulated to control release of fertilizer. Aside from helping it last all season, it can also help prevent plant injury. For flowering veggies, I always wait until flowering has been initiated. This signals that the plant has switched resource allocation from vegetative growth to reproductive growth and fertilizer additions will aid the plant in setting and filling fruit. Fertilizer additions prior to flowering will only increase vegetative growth. Topdressing, i think is more of a turf thing - basically broadcasting. I like to sidedress fertilizer by hand to get the material alongside (but not touching the stem) of the plant. Foliar feeding is another method of nutrient application that I've heard discussed, but I don't subscribe to. Plants take in most nutrients via the root system, so I figure its best to put fertilizer on the ground, where roots can get to it. I also like timing my applications before a good rain, so the fertilizer has a chance to soak in and start dissolving as soon as possible. If its a drought year like this one, irrigation will serve that purpose as well.
