Never apply fertilizer to dry soil. When soil is parched, even dilute solutions of fertilizer may be strong enough to burn roots. If your plants are dry and it is a convenient time for you to feed them, give a light, quick watering before you prepare the fertilizer. Moisture will have penetrated to roots while you are mixing the solution, and trouble will be avoided.
Some gardeners give weak solutions of fertilizer as soon as seedlings show the first true leaves and gradually increase the strength according to the manufacturer's directions. I prefer to wait until seedlings have been transplanted and perk up on their own. I find the soil they are sown in is usually adequate to start them into vigorous growth. However, if you sow seeds in sphagnum moss or in one of the sterile mediums like perlite or vermiculite that contain no nutrients, by all means give a first light feeding as soon as seeds germinate.
On mature plants I have used both dry and liquid fertilizers. At present, I am partial to fish oil emulsion. I mix it with water, and use it as a general fertilizer everything in the greenhouse gets it, and everything seems to thrive on it. It is a good idea to establish a schedule for fertilizing. Every two weeks during periods of active growth is the usual recommendation. If this is not convenient, don't be alarmed. Most plants will thrive with less frequent and less regular applications.
While it is true that certain nutrients are necessary and beneficial, it is also true that over fertilizing can be harmful to plants. Keep an eye on the reactions of your plants to fertilizer. When they want to rest, let them, as during stretches of dull weather. When they show signs of new life, particularly in sunny weather in late winter and early spring, encourage them with the recommended fortnightly feedings.