For the greenhouse, both dry and wet forms of fertilizer are used.  The dry is usually mixed with the growing medium, while the wet is diluted with water and applied with a watering can or by a special attachment to a garden hose.  "Slow-release" fertilizers are valuable because their nutrients are released gradually, providing a little at a time.

Keep in mind, though, that fertilizer comes in concentrated form.  Too much is worse than too little.  If over-applied, it can burn roots and stems or result in plants that produce all leaves and no blossoms or fruits.

As you become more and more familiar with your plants and how they look when hungry or dried out, you can adjust your feeding schedule to individual needs.

Here are some hints to keep in mind:

Make sure the soil is moist before applying fertilizer.  If soil is dry, plant food can burn roots.

Mix fertilizer at half strength for plants that demand less feeding, such as geraniums and most herbs.

Do not fertilize dormant plants.  Most growing things lapse into a period of inactivity when days shorten and nights lengthen.  (Christmas cactus is a notable exception.)  When days begin to lengthen, plants renew activity and will welcome a fertilizer.

Avoid the build-up of salts (whitish deposits on soil and pot rims) by periodically "leaching."  To leach, drench soil with tepid water until excess runs out at bottom.  If deposits are heavy, repeat two or three times at half-hour intervals.