[For 2009, I'm using a custom organic 10-10-10 fertilizer. More here]
I'm using 330 grams/12 oz (about 3 cups) of Bradfield Organics Tasty Tomato fertilizer (3-3-3), per box/two tomato plants, that I bought at Lake Street Landscaping (great source for city gardeners). Applied two times per growing season, once when I planted the seedlings and again halfway through the summer. Last year I followed the Official Directions and dosed them with a strip of Miracle Gro Organic (7-1-2) just once, when I planted. And then panicked in mid-July when the plants started to turn yellow. My improvised solution was to add water soluble Miracle Gro via the watering tubes for rest of the season. Really tedious.
This year I was ready when the plants started to quit. Obviously it's trickier to add fertilizer when there's a plant in and a plastic bag on, the box. Three days ago I made a vertical cut (up the side of each plastic bag that covers each planter) in line with the watering tube, from the bottom of the bag all the way up to the tube. Then I peeled back the bag, removed the old fertilizer strip, mixed in 90 grams/3 oz., (about 1/2 cup) of garden lime (to ward off potential blossom end rot) into the potting mix, added the new fertilizer, and then rolled the bag back over the sides of the box. I patched up the cut bag with a little duct tape.
I'm seeing new growth already.
Before I replaced the fertilizer I pruned the tomatoes. I'd done it once before, about a month ago, and it was time to do it again.
From an article in Fine Gardening I learned this:
A properly pruned and supported single-stem tomato plant presents all of its leaves to the sun. Most of the sugar produced is directed to the developing fruit, since the only competition is a single growing tip. The result is large fruits that are steadily produced until frost. If more stems are allowed to develop, some of the precious sugar production is diverted from fruit to multiple growing tips. Fruit production, although slowed, never stops. The result is a nearly continuous supply of fruits throughout the season. In general, more stems means more but smaller fruits, which are produced increasingly later in the season. (This is much less applicable to determinate plants, due to their shortened growing season and better-defined fruiting period. Therefore, determinate plants require little pruning.)
Get plants off the ground.
Give plants room.
Never prune or tie plants when the leaves are wet.
Pruning also affects plant health. The leaves of a pruned and supported plant dry off faster, so bacterial and fungal pathogens have less opportunity to spread. Soil is less liable to splash up onto staked plants. The bottom line: Upright plants have fewer problems with leaf spots and fruit rots because their leaves stay drier and free from pathogen-laden soil.
The way you choose to train and prune your tomato plants will affect how you space your plants, as well as the best method of support . There's no one right way to do it. Instead there are a few good patterns to follow.
The whole article is worth reading.
Here are a couple of other links on tomato pruning.