Advice on Feeding Early Tomatoes

Now that my tomato plants have settled down after being planted out in the poly-tunnel, they are really starting to put on a growth spurt. I have given them a a couple of high nitrogen feeds as opposed to the normal high potash feed for tomatoes, which is for fruit production. I have found that by the time the plants reach the tie wire just before my first picking that the heads are quite thin as the plants are so ladened with fruit, so I am trying to get a good balanced plant early on in the season. 

This shows a 1.2.3 NPK ratio

If you have trouble finding a high nitrogen feed then look for flower feeds some of which have a higher nitrogen content. On the label you will seen the NPK Nitrogen, Phosphate and Potash. All feeds usually contain extra elements in smaller amounts, but NPK are the major three. 
A normal tomato feed will  roughly be a 1.1.3 ratio, so more potash than nitrogen.So you are looking for 1.1.1 or less potash than nitrogen.
At this time of the year it is best to apply a diluted liquid feed, with a watering can. On very young plants apply half strength for the first few weeks.
 The three major elements essential for plant growth do different jobs, so here is a quick run through.
N – Nitrogen Is for stem and leaf growth. Nitrogen deficiency results in older leaves turning yellow with new growth being weak and spindly. Basically nitrogen in responsible for the strength and vigour of you plants.
P – Phosphorous – This is for root growth and photosynthesis. Symptoms of deficiency would include poor germination and establishment of seedlings, mature plants showing stunted growth and dark blue/green leaves or reddish-purple stems or leaves.
K – Potassium (Potash) – For flower and fruit production. Lack of Potassium results in yellow areas along leaf veins and leaf edges. Fruits like tomatoes may be stunted and lacking in flavor.
For growth the plants do requires nitrogen which is easily leached out of the soil, so is not always present even with adding manure or granular feeds to the soil earlier on.  There is also little or no reserves of nitrogen in the soil and it will leach out of pots and grow-bags quite quickly.
Home-made Feeds - While home-made feeds are a very cheap alternative to commercial products, I would not recommend using them on young plants as you will not know the exact strength and could burn the roots and foliage.

This Coeur de boeuf beefsteak tomato plant is forming nice trusses, but the head of the plant is getting a little thin, so good nitrogen feed should sort this out.