Hello tink - bear in mind that nitrogen promotes leaf growth; phosphates promote healthy root and tuber growth and potassium promotes the production on flower,seed and fruit. Now,having said that, it does not mean that the other two in combination with any one are not important. Just err on the side of a higher number for the element that will help to give what you require from your plant.
On growing lettuce, cabbage spinach etc a compound with a high N figure could be used.
On growing potatoes, carrots, beet, a compound with a high P figure could be used.
On growing tomatoes, squash, peas, beans a compound with a high K figure could be used (preferably at flowering time)
A phosphatic fertiliser such as superphosphate is an ingredient in good seed composts as it will promote healthy seedling roots.
It is also a fact that in most garden situations a general balanced fertiliser is adequate. If the soil is naturally fertile and contains humus (decayed vegetable matter)it is unlikely to be difficient in any one of the three main nutrients.
Fertilisers and their application and effect is a big subject. These are just guidelines.
PETER THANKS, YOU SUMMERIZED WHAT I'VE READ ON TWO PAGES,I FIGURED THAT A BALANCE OF THE THREE WOULD BE A GOOD PLACE TO START. WE HAVE ALKALINE SOIL HERE, SO PROBABLY WOULDN'T NEED AS MUCH PHOSPHOROUS..NOW I'M WONDERING HOW MUCH I SHOULD PUT ON A 1/4 ACRE PLOT. GOTTA' GO READ THE BAGS :0
For most veggies, we use 13-13-13. That's because our soil is plenty rich in nitrogen. For iris, daylilies, flowering shrubs, black berries, raspberries, grapes, etc. I prefer 6-24-24. If I need to rot down some leaves or wood chips quickly I may sprinkle liberally with 33% ammonium nitrogen and then keep it moist. It takes about a week to 10 days to tun them into pretty good compost this way. Then I'll add the 6-24-24 in that to plant. If using good compost to plant in I may not fertilizer at all.