Posted: 21/05/2012 at 07:50
Different veg have different requirements.
The key to understanding fertilisers - organic and chemical - is the NPK code that you'll see on the packaging. N = Nitrogen; P = Phosphorous; K = Potassium. Each of these three have different purposes in the fertilising process.
Nitrogen encourages leaf growth. In short, anything that is grown for its leaves - salad veg, silver beet, cabbage, etc - benefits from nitrogen. On the other hand, veg that produces fruit - toms, cukes, eggplant, etc - doesn't need more than a minimum amount of nitrogen. Too much nitrogen and they will produce foliage at the expense of fruit.
Phosphorous encourages root growth, flower development and general plant health. Leafy veg doesn't need a lot of it, but the fruit-bearing veg benefit from it.
Potassium (Potash) supports flower production and fruit development. No use to leafy veg, essential for fruit-bearing veg.
The actual NPK figures on the packaging tell you the percentage of each contained in the overall fertiliser. Eg, 7:7:7 means 7% each of nitrogen, phosphorous and potassium. It's called a balanced or all-purpose fertiliser, for obvious reasons.
So, in very simple terms, it's basically a matter of determining into which category your veg falls - leafy or fruit-producing - and using a fertiliser whose NPK ratio matches its needs.
Toms, for example, need a small N figure, a higher P figure, and an even higher K figure. Little nitrogen because you don't want to encourage too much leaf growth at the expense of fruit; more phosphorous to encourage root and flower development; more potassium to encourage fruit development. A ratio of something like 3:7:10 for toms would be ideal.
Then you get the idiosyncratic veg like peas and beans. They need next to no fertiliser - provided the soil is good to start with - because they manufacture their own nitrogen.
Beyond the NPK component, fertilisers also contain various minerals and micro-elements that some plants need more than others, but I'd need a book to detail them all.
The bottom line, though, is that if the garden soil - or whichever medium you use - is good and healthy to start with, veg shouldn't be over-fertilised. About the only exception might be salad veg - lettuces, etc - which do actually benefit from being pushed along. I use an ox blood-based fertiliser, almost pure N.