On a fertiliser pack, NPK stands for nitrogen, phosphorus and potassium. They are always in this order. The numbers that accompany them are ratios of each in the fertiliser.
A nitrogen-rich fertiliser will be, for example, 3-1-1.
Nitrogen promotes leaf growth so it's important for leafy crops such as lettuce, rocket, cauliflower, broccoli, spinach and silverbeet.
Basil just loves nitrogen, but for some plants too much nitrogenous fertiliser can produce large, heavy leaves on short stems. If your rhubarb is all leaf and not enough delicious stem, lighten up on the nitrogen and apply some sulphate of potash.
There's plenty of nitrogen in compost made with lawn clippings, sheep or horse manure or blood and bone.
If you want extra there's Thrive Soluble or Nitrosol.
Plants need phosphorous for root growth, bud growth and general plant health. Superphosphate is the most common fertiliser rich in phosphates.
Potassium is necessary for fruiting crops like tomatoes, peppers, beans and peas, and potash has lots of it. Banana peel and wood ash (not tanalised) are good sources, too.
Once tomatoes begin to swell, feed the plants with Tui Tomato Food or Phostrogen. Apply it on an overcast day.
Don't overwater, and scatter plenty of sheep pellets and blood and bone to encourage good crops.