Plants need Air, Water, Sunlight and Nutrients to grow.
The top 3 nutrients are Nitrogen N Phosphorous P and Potassium K
Various plants also need other smaller amounts of minerals or trace elements such as Sulphur, Calcium, Iron. Magnesium, Copper and Zinc.
Liquid fertiliser is available as a concentrate or powder that is diluted and applied to the roots and surrounding soil. At larger dilution levels it can act as a foliar feeder adding nutrients through the leaves.
Solid Fertiliser is usually granular, pellet or powder form. They release feed over a longer perion as in Bone Meal, blood Fish and bone or Growmore. They can be incorporated in a planting hole or scattered on the surface to be washed in to feed existing plants.
Green Manure is a crop that is dug back into the soil to improve fertility and soil condition. Clover and pea plants fix nitrogen into the soil whilst grass clippings need nitrogen to rot down.
Apply fertilizer in spring at the start of the growing season and give a boost to vegetables in summer with a granular or liquid fertilizer
General Purpose fertilsers have a combination of all 3 key nutrient NPK and this is shown on the packaging in proportions 2:2:6 would have the same proportions of nitrogen and phosphorus but twice as much pottasium so would be good for flowers and tomatoes.