I came upon this post and it's so useful I had to reproduce it here with full credit to the Author and source
By Dean Mears on Facebook Group Friendly Gardeners
As growth commences thoughts turn to feeding. Below is likely to be of interest to all gardeners and is an update of my research posted this time last year with more products added as a result of various Groups feedback.
I have been doing a little research and you may find this of interest and enlightening with regard to strength and cost of feeds. This list is not to promote any particular fertilzer, but to provide an insight. You may find your current favoured fertiliser has almost the same content as a far cheaper alternative.
The proportion of nitrogen, phosphorus and potassium in any fertiliser is known as the NPK ratio and is shown on the packaging as a list of three numbers. For example, the popular tomato fertiliser Tomorite has an NPK ratio of 4 - 3 - 8. This means that it contains 4% nitrogen, 3% phosphorus compounds and 8% potassium compounds. On the other hand, a balanced fertiliser will have roughly equal amounts of each element.
Plants will thrive throughout the growing season if provided ample amounts of nitrogen 
, phosphorus (P), potassium (K).
, phosphorus (P), potassium (K). Nitrogen: used by plants to ensure they have the energy to generate new growth in stems and leaves.
This makes it great for lawns, leafy vegetables and big leaf plants. Lack of nitrogen: leaves discolour (often turning yellow), slower, stunted growth.
Phosphorous: Particularly important for seeds to germinate.
It will promote stronger root systems and also make plants hardier during winter. Lack of phosphorous: spindly, thin stems, stunted growth and mature leaves that turn a dark bluish green and reduce the ability to produce flowers, fruit and seeds.
Potassium aka potash: essential for producing flowers and fruit, helps with photosynthesis and the movement of sugars within the plant.
It will increase cell strength, the regulation water intake and help make the plant more resistant to disease. Lack of potassium: older leaves appear ‘burnt’ due to lack of water regulation, affects root growth and stem strength less fruit/ /flower production and fruits will be less sweet and juicy.
(P), potassium (K).
These are some approximate examples of NPK bearing in mind that it varies somewhat, according to the source. Also, manure, compost, homemade compost, mushroomc compost etc will also add to the health of your soil;
Seaweed 1.9 - 0.25 - 3.68
Baby Bio 10.6 - 4.4 - 1.7
Baby Bio Organic 3 - 2 - 5
David Austin rose food 9.5 - 7.5 - 10
Vitax Q4 5.3 - 7.5 - 10.
Thompson and Morgan Incredibloom 14 - 5 - 19
Miracle-Gro Water Soluble All Purpose Plant Food 24 - 8 - 16
Ironite Plus 12 - 10 -10
Miracle Grow soluble lawn feed 36 - 6 - 6
Miracle-Gro Push and Feed pellets 14 - 9 - 11
Chempak Formula 2 High Nitrogen 25 -15 - 15
Chempak Formula 4 High Potash Feed 15 - 15 - 30
Chempak Formula 8 Low Nitrogen 12.5 - 25 -25
Elixir Bumper Crop High Nitrogen 28 - 7 - 14
Richard Jackson Flower Power Premium Plant Food 12 - 12 - 36
Solabiol Top Rose 5 - 6 - 12
Osmocote 14 - 14 - 14
Tomorite 4 - 3 - 8
Phostrogen 6 - 3 - 6
Growmore 7.3 - 1:5 - 8.00
HSK Tree Fern Feed 6.1 - 1.5 - 3.0
Palm Focus 2.1 - 0.9 - 3.6
Palm fertilizer (The Palm Centre) 15 - 5 - 15
Slow Release Palm Fertilizer (MyPalmShop) 17 - 6 -18
Plantgrow all purpose plant food of 3.9 - 0.7 - 4.3
Liquid Gold Leaf 9.0 - 5.0 - 16.0
Vitax Acer Feed 4.5 - 4.5 - 4.5
Vitex Summer citrus feed 25 - 15 - 15
Vitex Winter feed 20 - 20 - 20
Elixir Gromore Special Blend 7 - 7 -7
Elixir Spuds Galore 15 - 9 - 20
Compost tea 0.1 - 0 - 0
Horse manure 0.7 - 0.3 - 0.6
Alpaca manure 1.5 - 0.2 - 1.1
Cow manure 3 - 2 - 1
Mushroom compost 1.0 -1.0 - 1.0
Worm castings 1 or 2 - 0 - 0
Compost varies from 1.5 - 0.5 - 1.0 to 3.5 - 1.0 - 2.0
Poultry pellets 3 - 2 - 2
Duck manure 2.8 - 2.3 - 1.7
Sheep manure 0.7 - 0.3 - 0.9
Turkey manure 2.8 - 2.4 - 1.2
Rabbit manure 2 - 1 - 1
Vitax 6X poultry pellets 5 - 3 - 2.2
Blood fish and bones 5 - 5 - 6
Bonemeal 3 - 15 - 0
Vermicompost is 1.8 - 3.8 - 1.3
Fresh banana peel 0.1 - 0.1 - 2.3
Dried banana skins 0.6 - 0.4 - 11.5
Comfrey 3.0 - 1.0 - 5.0
Nettles 5.6 - 0.7 - 3.7
Coffee grounds: 2.0 - 0.4 - 0.7
Egg shell 1.2 - 0.4 - 0.1
Tea 4.4 - 0.24 - 0.25
Wood ash 0 - 1 - 3
As you can see that some stand out as the strongest, but you may want one with a higher individual nitrogen, phosphorus or potassium for individual plant types, but you could further dilute your choice from above as strongest may not be the best. Bare in mind that the list does not include trace elements, copper, boron, zinc etc, but these should be present anyway in most good soils. The goodness of seaweed gets very esoteric when you look into it and, as a natural free fertiliser, it may have additional properties very beneficial to plants. Some seaweed products claim added benefit because they are said to contain natural plant growth regulators such as cytokinins (plant hormones that stimulate cell division). Be wary of any fertiliser that does not provide the NPK or hide it in the small print. Also, any product that promotes itself on the strenght of the insignificant trace elements.
