If you are lucky enough to have a soil which is rich in nutrients and of a high quality thanks to all the compost and manure you have managed to dig in, then the chances of having to feed your plants often is greatly reduced.
Fertilizers should really only be used as a supplement rather than a replacement for organic matter, they are best used for plants that have their growing space restricted i.e. plants in containers, hanging baskets or grow bags or to give a sickly weak plant a bit of a boost.
The best food you can put on your soil is your own compost, other types of organic matter like leaf mould and green manures tend to save the nutrients rather than add them. Plants do best in a well-structured soil.
WHAT MAKES A GOOD LIQUID FERTILIZER?
COMFREY - Alkaline so don't use it on chalky soils.
An excellent all round plant that has many uses, its long root draws up potassium from the subsoil. Bees and other insects love its flowers and it makes a superb liquid feed (if you can stomach the terrible smell).
How to grow comfrey - Comfrey very rarely sets seed so see if you can get some off a fellow gardener, if not buy some root offsets from a garden centre or seed company. Choose a spot where it can grow away happily for a good few years. The roots of this plant go down a long way and will be almost impossible to dig out. Plant in the early spring/autumn as the plant is dormant during the winter months. Don't harvest in the first year but remove any flowering stems, this allows the plant to build up its strength. use sparingly in the second year but after this it can be cut back up to four times a year when it reaches about 60cm in height.
How to make a comfrey feed
Use a water butt or similar sized container and drill a hole approximately half an inch in the middle of the base. Place some crocks or a stone over the hole to stop it getting blocked. You will then need to raise the barrel up on blocks in order to be able to place a smaller sized container under the hole to catch the liquid. Make sure you also have a snuggly fitting lid.
Fill the barrel with fresh comfrey leaves, put the lid on and wait for a couple of weeks. after this time a dark fairly odourless liquid should start dripping out. Dilute this liquid 15 parts water to one-part concentrate and you will have a wonderful potassium rich feed which is excellent for tomatoes and best of all it's free!
Nettles make a good general feed although they contain less phosphate and should not be used on alkaline soils. The shoots of nettles gathered in spring contain the most nutrients. You can use the same barrel system as above for making nettle feed.
Sheep manure has the highest phosphate content and you can make a great 'manure tea' with this. Simply fill an old perforated sack or bag with the manure, tie the top and simply dangle it into the water butt and leave it for about a week or so. This mixture can be used neat and makes an excellent feed.
ORGANIC DRY FERTILIZERS
Organic fertilizers are of plant, animal or mineral origin and are totally natural and best for the organic garden. All the fertilizers below should be sprinkled on the soil or around plants and lightly forked in.
Blood Fish and Bone - A good all round fertilizer
Blood Meal - A good source of nitrogen, use to boost your spring crops
Bone Meal - Rich in phosphate, good for root growth, use when planting shrubs.
Fish Meal - Contains phosphates and nitrogen.
Hoof and Horn - High in nitrogen, but needs time to break down.
Rock Potash - Good for light or poor soils, it is a slow release fertilizer which is especially good for fruit and vegetables.
Seaweed Meal - This is a slow release fertilizer and a good all-rounder. Helps build up humus content in the soil.
THE MAJOR ELEMENTS AND THEIR MEANINGS
NITROGEN (N) - Promotes shoot and leaf growth, helps give plants their green colour. If a plant is deficient in nitrogen it can be stunted and pale. If there is too much nitrogen in the soil, plants are more likely to attract pests and growth will be sappy.
PHOSPHOROUS/PHOSPHATE (P) - Encourages healthy roots and all round growth. Use small quantities. If a plant is deficient in phosphate older leaves will appear discoloured and growth will be stunted.
POTASSIUM/POTASH (K) - Good for producing good sized fruit or flowers, it helps plants fight pests and diseases, a deficiency will show up as small flowers and fruit and yellowing of leaves.
MAGENESIUM (Mg) - Helps the plants produce chlorophyll which gives them their green colour, a deficiency shows up as a yellowing of the leaves which starts between the veins.