Unless you have exceptionally rich soil, you'll need to give vegetables extra nutrients to grow their best. Organic fertilisers have the advantages of being slow to act and complex, containing a variety of micronutrients as well as major elements. Sometimes, however, plants need the quick boost of a liquid fertiliser to get started, especially in cool weather when nitrogen--used for making lush growth--is less available.
Liquid fish fertiliser, made from partially decomposed ground fish, is an excellent source of micronutrients. Usually a 5-1-1 formula (5 per cent nitrogen, 1 per cent phosphorous and 1 per cent potassium) is best for leafy vegetables like chard, lettuce and spring onions. A 5-1-1 formula has too much nitrogen, however, for regular fertilisation of fruiting plants such as tomatos and squash. Fish emulsion can be sprayed directly on leaves for quick absorption but will burn if used in concentrations higher than those recommended on the package. The smell can be a deterrent but usually dissipates within a few days.
Another good source of trace elements is seaweed extract, more expensive than fish emulsion but without the strong smell. Not strong enough to correct obvious deficiencies of micronutrients, it's best thought of as a tonic that also includes naturally occurring plant hormones. A 9-2-7 formula, seaweed extract is more balanced and would be a better choice for foliar feeding of tomatoes, squash and root vegetables than fish emulsion. Seaweed is also a good source of trace elements. Its odour is not as strong as fish emulsion, but it is more expensive
Manure is high in nitrogen and other major nutrients, but the exact composition varies depending on the source. Soaking a bag of manure in water for several days can produce a valuable liquid fertiliser cheaply and easily. Use composted or sterilised manure for best results.
You can also steep homemade compost in water and use it like manure tea, but one of the best liquid products to apply to your garden is the specially brewed commercial compost tea now available in many areas. Depending on the recipe, it can be high in beneficial fungi or bacteria, best for trees or perennials. Specially designed equipment oxygenates the "soup" to get the best results--the richest variety of microorganisms to add to the soil. The machines are generally sold to growers, but you may be able to buy a small quantity, a gallon or two, of tea from a local nursery or other source. Each gallon of tea can be diluted into 10 gallons of spray.