Foliar fertilisers are absorbed through plant leaves. They may sometimes be applied to soil, but are more usually applied directly to foliage. When used every two - four weeks during the main growth period, they are a good way to provide an extra boost of nutrients and growth hormones, just when the plants need them.
Foliar fertilisers can supply nutrients when they are missing from the soil, or unavailable due to chemical "tie-up" in the soil, or when roots can't take them up due to cold soil temperatures or stress.
Some foliar fertilisers are an excellent source of micronutrients, especially the products which are made from kelp. The fish emulsion products are also a good source of micronutrients. These days there are many "high tech" liquid fertilisers, which can deliver precise amount of nutrients, in different mixtures. Although these are very good for correcting production problems, the old style seaweed fertilisers still have a role as a general tonic and trace element snack for the plant.
Observe the label closely if you want to remain totally organic, and avoid the products which have very high nitrogen levels.
Critical times for foliar fertilisers are after transplanting, when the plant hormones greatly assist root development (and lack of roots means plants can't readily access nutrients from the soil), and and peak growth or stress periods such as the lead up to flowering and fruit set.
Leaf crops can tolerate dilute applications every two weeks. Don't use the same sprayer you use for herbicides, and set the nozzle to a fine spray.
The best time to spray is early morning and the next best time is late afternoon. An overcast day is fine, but not if rain is expected. Avoid applying foliar fertilisers in very cold or very hot weather, as leaves close down during extreme temperature conditions, and won't make use of the beneficial nutrients you are applying.
Apply the fertiliser until the leaves are dripping, and don't forget to spray under the leaves too, as there will be more open pores on the underside than on top.
Foliar fertilisers can be used to drench the roots of plants which are stressed by weather or disease, or when they are recovering from transplanting. They can also be used through drippers, especially the kelp ones (some fish emulsion products have too much solids to work this way).
Good kelp based seaweed fertilisers supplies about 60 nutrients which plants need. They also contain plant hormones, vitamins and enzymes, which play a role in regulating functions such as root extension and branching, and flowering. They are quite safe in the recommended dose.
It is also possible to purchase powdered kelp, which is a great slow release, leaching resistant fertiliser. I recommend its use in potting soils.
You can make your own liquid fertilisers too. If you are lucky enough to live near a beach where kelp is washed up, it is well worth collecting for this purpose, or for the compost. Other seaweeds are also useful, but kelp is the best. Liquid fertilisers may also be made from fish waste, animal manures and weeds.
You need to be very careful using fish waste and animal manures in large quantities. They can stink, attract vermin and supply too much nitrogen for your organic soil to utilise safely. Certified growers should check their organic Standards carefully for restrictions on the use of animal manurte tea. The seaweed products which you buy have had most of the sodium removed by chemical processes or filters. They won't be a concern in friable soils with a high rainfall or irrigation regime, but be aware of total salt additions in saline or very tight soils.
I use the following recipe for making liquid fertiliser from weeds and seaweed.
Loosely fill a large plastic drum or metal drum with weeds.
Fill with water and cover.
Stir every two - four days.
Leave to stand for at least two or three weeks, or until any pungent ammonium smells have dissappeared.
Filter and dilute at least three to one with water.
Apply to plants or soil.
Use the old weeds in the compost.