Liquid
V's granular or bulky feeds
In general granular or bulky feeds like seaweed, manure or compost take longer
to have an effect as they need to break down n the soil first. Liquid feeds, on
the other hand, are readily available to your plants and will have an immediate
effect.
It is a good idea to apply bulky organic feeds at any time of year to improve
the long term health of your soil. If treating a specific issue or to give a
plant a boost it will be more beneficial to use a liquid. Remember a liquid
doesn't build the soil so they should always be used as a addition to your
broader soil feeding regime.
The exceptions to this rule are container grown plants. They will always need
direct feeding because of the limited root space available to them and the fact
that they are growing in a simple growing medium like peat. Peat, coconut fibre or peat free compost has has very low nutrient levels so your plants will need
a supplementary feed.

NPK
It is a good idea to start is with a home soil test which will show the levels
of the major nutrients Nitrogen (N) Phosphorous (P) and Potassium (K).
Potassium can also referred to as potash but it is the same thing. Wood ash
from a fire or wood burning stove is also a good source of potash with ash from
hard woods like beech or oak being best.
There are also a range of micro nutrients but you will need to get a
professional test for these to show up. Any micro nutrient deficiencies will
show up in your plants with symptoms like yellowing between leaf veins
(magnesium deficiency) or brown curds on cauliflower (boron deficiency) but
most good garden soils will have sufficient quantities.
Plants need nitrogen
for leaf growth, phosphorus for root growth, and potassium for fruit growth.
All plants need all three, but leafy crops particularly need nitrogen, and
fruit crops won't develop well without enough potassium. When you look at a bag
of fertiliser you will see the NPK values displayed as 3 numbers.
For
example an average tomato feed is labelled as 6:3:10. It has 6 nitrogen (N) for
foliage, 2 phosphorous (P) for healthy roots and a high 10 potassium (K) to
produce all the fruit we're hoping for. Cabbage, broccoli, chard and other
leafy crops will benefit from a high nitrogen feed. Feeds high in nitrogen
should be avoided on fruiting plants, however, as they will grow lots of lush
foliage at the expense of the fruit.
The major NPK nutrients are in a refined,
soluble form in chemical fertilisers which means they don't benefit the soil
and are easily washed through into the ground water. NPK is also present in
organic fertilisers but they tend to be in a slow release form, usually with
some extra bulk which feeds the soil life as well as your plants. Where
possible it is always better to use slow release organic fertilisers than
artificial for this reason.
Use
the minimum possible
With a direct feed, whether artificial or organic, you should use as little as
you can. It does not follow that if some is good then more will be better as
plants can only use a certain level of nutrients. The excess will be wasted or,
worse, run off into water courses and cause algae growth and problems down the
line.
Nitrogen will result in lush, leafy growth. Adding too much causes fast and
weak growth which leaves your plants open to attack by pests or disease. In the
case of fruiting plants excess nitrogen will also reduce fruit yield and
quality.