Put your fertilizer on in the spring, as soon as the new leaves start to appear. It will begin to fuel their growth as they get moving. Follow the instructions for each type of plant for amount of feed. No point fertilizing at this time of year. Plants are all dormant and too deeply asleep to use it. I do my whole garden with Miracle grow slow release granules as growing starts. Soil improvers are an excellent approach too. I really do believe we need both.

I use good old fashioned growmore, it's as good as any, and cheaper than most, plus manure and garden compost to add humous, Derek,

I use granular growmore type stuff in mid spring but will give the camellia a sequestrine tonic as I garden on chalk. Plants in pots get slow release fertilizer when i top dress them in spring.

I also mulch with leaf mould [autumn] and home made compost in spring.

Please would someone remind me what Growmore is? Is it the slow release (six month) granules? Also known by other brand names, ie, Miracle-grow? Are they the same thing?

Growmore was developed in ww2 when everyone was encouraged to grow their own veg, it's a basic, general purpose fertilizer, used to be called national growmore, it doesn't contain a lot of trace elements, but does what it "says on the tin" so to speak, and was never patented so any chemical company can produce it, which makes it a bit cheaper than most of the "designer" fertilizers, Derek

Just to add to Derekm's reply it looks like a white granular grit, The slow release stuff is in a permeable capsule and are often mistaken as slug eggs etc.

We use a lot of well-rotted cow manure on our vegetable garden and round our roses in winter, but not on the rest of the garden where we grow our flowers. By itself, it doesn't seem to be enough, and we put the equivalent of "growmore" fertiliser on in Spring to supplement it. We spread what compost we have around the flower beds, and sometimes add woodash, too, but not where we have acid lovers. Seems to do the trick.

Jonah, Growmore is sold as Growmore, and takes six weeks to break down, so it is a slow release, but needs replenishing after 6 weeks, if necessary.

I'd use Growmore, and add humus rich materials such as good garden compost you've made yourself, composted animal manures, soil conditioning compost, leaf mould, anything like that, as well. Chemical feeds such as Growmore or miracle gro or chempak, etc., release feed directly to the plants, but humus rich materials improve the condition of the soil, and the bio diversity within it, thus creating a rich environment in which plants will thrive, with or without chemical feeds.

Vitax Q4 for flowers.
Fish, Blood and Bone for general use.
Elixir Organic fertiliser for green vegetables.

To improve all soils, Calcified Seaweed.

Your last recommendation intrigues me, Scrumpy. I've just googled it. I must say the "calcified" bit has alarm bells ringing, since we have shallow alkaline soil over rock, which we feed with as much rotted manure as we can, and add specific granular feeds to trees and vegetables. It gets described as a "pH regulator" but I can only think it would regulate ours even further up the scale.
I am ashamed to say I've not heard of it before, but apparently you CAN buy it here. (At a price) If it makes a useful organic soil conditioner, I'd be interested in searching some out, but not if it's only suitable for acidic soils.
(They must do something with all the stinking, rotting green and red algal bloom they scoop off the Adriatic every year when the weather gets hot. ;o)

By all soils I meant clay and sandy, as opposed to acid and alkaline. Sorry for any confusion.
Yes, it raises the PH of soil, so on very alkaline soils i'd use ordinary seaweed for the trace elements that you can't get from ordinary fertilisers like growmore.
The benefits of calcified seaweed can be found on google and yes it is a pure organic conditioner. Environmentalists were opposed to it's use, but it has become available again.
Another expensive option is nutrimate. Google that one.
I use that in my home made composts, and also sprinkle it in planting holes at planting time for my sweet peas and dahlias.
The idea behind using the above two is that they both improve the availability to plants of nutrients already in the soil.

I've been googling Nutrimate for the past half-hour, and it doesn't seem to be available in Italy - well, not by that name, anyway. It does look like a wonder product if all the claims are true. Maybe I should keep looking. Thank you for the advice, Scrumpy.