Inline feeders? Any ideas?

I want to water the garden from a constant water supply on a timer, but I don't want a water butt or anything like that involved.

Are there are bits of gismology I can buy that'll feed into a water supply a measured amount of liquid feed?

Here's an example of what I mean;

http://www.watermate.co.uk/tapacc.html

What I don't like about it is that I'd need to pack it with solid material. That means using basic NPK nutrient as far as I know. If I was to put something like chicken manure pellets in it, it'd most likely clog the sprinklers with insoluble solids. And I want to use some form of organic nutrient to make sure I get all the trace elements.

I've also seen those hose fittings you can get to go on the end that feed the garden. Those usually have a spray nozzle right on the end though, rather than a fitting that'll let me attach it to multiple sprinklers.

So I need one of two things, a feeder that'll drip in a measured amount of liquid feed, or some form of solid feed that's 100% soluble. Are there any nutrients out there like that? Solid with traces and 100% soluble? Basically, solid hydro feed.

Any ideas would be great! Thanks in advance for taking the time

Discovered they're called 'syphon mixers'. Excellent, a search term.