I planted loose leaf, romaine and baby spinach. Can I, or should I fertilize them with miracle grow or smothing else?
All are big nitrogen lovers so yes you will have to feed them regularly with something. How much and how often will all depend on your soil preparation.
Hopefully you amended the soil with a well balanced fertilizer before planting? Maybe even worked in some good compost? If so then you can either side dress the rows/plants with either a synthetic granular fertilizer or an organic such as blood meal or alfalfa meal - both high in nitrogen - mid-way through their growth or you can use which ever liquid fertilizer you prefer once every week to 10 days when watering. The problem with the liquid synthetics is the plants don't need all the other stuff in them so the excess is wasted and runs into the ground water.
Using lots of compost and compost tea to feed them is another option and my personal preference.
I would second the idea of liquid fertilizer. Use something organic. A couple of things to consider:
First, lettuce and spinach grow very quickly, and you should accommodate that rapid growth cycle by providing them all they need in a readily available form. Granular organic fertilizers generally take too long to break down. That's not to say that they take an unreasonable amount of time to do so, just that these particular plants have such a quick growth cycle that waiting even just a couple weeks for the nitrogen in granular fertilizers to become bioavailable is too long. The second thing to consider is that spring soils are still cool, and the bacteria that fix nitrogen and convert it from one form to another are not fully active yet. What's more, nitrogen does not cling to soil particles very well, and thus gets flushed out of soil by spring rains. Putting these factors together makes a pretty strong case for providing extra nitrogen in a quickly available from for lettuce, spinach, and other fast growing spring greens.
Fish emulsion works extremely well, or if you're particularly beholden to Miracle Grow, then try their new Organic Choice All Purpose Plant Food. It's high in nitrogen (8-0-0) and is made from beet molasses. I never thought I would recommend a Miracle Grow product, but in fact I have tried it and it works quite well. Walmart and the big box stores usually carry it. See here: http://www.scotts.com/smg/catalog/productTemplate.jsp?proId=prod10280006&itemId=cat70048
Here is a link that might be useful: Garden Imperative blog
I have an endless supply of horse manure which is organic and high in nitrogen. What do you think of side dressing with a fresh horse manure tea?
Fresh - no. Fresh manures in any form are not recommended for use around food crops and haven't been for many years. This is because of all the pathogens and any medications they contain.
Well composted or 90-120 day well aged manures only go into the garden. This is especially important with low-growing leafy greens where direct contact is common.
So if you have some old composted manure and want to make tea from it fine but compost the fresh stuff and then add it to the garden this fall.
What do you mean by make a tea?
I use Teracycle's Worm Poop fertilizer on all my plants. It's organic, comes in recycled/reused containers, and works great. It's a bit pricey, but I don't mind since it is made by a very eco-friendly company right here in the U.S.A.
everyone does there own thing buty I'd prefer somthing free and organic :)