You're talking about cover crops. I plant winter rye here. It's a good one for germinating after your plants are done in by the first freeze. It will come up and then pretty much just sit there till the temps warm up in
spring. I've tilled some of mine in, the rest is about a foot and a half high.
One good organic liquid fertilizer is Fertrell 3. It used to be called Sea Mix. It's a blend of fish emulsion and seaweed so it's fairly high in nitrogen. Maxicrop is straight seaweed. It's excellent for micronutrients. Another is Omega 1-5-5 which is good for promoting flowering, fruiting, and
seed formation. It's made from soy protein, casein, fish meal, rock phosphate, and potassium rock.
You can also make compost tea by steeping a shovel-full or two of compost in a 5-gallon bucket of water for 5-7 days.
Be careful with the Miracle-Gro, it can cause a build up of salts which is harmful to plants. Organic fertilizers don't have this problem.