i have been making my soil since 1983, here in the piedmont region of georgia. i have tried and often used many ways to add organic nutrition to my soil, without the longterm time-frame in waiting on cover cropping. whatever the size of your garden, there is always time for compost.
i usually make a pile in late fall, for spring planting and mulching. i keep one sort of cooking slow into late summer, add in spent plants and the fall leaves, and return it to beds in the fall season. also liberally placed around perennial plants and shrubs, and fruit trees, if i have enough of it.
there is NEVER enough compost...#1 organic rule of thumb.
certainly a small plot would benefit from organic matter, even digging in the leaves and mulch at the end of each growing season adds good texture to the dirt, and adds food for worms, and the soil bacteria and mycorrhizae so important to 'living dirt.' if you cannot devote any room to lie fallow, or a don't have large enough space to till in green manure, there are many available soil amendments and additives that do nicely to organically improve your garden's fertility.
my favorite shortcuts to dig into your dirt are as follows. any and all can be used in any combination, so as to produce a gentle combination of N/P/K....most blended organic additives are in the 5-5-5 range. you can pick and choose what you can afford to return to the soil.
remember that any improvements can sometimes be pricey, but consider the price of conventional 10-10-10, as well as the damage that concentrated nitrates can have on living organic dirt. all of these organic additives are used over a much longer term, as they slowly break down. so do consider the longterm investment in your land
alfalfa pellets - horse feed- costs about $ 14 a bag, for 50#. sprinkle down the plant row and till or dig in. adds good organic stuff slowly, esp. calcium, grows great greens and broccoli, etc. and breaks up all into the dirt, drawing worms, and quickly changing the texture of the dirt. i sprinkle it down the rows added to the mulch, also add put in tomato and pepper transplant holes (say, a cup) as well as shrubs and trees (more like 1/2 #) for a larger hole. alfalfa is rich in nitrogen, but you cannot burn a plant with it. i use it freely, even sometimes in potting mix. i go through about a bag a year.
wood ash- those of you that have wood heat already appreciate this wonderful byproduct of the fireplace. since georgia soils tend to be acid, i use a goodly layer every year to condition the dirt and balance ph. my asparagus especially appreciate a winter ashing, it makes production jump dramatically come spring. just a note: i ate my first spears today! 3/17/09
oystershell chicken grit- @ feed stores, maybe $ 5 for a 30 # bag. these roughground shell bits are a gentle way to add calcium and lime to the garden, very slow release and nice texture for well draining dirt. another longterm additive, as a little goes a long way.
kelp meal- sometimes available in smaller bags 5 or 10# at nurseries, i ordered a 50# bag one year, and the shipping was higher than the price, but i had years of it.. a good addition to all types of soils, so dig it into the mix liberally. sprinkle handfuls down the row, and add to your transplant holes. rich in potash and ocean minerals. feed a bit to your animals as a supplement, they find it quite palatable, and it's very nutritious.
last, but not least, rock powders. with very slow release into the soil, they provide phosphorus to balance the nitrogen, and make for good strong roots and good blooming. phosphate rock is available in some places-as an organic additive- but for larger amounts, you can usually locate a pile of rock dust where a new well has been put in. around here its usually granite. nobody usually minds you getting it all, for most new homeowners it's just in the way.
i no longer make compost as regularly, and i have come to appreciate these shortcuts,
as another way to keep making the dirt. they are not as fast acting as the flash of rapid green growth that accompanies a chemical fertilizer, but the lasting affects of large amounts of organic material and good mulch will be present and working long after that chemical fix is long gone.