Get a soil test from your state agriculture extension agency and add what's recommended/needed.

It's free in some states.

It's worth doing at least once no matter how small your plot is just to find out what you're starting with.

Where I'm at it's free, but I use a small two-raised-bed setup and check it every 3 years because it's not in intensive production.

Most of the store-bought tests are not worth it vs. getting the real deal out of a soil analysis.

The addition of well-aged garden compost in spring and fall will improve almost any soil in a few years time and will provide all the major and minor nutrients and trace elements your garden soil will need to support your crops. Even if your soil test show the garden is lacking, the addition 1 or 2 inches of well-aged compost twice a year will turn the garden around in a couple of years. Well-aged manure can be added in fall along with the compost and also in spring for many crops--but not all. If you have the soil test done by the cooperative extension ask them to recommend fertilizers for your soil--you won't have to guess.

Here's a general soil care calendar:

FALL: Work in any summer green manure and or plant a green manure winter cover crop. Add two inches of well-aged compost and manure. Send soil sample to the cooperative extension for testing (or do it yourself chemical soil test). Apply lime or phosphorus fertilizers and organic matter as dictated by soil test. Leave a rough surface particularly if the soil is fairly heavy and if the soil in poor condition and would benefit form frost heavage.

WINTER. The garden soil will do its own work during winter months provided organic matter was added in the fall. Allow early winter freezing to kill off harmful grubs. A thick mulch applied along with hot fresh manure on a cold day in late winter will encourage the soil to warm up that much earlier in spring.

SPRING. During early spring months, soil must be aired out and allowed to dry before plants and seeds are started. Incorporate winter green manures at least two to three weeks before planting seed. Summer green manures can be planted for early tillage in late spring or for summer or fall tillage. Make a final pH test of the soil to be sure that any problems were corrected by fall liming or organic matter amendments. Apply potassium and nitrogen fertilizers along with additional organic matter as necessary. Level the soil and rake smooth and add a 1 or 2 inch top layer of well-aged compost. Gently water the soil in preparation for seed planting.

SUMMER. Mulch with a thin layer--an inch or less--of garden compost or other organic mulch after plants have developed several sets of leaves. Periodically check soil moisture with moisture gauges.

GROWING SEASON SIDE-DRESSING: Compost or manure tea will give most plants the boost they need during the growing season. An early and mid-season application will suffice for most plants; but heavy feeders would probably appreciate compost tea every few weeks up until fruit set.

Here is a link that might be useful: HarvestToTable.com

Thanks for the tip NC. Thanks A LOT for all the info Steve, very helpful. Just a couple of questions. Do you think there is any benefit from mulching all year round? I know some organic permaculture types recommend doing this. Also wondering what you think of foliar sprays of seaweed or fish emulsion?

You plants will get most of their nutrients to grow through their roots so make sure you mix in compost before you plant and try to maintain a thin layer of compost on your beds throughout the growing season. Mulching when the garden is resting will protect beneficial organisms you've attracted to your soil.

Seaweed, fish, and compost tea foliar sprays give plants an added boost. I like to use it at flowering and fruit set for tomatoes, peppers, cukes, melons, and strawberries. But because its gentle, it can be used at any time during the year. I don't use it more than once or twice per plant per growing season though.

Keep in mind that anything that you are adding to the soil surface to provide nitrogen or potassium will need to be leached to get the N and K into the soil. If you live in a dry climate and/or drip irrigate the water soluable N and K in the amendment will not leach into the root zone. That isn't a problem if you overhead water or have incorporated the material into the soil deep enough for the roots to get it. Manure tea is a way around this as the N and K are leached into the tea water from the (usually) manure which can then be applied to the soil near the plants.