One of the great things about organic gardening is that it costs so much less. You don't need to buy expensive pesticides, herbicides, and soil amendments. But that doesn't mean you don't need to amend your soil!
My own garden is managed pretty intensively, I try to keep it growing something as often as possible when there is sun available. Because of this I have to be very careful about rotation, adding nutrients, and managing pests. Here are some tips to help keep your soil healthy.
Follow with Fallow: If you have the space to let beds lay fallow for a year or two, by all means do so. Since I'm working with so few square feet I don't have the luxury of letting it lie fallow. Instead I make a point of rotating my veggies from bed to bed each year. Certain ones have to stay in place, such as the pumpkins, but I can skip a year of pumpkin growing if needed to manage pest problems.
Rotate Plantings: To keep your garden soil healthy and productive you should always move plant families from one space to another each year. Not just individual plants, but plant families! If you grew broccoli in a space one year, don't follow it with cabbage the next, instead try to follow it with a plant from a completely different family such as onions, carrots, or tomatoes. Soil dwelling pests that were attracted to the broccoli initially may have laid their eggs in the soil, when they hatch they will be sorely disappointed to find there is no broccoli (or anything similar) there anymore. Similarly molds and diseases that developed around tomatoes will have no nearby victims if that space is followed with a planting of garlic, and your garden won't be as likely to suffer from them. In addition, following fruit-bearing plants like peppers and tomatoes with root- or leaf- crops can keep your soil going longer without amendment. Fruit crops use up a lot of nutrition, it requires much more effort to grow a plant, flower it, and ripen the fruits than it does to grow just the leaves, or the leaves and roots. This can give your soil the break it needs between fruit crop plantings.
I Plead the 5th: There are 5 primary organic practices to amend your soil, all of which add organic matter and nutrition to it. Bio mulch of some type; fallen leaves, hay, etc. Use these to mulch your summer garden to keep the ground moist. After the crops are gone leave the mulch on the beds to rot then dig it into the soil in late winter or early spring. Breaking down this mulch will use a lot of the nitrogen from your soil, but once it is decomposed the nitrogen levels will return to normal, so be sure to give the soil bacteria a couple months between digging it in and planting new veggies. Hay costs between $3 and $6 per bale from farms, and can generally be found by checking Craigslist or asking at a local farm in the spring. Growing a cover crop over the winter season such as oats or winter wheat also keeps your soil protected from erosion over the winter and then is turned over into the soil just like a mulch would be. Adding compost is amendment number three. The very best compost is made of a blend of leaves, grasses (lawn cuttings or hay), fruits or veggies that were not eaten, and plant waste from the kitchen. For some people composting is an art form, I am not one of them. My compost pile is not likely the most nutritious in the area, but it does the job. Jack-o-lanterns, pulled out plants, peels, stems, cores, rotting hay bales, raked up leaves, all of these things go into my compost pile. Why bother with a compost pile? Because nature has a cycle. Things grow from nutrients in the soil, they feed us, they die, they are decomposed and the nutrients are returned to the soil. Why would you want to screw that up? It's a perfect cycle in which you can dispose of all the kitchen waste from your harvests, then use it a few months later to feed your new plants. We'll go in depth on composting at another time though. The last two items on the organic 5 amendment list are tea and nutrient specifics. Tea, or compost tea in this case, is nothing more than water that has had a "tea bag" of compost sitting in it for a short time. The nutrients in the compost are leached out into the water making a muddy brown "tea" that can be spritzed directly onto plants or poured into the soil to add a boost of nutrition. If a plant is struggling to survive an insect infestation, or you are transplanting a new plant a splash of tea can really give them a boost. Nutrient specifics are a little more challenging, these are particular items that add a specific nutrient to your soil or compost. Before worrying about these you should do soil testing to see if your soil is already in good shape. Some examples of nutrient specific amendments are listed below.
N-P-K, often listed on fertilizer bags stands for Nitrogen, Phosphorus, and Potassium; each of which aids different types of plant growth.
The macro nutrients NPK; generally a well balanced compost will supply all of these without any additions.
Nitrogen (N) - legumes (beans, peas), cow and poultry manures well-rotted only.
Phosphorus (P) - banana peels, crab shells, shrimp peelings, most grains and nuts, and guano
Potassium (potash, K) - fruit skins, esp. banana peels.
Micro nutrients, these can be more challenging to get in compost and commercial fertilizers don't generally contain them, which means an organic garden is more likely to be healthier!
Calcium (Ca) - eggshells, dolomitic lime
Magnesium (Mg) - manures, compost, dolomitic lime, and epsom salts.
Iron, Manganese, Zinc, Copper and Cobalt (Fe, Mn, Zn, Cu, Co) - kelp in its many forms; meal, powder, liquid.
pH amendments are used to adjust the soil acidity, they either sweeten or acidify soils. Generally a good organic matter rich soil will not need these but certain plants prefer acidic soils, and some areas naturally have more acid soils and need to be sweetened.
Sweeteners - dolomitic lime, wood ashes.
Acidifiers - coffee grounds, vinegar, compost and manure.
While all of these things work to improve soils you may choose between them based on your own preferences; for example I prefer not to buy soil improvements if possible so would be more likely to use wood ash to sweeten soil rather than buying dolomitic lime. Also some soil amendments sold at stores like blood meal, bone meal, and fish emulsion are by-products from other industries. Personally I have no problem with using by-products in my soil; but I don't trust that the animals used to make the by-products were in good health and don't contain chemicals that could leach into my soil, so I stick with plant by-products as much as possible. I also try to make sure they come from varied sources, using one plant's compost (i.e. mushroom compost) could leave your soil short on nutrients that particular plant doesn't use as much.
Remember to test your soil before adding amendments to it, except maybe for compost. Compost is just all around good, and won't throw your soil out of balance.