Test your soil
The first step to amending soil is to measure its nutrients, pH and texture/drainage, which you can do with sample holes 3′ deep in several areas of the yard and a soil test. Because a good lab soil test can be expensive and you may have many micro soil environments, consider buying a good soil testing kit.
Two approaches
The cheap and easy approach is to choose plants suitable to existing soil and growing conditions. Amending soil can be expensive and there are many fine unusual and common plants suitable for the range of soils. For a list of such plants call extension office in your state. This site focuses primarily on the second approach which is to choose your plants and adjust your soil as necessary. In larger gardens you will probably want to use both of these approaches, perhaps amending the growing areas close to the house and limiting your plantings further away to those which can tough it out.
An analogy
Soil functions in many ways as a stomach and the analogy provides a general basis for decisions. As our stomachs require food to support a the organisms in our digestive system, which in turn releases energy to our body, soil requires a steady supply of organic mater to support the worms and micro organisms which digest this into available plant nutrients. Trace minerals and other supplements, like vitamins, complement but are no substitute for the staple of organic matter. Balance and moderation are key.
Soil facts
Studies on amendment techniques suggest the best approach is to amend the entire bed rather than the planting hole. This is particularly important for trees and shrubs who’s roots will otherwise become root-bound, less drought tolerant and the plants more likely to fall over. Long term, trees and shrubs will do better with no amendment at all than amending only their planting hole, although they will grow slower in the first few years. Similarly, plants will stay in the top layers of the soil unless the amendments you add are tilled into the subsoil.
Adding nutrients to Soil
Depending on the results of your soil test you will want to add varying quantities of Nitrogen, Potassium, Phosphorus and trace minerals. This applies to either sandy or clayey soils. Although chemical methods can do this effectively the following is an organic approach. For Nitrogen add Cottonseed meal; for Phosphorus add Rock Phosphate and for Potassium, Kelp meal (kelp also contains helpful trace minerals). To adjust your pH use dolomite lime if soil is too acid or Ferrous sulfate to raise acidity. Amendments are best applied by evenly covering the surface of the area to amend and tilling in as deeply as possible.
Amending clay soil texture
Clay has the asset of holding water but is easily compacted to hard pan or turned into a gooey mush. For this reason the best time to amend clay soil is when it is damp (not too wet). Coarse organic material such as aged bark or rough compost is the best amendment, although if drainage is the only issue 5/8″ clean crushed gravel or a similar material will also work. If adding bark you will need to compensate for the nitrogen it will tie up in the soil by adding additional nitrogen and possibly lime to adjust the pH. A 2-6″ layer of amendment (the amount depending on how extreme your soil is) tilled in well is usually adequate for planting.
Amending sandy soil texture
Sandy soil is easier to amend then clay. The goal in amending sandy soil is to add sponge like organic matter in the form of peat, composted bark dust, manure and/or compost to help it to retain more water as well as provide nutrients. Again, 2-6″ is usually adequate.
Double digging
This arduous approach to soil amending will greatly help in situations of poor drainage or hard pan where deep rooted vegetables or trees and shrubs are planned, although it will improve any growing situation. After rototilling your texture amendments and nutrients into the soil use a shovel or garden fork to clear a trench about 5′ by 1′ as deep as you have tilled, placing this soil in a mound the length of the trench behind you. Now fork or dig a trench inside the trench an additional 8″-12″ deep placing this subsoil behind you on top of the soil from the upper trench. Move forward 1′ and dig another upper trench, filling the lower part of your first trench with the soil. Again, create a trench within this trench, adding the subsoil to the topsoil now in your first trench. Repeat this pattern until the area is complete. Now repeat whatever steps you took to amend the first 8-12″, adding texture and nutrients before tilling in. An easier way with less beneficial results is to simply loosen the subsoil in the lower trench, keeping the subsoil below and not rototilling again.
Maintaining your amended soil
A 2-4″ layer of bark or other coarse organic mulch topped of yearly will not only maintain the supply of organic matter, provide protection from the sun and reduce erosion but can also reduce water needs by 30% and weeds by 80%! This makes mulching the most important maintenance saving thing you can do and the best way to maintain healthy soil.