There are three essential layers of soil in the ground. Parent rock is the very underlying layer. This layer partly determines the type of soil you have. Sandstone yields sandy soil; impervious parent rock can lead to a soil that drains poorly. Porous or fractured parent rock can contribute to good drainage. Sub surface soil or, subsoil, is the second layer. This layer is mineral rich and mostly undisturbed by cultivation. This layer contains little or no organic matter and few microorganisms. Finally the first layer or top layer (topsoil) lies above the subsoil where plows, gardeners, and weather can alter it. Along with small mineral pieces, topsoils contain organic matter, air and water. This is the layer that is crucial to the growth of your garden plants and the proper balance of these components is vital to the outcome of your garden.
Knowing the type of soil you have can help you decide the right ways to enhance your soils good qualities. A good balance for soil is 5% organic matter, 25% water, 25% air, and 45% mineral (gravel, sand, silt, clay). The spaces between the soil particles are the pores of your soil - water moves through these pores along with the air that the roots and microorganisms depend on. Too little pore space and the water and air can't move freely. If there is too much pore space, (i.e.) sandy soil, these elements pass thru too quickly and few nutrients are left behind. You can do a "Clump" test to see what type of soil you are contemplating planting in. In a slightly damp soil take a small amount into you hand and make a fist, if the soil doesn't hold together it is too sandy of a soil for planting most of your plants. This type of soil would be good for cacti or other desert succulents. If the soil stays in a clump and doesn't crumble at all it is too dense and will need to be mixed with a medium to open up the pores. A good soil will clump in your hand and crumble slightly when you wiggle your fingers.
Now we move on to the fertilizing of the soil. This is a crucial process to organic gardeners. If you feed the soil it will take care of your plants. With organic gardening this is a delicate process. Plants need smaller amounts of so called micronutrients and there are eight nutrients considered essential for plant growth. These nutrients are iron, manganese, boron, copper, zinc, molybdenum, chlorine and nickel. All of which occur in very small quantities in most soils. Sometimes referred to as trace elements.
Organic fertilizers contain an abundance of trace elements. Organic matter is plant and animal matter that is in the process of decay. Most importantly grass clippings, leaves, hay, straw, pine needles, wood chips, sawdust, and manure. For some easy ways to add organic matter to your garden try the following: Till compost into vegetable gardens, leave grass clippings on the lawn, and try adding wood-chip mulch to perennial beds.
Good healthy soil is not a one time deal, it is an ongoing process of providing the organisms your plants depend on.
The best organic matter is compost, which is organic matter that has decomposed until they resemble loamy soil. If you are buying your compost you should look for the words "certified organic", or "OMRI Listed" seal, or anything that states it is approved for organic growing. You don't want to buy something that comes from a contaminated source like, sewage sludge. Compost should look dark in color and have a crumbly texture with no large pieces of undecomposed matter. If water oozes our when you squeeze a handful of the material it's too wet; if it blows away easily it is too dry. Last but not least it should have an earthy smell without a strong ammonia or sour smell.
Of course making your own compost will be the best way to ensure high-quality compost and save some money. For the last two growing seasons we have been using colloidal humus with excellent results.