Have you ever wondered what dirt is made of? The simple answer is that dirt comes from rocks and organic matter. The characteristics of the dirt depend on the weather, which rocks and where they are located, what kind of organic matter and how much time has elapsed.
You might ask how in the world does this happen. Well, the climate has a major effect on the breakdown of solid materials. Alternate freezing and thawing as well as wet and dry cycles will accelerate the breakup of rocks and over time smaller and smaller particles are formed. The rocks are responsible for soil texture and sometimes alkalinity. Soils that are produced from limestone are often finely textured, neutral to alkaline and fertile. Soft shale rock can develop into a heavy clay soil and sandstone becomes a coarse sandy soil. Granite usually turns into a sandy loam, slightly acidic soil.
The topography or shape of the land surface also plays a role in soil development. Soils are often deeper in low areas and on flat surfaces and shallow on slopes. Low places that are in dry regions can often turn into salty soil where as low areas in wet climates can become peat bogs.
Organic matter is the source of many nutrients in the soil. Nutrients are released into the soil during the process of animal and plant decay or in the form of animal wastes such as manure. Living creatures also have an essential part in soil development. Earthworms and moles help aerate the soil by burrowing. People play a role by moving large amounts of dirt or by simply turning over the soil in their own gardens. Over the span of thousands of years these factors work together to create the soil we see today.
But wait, only about half of the dirt or soil is made up of solid material like rock, organic matter and living soil creatures (worms, ants, fungi and bacteria). The other half of soil consists of air and water. If you’re lucky these will be in approximate equal amounts. Air is essential because it supplies oxygen to soil creatures and plant roots, without it both would suffocate. Air also makes good drainage possible.
Many of the nutrients and other chemicals vital for healthy plants are dissolved in the water. This nutrient filled water is held on the surface of the soil particles. Clay soils will hold more water because the soil particles are smaller and there is more surface area. Sand particles are larger, but more porous so they have less surface area and therefore do not hold water very well. This is why clay soils stay wet longer and sandy soils dry out quickly, requiring more irrigation and fertilization. A good mix of soil will include 50% solid material, 25% air and 25% water and include soil particles of various sizes.
Sources:
Secrets to Great Soil by Elizabeth P. Stell
"A Storey Publishing Book" 1998