Soil is made up of an extensive variety of substances, minerals, and rocks. These substances can be categorized into four main groups. These groups are organic materials, inorganic materials, air, and water.

Organic Materials In Soil

Organic matter makes up only a small fraction of the total materials in soil, yet it is extremely important. This organic material is made up of living organisms such as plants and animals, dead plants and animals, and nutrients that have come from decomposed plants and animals.

The amount of life present in the soil is surprisingly large. Take for example just a small handful of dirt, that you might pick up outside in your backyard. In your hand you likely hold several hundred trillion different microscopic life forms. Now consider your entire back yard. Within that small portion of the Earth’s soil, are likely hundreds and hundreds of trillions of life forms, including over 100,000 earth worms in a typical sized yard.

Inorganic Materials In Soil

The inorganic materials found in soils account for about half of the total mass of most soil. These inorganic materials take the form of sand, silt, and clay, and are referred to commonly as dirt.

They form as rocks are eroded by the forces of weather, both wind and rain.

Much of the sand, and silt is small, but visible grains of rock. However, some of it can become so fine that it is microscopic, especially when it comes to clay.

Clay is an extremely important part of soil. Because of its porous nature, water is able to seep through clay with ease. Most clays have a negative charge, which attract positively charged particles suspended or dissolved in water. As a result, many important chemical reactions take place in clay.

Air In Soil

About half of the total mass of soil is made of up vast interconnecting cavities, or holes. These holes are filled with both air, and water. The amount of air and the amount of water filling these spaces varies dramatically throughout the year, and from location to location, but averages about 50% each.

The air found is soil is very different from the air found in the atmosphere. The air in soils is not exposed to moving air currents, and is much more moist, or humid than atmospheric air. It also tends to be very rich in carbon dioxide, and poor in oxygen.

Oxygen is removed by living organisms, and carbon dioxide is left behind. This carbon dioxide leaks out of the soil, slowly replenishing the atmospheres carbon dioxide supplies.

Water In Soil

Water is one of the most important ingredients in any soil. Without water, soil formation would not be possible.

Water enters soil via a number of different processes. The most common are through precipitation, such as rain and snow. This precipitation enters the soil and drains down into it.

In many cases, water also enters soil from the dirt beneath it, as water climbs upward to fill the empty spaces between particles of dirt.

Many substances dissolve into water, and are carried from one portion of the soil to another. Water makes chemical reactions in the soil possible, and supplies micro-organisms with the water necessary for life.

Water leaves soil via evaporation, as well as through drainage. Excessive water can rob soils of their nutrients, by carrying them away to other locations. This process is known as leaching.

The maximum amount of water that a soil can handle is known as the soil’s field capacity. When a soil is at field capacity, it means that all available pores in the soil are already filled with water, and that there is no room for more water to enter.

In temperate areas, where four seasons are experienced, it is not uncommon for soils to reach field capacity during the Winter, when low temperatures inhibit plants from using water, and also decrease evaporation.

Conversely, during the Summer the opposite can take place. Plants require great amounts of water to survive, causing their roots to drain the soil’s water supply. Increased evaporation also decreases water levels, creating a deficit, where the soil has less water than it needs to support the life forms living with in it. When this happens, plants begin to wilt, and animals begin to die.