FOUR MAIN PARTS TO SOIL
1. Minerals – comes from the weathering (chemical, physical) of the parent material. The inorganic material in soil is made up of rock material mainly. It can be sand, silt or clay. The particles in each of these rock types are ordered from larger to smaller with sand being the largest and clay particles being the smallest. The best type of soil is called loam and has just the right mixture of sand, silt, clay and humus.
2. Bacteria and Organic Materials –bacteria break down organic materials (leaves, dead animals, bugs etc.), returning nutrients to the soil. This is called humus, and gives soil its dark colour (this part also known as topsoil). Topsoil formation is a slow process. Over the last 6000-10 000 years only 15-25 cm of topsoil have formed in Canadian forests, and 40-100 cm of topsoil in the grasslands of the prairies.
3. Air –plants need air around their roots. Air in soil can be caused by humus, burrowing worms, insects and small animals.
4. Moisture – water is needed for physical and chemical weathering to help weather rock and break down nutrients. It is also taken up by plant roots.
SOIL PROFILE – shows different horizons, or layers of the earth, from the parent material, to the topsoil. Each horizon has different physical, biological and chemical characteristics.
SOIL AND NATURAL VEGETATION
The main factors affecting the type of flora in any part of the landscape and its distribution are the soils, the prevailing climate and the relief. Although many plants can, in principal, tolerate a wide range of conditions, virtually all species will have a set of conditions that are ideal for their needs and beyond this set of ideal conditions the plants will struggle more to survive and compete against other plants that are more suited to those differing prevailing conditions.
The main soil factors influencing the flora in any place are acidity or alkalinity and the degree of wetness. Soils help to manage the distribution of water coming from precipitation (rain, snow and hail).
Surface flow: this is water which flows over the land’s surface to the river
Soil water: some water passes into the soil and remains there for some time. Here it is available for plant roots.
Percolation: this is water which soaks down into the soil to the water table and the aquifer below. Sandy soils have rapid percolation, whereas clay soils have very slow percolation.
There is a huge combination of soil/climate/topographic conditions worldwide and this gives rise to a wondrous range of flora across the world.
SOIL FUNCTIONS
Soils provide many important functions for plant, animals and humans. The most obvious is for supporting growing crops, plants and trees in the wild, on the farm and in our gardens – but there are lots of others too. For example soil is very important as a home, or habitat, for the teeming millions of soil organisms. It is quite amazing how much variety of life exists in soil.
Soil is extremely important as a filter removing pollution from our drinking water and helping to regulate the flow of water through the landscape. Most rainwater ends up moving into the soil before it gets to plant roots, the aquifers or the river! Soil is also the foundation for our buildings and roads. Houses and schools are built on soil. The type of soil affects how buildings are made!
Soil also protects our history and past – archaeological finds are dug from the soil. Soil plays an important part in the preservation of the earth’s history.
Finally, soils are important in the story of climate change. Soil organic matter is one of the major pools of carbon in the biosphere and is important both as a driver of climatic change and as a response variable to climate change, capable of acting both as a source and sink of carbon. Soils also helps regulate other greenhouse gases such as nitrous oxide and methane.
THREATS TO SOIL
Soil erosion is a major problem affecting soils all over the world. It is a process that involves the removal and transport of soil by wind and water. While erosion is a natural process, human activities such as changes in farming and land management practices, can cause it to occur much faster than under natural conditions.
The rapid growth of the world’s population has resulted in increased cultivation of land. This puts more pressure on land and leads to soil losing its structure and cohesion, which means that it can be eroded more easily. Heavy farming machinery can also ‘compact’ soil, which causes water to run straight off the surface after rain, taking soil particles with it, instead of infiltrating into the soil. Changes in farming methods are not the only ways in which humans are contributing to soil erosion; activities such as construction and even recreation, such as hill walking, have led to severe erosion. \
SOIL NUTRIENTS
The most important parts of the nutrient cycle relate to the exchange of nutrients between three main sources (See the website diagram):
(i) that in the above ground plants and animals;
(ii) that within the soil, specifically within the organic matter;
(iii) and that in inorganic form in the soil consisting of inorganic ions derived from various sources including weathering of minerals in the soil, ions in solution in the soil and ions absorbed onto the surfaces of minerals in the soil. The cycling of nutrients between these main sources constitutes a major part of the nutrient cycles and is one of the main reasons why plants, animals and humans are able to live on earth. List the 6 sources of nutrients for soil shown in the website diagram.
