Most of the dry land on the earth is covered with soil.
Soil is made when rocks break and crumble.
Plants grow in soil.
Soil is the home for millions of animals.
Soil erosion is the wearing away of soil by wind and water.
What is soil?
Rocks are broken by earthquakes and volcanoes. Rain, frost, sun, wind, water and ice all help to break up the rocks. It takes thousands and thousands of years for the rocks to turn into soil. This is called weathering.
When plants die they rot in the soil. Rotting plants change into humus. Humus and plants hold the soil together. Humus makes the soil healthy for new plants. The healthier the soil, the healthier will be the plants that grow in it.
The soil is the home for millions of animals. These animals make burrows and tunnels under the ground. Burrowing and crawling through the soil, the animals turn it over and mix it up. Air for the animals to breathe gets into the soil. The roots of plants need the air too. Rainwater runs into the soil and waters the plants.
Before a farmer plants crops, the soil is ploughed. Ploughing beaks up the soil and makes spaces for air and water which the plants will need as they grow.
Types of Soil
Soil is made up of finely ground rock particles. There are three main types: clay, silt, and sand. Each kind can be identified by the size of its particles, by its texture and by tests that show:
(a) how long particles stay suspended in water
(b) the ribbon test, which tests whether or not the soil stays in a ball when moist and squeezed tight.
Most soils are a combination of the three types of particles.
Clay particles are the smallest and lightest. When suspended in water, clay particles are generally the last to settle on the bottom. The texture is sticky when moist, but hardens and cracks when dry.
There is not much space between the clay particles, so it is difficult for plants to grow and survive because it is hard for the roots to penetrate the clay, and for air to circulate through it.
Moist clay can be pressed into a ball, and because the particles bond tightly, it is good for making pottery.
Silt particles are medium-sized. The texture is smooth and slippery. When suspended in water, silt particles will settle after sand, but before clay. When moist, silt cannot be pressed together to form a ball. Silt particles do not hold together as strongly as clay and are more easily eroded (washed or blown away) by water and wind than clay. Silty soils hold water and nutrients well and are good for growing plants.
Sand particles are the largest of all soil particles, and the texture is gritty. There are larger spaces between the particles. When suspended in water, sand generally falls to the bottom first. When moist, the sandy soil will not form a ball when squeezed and falls apart. The soil does not hold much water.
Loam is a combination of soils
Loam is a combination of soil types so the particle size varies and the texture is neither gritty nor smooth. When loam is suspended in water, the different soil particles settle in fairly equal layers. Loam is excellent for growing plants because the sand helps lighten the soil, the clay helps bind the particles together and holds water, and the silt helps hold nutrients needed by plants and animals.
Erosion: the wearing away of soil
Erosion is the wearing away of the topsoil. Topsoil is the top layer of soil. It contains the most organic, nutrient-rich materials and is the layer needed crops.
Erosion happens when there is too much rain and there is a flood. The flood waters wash away the soil. Winds also blow the soil away. This is erosion too.
Erosion also happens when too many trees are cut down. This is called deforestation. The trees were holding the soil together. When they are gone, the winds and water, blow and wash the soil away.
Erosion is also causes by some farming practices. Farmers need to protect farmland from the wind by planting windbreaks around their land. They should move herds of sheep and cattle around so that grasses will have time to grow back and help keep the soil free from erosion.
There are several types of soil erosion.
Sheet and rill erosion occurs on sloping land with little ground cover. When it rains water runs down the hill, and without plants to hold the soil in place, the flowing water washes the soil away.
Sheet erosion is when the water removes even layers of soil.
Rill erosion is when the water makes channels up to 30 cm deep.
Gully erosion is when water makes a deep channel that washes away soil when it rains. Each time it rains, the channels get deeper as more soil is removed. The soil can wash away into creeks and streams and block the water flow and dis-colour the water, or damage roads. The loss of topsoil reduces the amount of area available for farming.
Mass movement is when the erosion is helped by gravity, including landslides and avalanches. Mass movement not only removes a great deal of earth and rocks, it can destroy houses and farmland.
Wind erosion is when the wind lifts and removes topsoil. In dry areas in particular, soil that is not kept in place by plants is easily removed by the wind. Where crops have been grown repeatedly without giving the land a rest, the soil becomes less bound together and easily breaks down and is removed. Where animals have grazed too much or have trampled the earth hard, the plants are no longer holding the soil together, and it can be removed by the wind. The wind dumps the soil elsewhere, and can clog rivers and streams, other farmland and roads. It can cause dust storms.
Wind and water erosion is not confined to farmlands. The coastline of Australia shows the effects of wind and waves on the rocks.
Conserving the soil: How can erosion be controlled?
Farming methods used should be suited to the conditions. Crops need to be rotated around fields so that the soil is less disturbed by constant ploughing.
Stubble left in the fields, particularly canola stubble, helps prevent wind erosion and are useful as grazing land.
The numbers of stock on the land should be enough so they do not over graze the land, but are spread evenly over the whole area.
Where there is bare land suffering water runoff, plants such as grasses should be planted to hold the soil and keep it from being washed away.
To stop erosion, people plant trees to hold the soil together.
Grasses , shrubs and trees planted alongside waterways will stop water from eroding the banks.