Soil is a loose mixture of rock fragments, organic material, water, and air that can accommodate the growth of vegetation. Soils differ according to what type of rock that the soil is weathered from (an example, Bedrock). The rock formation that is the source of mineral fragments in the soil is called the parent rock. Soil that is stationed above its parent rock is known as residual soil, which soil that has been moved away from its parent rock is known as transported soil. Causes for the transportation of soil are rivers, wind, or movement of glaciers.
Some soils can't support the growth of vegetation (forest soil) and some soils can support the growth of vegetation (temperate soil). To better comprehend the subject of soil, you will need to study its [soil] properties, such as soil texture, soil structure, and soil fertility.
Soil Texture and Structure
Soil is made up of sized particles with different sizes. These particles can be as large as sand (2 mm). Other particles can be too small to the point that we will need the help of a microscope to see them! Soil texture is the soil quality that is based on the proportions of soil particles.
Soil texture affects the soil's consistency, which is the soil's ability to be worked and broken up for farming (ex: Soil Texture that has a large proportion of clay can be hard and difficult for farmers to break up). Soil Texture influences the "infiltration", or ability of water to move through soil. Soil should be able to allow the access of water [to get to the plants' roots], but, the soil should be able to allow ENOUGH water, meaning: not to completely saturate the soil.
Water and air movement through soil is also influenced by soil structure, which is the arrangement of soil particles. Soil particles are not always evenly spread out. Often, one type of soil particle will clump in an area. A clump of one type of soil can either block/help water flow, which affects the soil's moisture, or simply to put: "soil moisture".
Soil Fertility is the ability for soil to keep a hold of nutrients and to supply nutrients to a plant. Some soils are rich in necessary nutrients, like iron, while some soils do not have the necessary nutrients and aren't able to supply [the necessary nutrients]. A lot of nutrients in soil come from the parent rock, while others come from a organic material named humus. Humus forms from decayed remains of plants/animals. These remains are broken down into nutrients by decomposes, such as bacteria and fungi.
Due to the formation of soil, soil is usually in a series of layers: Humus-rich soil, sediments, and then, bedrock. Geologists, or an expert in studying the Earth, call these layers "horizons".
The top layer of soil is usually called the term, "topsoil". Topsoil contains the most humus out of all the other layers, so that is why it is ESSENTIAL for farmers to have good topsoil, because humus is rich in the nutrients for the plants to be healthy!
- A Horizon - Consists of the topsoil, which contains a lot of humus. Soils in forest have an O horizon.
- B Horizon - Collects dissolving substances and nutrients deposited from the upper horizons.
- E horizon - The A and the B horizon = combined!
- C/R horizon - Bedrock: Partially-No weathered/weathering.
Soils can be acidic or basic. The pH scale is used to measure if soil is acidic or basic, and how acidic and basic it is! The scale ranges from 0-14, 7 being neutral. Soil that has a pH below 7 is acidic, while soil that has a pH above 7 is basic. Different plants need different nutrients, so the right pH for a soil depends on the plants growing in it. Soil that have a high or low pH deprive their plants of certain/different nutrients (depending on the pH).
Soil and Climate
Soil types vary from place to place, typically because of the differences of the climate.
Tropical Rain Forest Climates
In tropical rain forest climates, the air is very humid and the land receives a large amount of rain. Because of the warm temperatures, crops can be grown all year long! The warm soil temperature also allows dead plants and animals to decay easily, which provides humus to the soil. Because of this, we may think that the tropical rain forest contain some of the best soil.
But in reality, the tropical rain forest soils are nutrient poor:
- The heavy raining [that goes on] in this climate leach important nutrients from the topsoil into deeper layers of soil, causing the topsoil to be extremely thin.
- Lush vegetation has a great demand for nutrients. The nutrients that aren't leached away are quickly taken up by plants and trees that live off the soil.
Deserts barely get any rain, which causes many problems, such as very low rates of weathering and less ability to support plant/animal life. Low rate of weathering = soil is created at a slower rate.
Though barely any rainfall, some water is available from the groundwater. Though, when groundwater seeps to the surface, the water evaporates. When the water evaporates, it leaves off any materials that were dissolved in the water. Often, the materials left behind are various types of salts. These salts can become toxic when they are concentrated to a certain point, which can poison plants. Death Valley is an example of this.
Arctic areas have little to no precipitation. In arctic climates, chemical weathering occurs very slowly (possibly due to the lack of rain!), which means soil formation occurs very slowly. Slow soil formation = Thin, unable to support many plants. In addition to the unfortunate mess, arctic climates have low soil temperatures, which means the decomposition of plants and animals happen slowly/or they just stop completely. Slow decomposition limits the amount of humus in the soil, which limits the nutrients available [for the plants].
Temperate Forest and Grassland Climates
Much of the US has a temperate climate (usually in the midwest), which has coined the term "breadbasket". This is because temperate soils are one of the best soils you can find:
- Get enough rain to cause a high level of chemical weathering (remember, lots of weathering = soil created at a high rate).
- Frost action (temperature changes), and as a result of frost action = thick, fertile soil.
Soil Conservation Techniques
In order to maintain the fertility of soil and protect them from erosion and nutrient, different techniques have been created in order to fulfill this need:
- Contour plowing - A farmer plows across the slope of the hills, creating rows [that act as a series of dams instead of a series of rivers].
- Terracing - Same method as contour plowing, but just applied onto steep hills.
- Cover crop - Crops that restore nutrients to the soil that is essential for other crops.
- No-till farming - Protective to soils by preventing erosion and making a cover.
In the 1900s in the southern part of the US, the soil [in the south] had lack nutrients by farming only one crop, which was cotton. George Washington Carver urged farmers to plant soybeans and peanuts instead of cotton. Some plants, such as soybeans and peanuts, helped restore essential nutrients to the soil. These plants are known as cover crops (mentioned above). These crops are planed between harvests to replace certain nutrients [needed for other crops] and prevent erosion (by providing a cover from wind and rain).
Then there is crop rotation, which is pretty much what it sounds like. Changing the crop that is being used on a certain field.