Biology : Soil
Humus
*It is a major source of the key soil nutrients like nitrogen and phosphorous. *It increases the water-holding capacity of the soil *It encourages the soil particles to bind together in soil crumbs. A soil with a good crumb structure has good drainage, is well aerated and easy to work.
What are the two types of fertilizers?
*Natural Fertilizers *Chemical Fertilizers
Sandy Soil
*Particle Size - close to 2.0mm *Air Content - good due to large air spaces between soil particle *Water Holding Capacity - poor water retention. water drains through the large air spaces quickly *Drainage - good drainage, water drains through large air spaces quickly * Mineral Ion Content - low, since there is rapid drainage, leeching occurs
Clay Soil
*Particle Size - less than 0.02mm *Air Content - poor due to small air spaces between tiny soil particles *Water Holding Capacity - very good water retention. large amount of water is held by capillarity *Drainage - poor drainage, soil is easily water logged *Mineral Ion Content - high since the soil drains slowly and attacks ions
Loam Soil
*Particle Size - medium: between size of clay and sandy soil particles *Air Content - Moderate due to air spaces being between those of sandy and clay soil *Water Holding Capacity - relatively good water retention due to moderate air spaces. enough water is retained for plant growth *Drainage - relatively good / moderate drainage * Mineral Ion Content - good enough for healthy plant growth. rich in humus and minerals
Natural Fertilizers
*generally inexpensive *may be difficult to use since they are bulky and sticky e.g. manure *longer lasting - they depend on decomposition which is a slow process *improve the crumb structure of the soil and therefore improve more qualities than just nutrients *if not decayed properly it may be toxic to the plants *encourage earthworms
Chemical Fertilizers
*generally more expensive *usually are available in powder and granular form and are easy to apply *dissolve quickly into the soil and therefore quick to act - however are short lasting in the soil *can lead to the breakdown of soil crumbs resulting in erosion: can also lead to acid formation *chemicals may stay in the environment for a long time and may harm other plants and animals *often harmful to earthowrms
Ways of Reducing Soil Erosion?
*terracing of hillsides *suitable crop pattern
What are the four main components of Soil?
*the mineral matter *the organic matter *the soil air *the soil water
Types of Soil
1. Clay Soil 2. Sandy Soil 3. Loam Soil
The Components of Soil
1. Topsoil 2. Subsoil 3. Bedrock
Clay Soils
A clay soil is made up of a very small particles packed tightly together. Because the particles are so closely packed, there are very small spaces between them.
Sand Soils
A sandy soil is made up of larger soil particles that cannot pack very closely together.There are large spaces between them. Sandy soils thus contain a lot of air. This means that they heat up more quickly than clay soils, because air is exchanged easily between the soil and the atmosphere.
Importance of Organic Matter
Adds mineral salts to the soil. Improves the crumb structure of the soil. A soil with a good crumb structure has good drainage, is well aerated, absorbs and retains water and mineral salts and helps to prevent erosion by binding soil particles together. It also helps to insulate soil against extremes of heat and cold.
Bedrock
Below the subsoil is the bedrock (third soil layer). This is the rock from which the soil above has been formed progressively over the years.
Subsoil
Below the topsoil is the subsoil (second soil layer). Only the deepest roots penetrate this far down and only few animals burrow this deep.
Capillarity
Capillarity is the tendency for water to move into very narrow spaces.
The Organic Matter
Dead organic matter and living organisms make up the organic content of soils.
Natural Fertilizers
Fertilizers may be organic in origin when they are derived from plants and animals. These are called natural fertilizers and include manure, peat, compost and treated sewage.
Importance of Mineral Matter
Forms the 'soil skeleton' providing support and anchorage for plant roots. Provides a shelter for soil animals and a surface for the attachment of micro-organisms.
Magnesium Mg
Function - essential component of chlorophyll Effect of Shortage - yellowed leaves, poor growth
Nitrogen N
Function - essential for protein production, vegetative growth and chlorophyll formation Effect of Shortage - spindly growth; pale leaves; reduced cropping
Phosphorous P
Function - essential for root development Effect of Shortage - slow growth and smaller crops
Potassium K
Function - influences chlorophyll production; controls the action of nitrogen and phosphorus in plant growth; regulates stomatal opening Effect of Shortage - leaf decay, poor cropping especially of grains and tubers
How is Humus formed?
Humus is formed from dead organic matter. It is a dark brown sticky material which forms a coating around soil particles.
Fertilizers (Plants)
If the soil is unable to provide these nutrients then plant growth will decrease, and eventually plants may die.
Soil Water (Leaching)
Leaching occurs when chemicals and mineral salts are washed out from the soil by rain. As the rain water passes through the soil.=, these substances drain out with the water.
Lime
Lime is calcium hydroxide, and is often added to clay soils. It makes clay particles dump together or flocculate into large 'crumbs'. The large soil 'crumbs' improve drainage and allow more air into clay soils.
The Organic Matter (Humus)
Living organisms that are found in soils include bacteria, fungi, worms, insects, burrowing animals and plant roots. Some of the smaller animals are detrivores which feed on the dead organic matter, and bacteria and fungi are decomposers - they help to form humus. Plants roots bind the soil particles together preventing erosion.
Loams
Loams are 'ideal soils' and contain about 50% sand, 30% clay, 12% humus and 8% lime.
Importance of Soil Air
Oxygen in the soil air is essential for respiration of plant roots and soil organisms, including decomposers. Nitrogen in the soil air is necessary for nitrogen-fixing bacteria to form nitrates.
The Organic Matter (Peat)
Peat is a special kind of soil that is formed from partial decay of organic matter, due to a lack or oxygen. It is almost totally organic in origin, and contains only a very small amount of mineral matter. Peat soils are acidic, and can be improved by adding lime which increases the alkalinity which is better for most plants.
Importance of Living Organisms
Plant roots bind soil particles together preventing erosion. Earthworms: *improve aeration and drainage by burrowing *improve fertility by decomposing of their waste, dead bodies and litter they pull into their burrows *bring materials to the upper layers *grind the coarse soil particles in their gizzard
Fertilizers (Plants)
Plants have basic needs just as animals and other living organisms. Examples of these needs include sunlight, water, carbon dioxide, nitrogen, oxygen, mineral salts (calcium, potassium, phosphorous). The soil is responsible for providing the basic needs of the plants. The soil may loose its ability to provide nutrients and offer support to the plants due to leaching or soil erosion or overuse. There are common practices by man that may cause leaching or erosion. These particles include deforestation, poor farming methods and overgrazing.
When does Soil Erosion occur?
Soil Erosion occurs when the topsoil (humus, air and living organisms) is removed from the land. It may occur naturally by wind and rain but, when land is cleared for cultivation or squatting, the soil can be eroded much more quickly. The subsoil that is left is much less fertile and the land can support less vegetation and fewer crops.
How is Soil formed?
Soil is formed by the breaking down of rock. Over many years, rocks are weathered by wind, water and drastic changes in environmental temperature. They are broken down into particles of different sizes. These particles are slowly colonized by small plants which add humus when they die. Larger plants and animals move in, and after thousands of years, soil, which is a complex mixture of inorganic and organic materials is produced.
What does soil water contain?
Soil water contains dissolved chemicals that have been produced by weathering and decomposing. There is a constant movement of dissolved chemicals, from the soil into the solution, and from the solution into plants.
Importance to Soil Water
Soil water is essential for photosynthesis, as it is taken to the leaves. It prevents desiccation of organisms like earthworms. It dissolves mineral salts for absorption by the plant roots
The Soil Water
Soil water, obtained mainly from rainfall, is held in a thin film around the soil particles. Any additional water drains away. The smaller the soil particles, the larger is their surface area and the more water is held.
The Mineral Matter
Soils are composed of inorganic rock particles of varying size, classes as gravel, stone and fine earth. The fine earth is divided into clay, slit and sand, which are classified by size: *clay: less than 0.002 mm *silt: 0.002-0.02 mm *sand: 0.02-2.0 mm Anything that is lager than 2.0 mm is gravel.
Topsoil
The layer (first soil layer) in which most plants roots are anchored and where the most soil-living animals live.
What is Soil Erosion?
The physical removal of the exposed top soil by wind and rain
The Soil Air
The soil air occupies the spaces between the particles and is readily displaced by soil water.
Chemical Fertilizers
These inorganic fertilizers are also called synthetic or artificial fertilizers. They usually contain nitrogen, phosphorous and potassium.
Leaching
To remove soluble or other constituents from by the action of a percolating liquid
Waterlogged
When the spaces in between the soil fill up with water and there is no room for air.
Drainage
With large spaces, though water is not held by capillarity and water drains from the soil quickly. Minerals are also no attracted to the sand particles and can be washed out of sandy soils more quickly.
