Earth Science

PEDOSPHERE LIVING SKIN OF EARTH (SOIL)

            Soil is made from portions of the geosphere, atmosphere, hydrosphere, and biosphere. It is generally composed of 45% mineral (gravel, sand, silt, and clay), 25% air, 25% water, and 5% organic matter (humus, roots, and dead and decaying organisms). These components reflect the process that formed the soil.

§  Soil Formation

            Soil forms when rock weathers. Weathering, or the breakdown of rocks, may result from physical or chemical changes. Soil formation may be slow or a rapid process, depending on the factors at play.

             Five factors the affect the soil formation:

1.      Parent Material - The material or source material is important in soil formation because its chemistry and type will determine the soil that will be formed. Soils formed on site through the weathering of the underlying rock called residual soils. However, most soils are transported from their source to another area by agents of erosion and transportation such as water, wind, and ice.

2.      Climate - Temperature, rainfall, and moisture affects the pattern and intensity of soil-forming process such as weathering, leaching, transportation, and distribution. Climate also affects the type of organisms, biological activity, and rates of chemical reactions. Thus, soil types vary depending on climate.

3.      Topography – The gradient of the slope water flow and erosion. Soils that form in steep slopes tend to be thinner because of the higher rates of erosion. Slopes aspect, which is the direction of the slope face, affects temperature and slopes as facing the sun are warmer. As a result, different types of soil form in different landforms.

4.      Biological factors - Organisms such as plants, animals, microorganisms, and humans affect soil formation. Animals and microorganisms mix the soil through burrowing, while plant roots aid in the weathering process as they grow. Microorganisms also affect nutrient and chemical exchanges between roots and soil. Human activities such as agriculture and urbanization have positive and negative effects on the quality and quantity of soil.

5.      Time - The formation of soil is along and continuous process which may take hundreds to thousands of years depending on the climate and environment. In moderate climates, it takes 200 to 400 years to develop 1 can of soil. The rate is faster in tropical climates and much longer in dry and cold climates.

§  Soil Texture

            Soil texture is defined as the relative proportion of the particle sizes in soil- sand, silt, and clay. Soil is naturally composed of a mixture of these particles and the proportion of which affects other soil properties such as porosity and water retention. The smallest of these particles is clay (<0.002 mm), followed by silt (0.002-0.005 mm), and sand of varying sizes (0.005-2.00 mm). Particles larger than coarse-grained sand are called gravel and rock if they are >75.oo mm.

            The proportions of clay, silt, and sand are plotted in the soil texture triangle to determine its soil type based on particle size. The ideal for agricultural use is loam. Loam provides good aeration and drainage properties of large particles with nutrient-retention and water-holding abilities of clay particles.

            Nicholas L. Galvez is considered as the “Dean of Filipino Soil Scientists”. He laid the foundation for the different fields of soil science in the Philippines. He trained Filipino soil scientists at the University of the Philippines College of Agriculture (UPCA) for 42 years. He is recognized internationally with his scientific papers on soil chemistry, soil mineralogy, and other aspect of soil science. Dr. Galvez obtained his bachelor’s degree in chemistry from the University of Minnesota in the United States and hid doctorate degree in soil science from the University of Gottingen in Germany.

§  Soil Profile

            Soil formation is a gradual process which involves the development of succession of zones or soil horizons. Each horizon has a distinct set of physical, chemical, and biological characteristics. The sequence of soil horizons from the surface down to the underlying bedrock is called soil profile. Soil profiles vary depending on climate, topography, rock type or parent materials, biological activity, and time.

            Soil scientists use the capital letters O, A, B, C and E to identify the soil horizons. Most soils have three major horizons: A, B, and C horizons.

1.      Surface Horizon (A) - is composed of mineral matter mixed with some dark organic humus.

2.      Subsoil (B) - is the accumulated clay and other nutrients from the layers above it.

3.      Substratum (C) - is composed of loose or partially altered parent material.

4.      Organic horizon (O) - composed of loose or partly decayed matter.

5.      Horizon (E) - characterized by a significant loss of minerals (eluviations) and leaching.

6.      Hard Bedrock (R) - is not a soil, while the layer of loose, heterogeneous, and superficial material covering the bedrock is collectively called regolith.

§  Soil Orders

Soil scientists also developed a soil classification system to identify, understand, and manage soils. The most general level of classification is the soil order consisting of 12 types.

1.      Gelisols - Frozen soils found in the coldest regions on Earth.

2.      Histosols - High organic content and wet.

3.      Spodosols - Sandy and acidic found in moist climates that often support dense forests.

4.      Andisols - Composed of volcanic ash.

5.       Oxisols - Very weathered and common in tropical climates.

6.      Vertisols - Claylike soils that shrink and swell.

7.      Aridisols - Very dry soils in arid regions.

8.      Ultisols - Weathered soils.

9.      Mollisols - Deep and fertile soils.

10.  Alfisols - Moderately weathered productive soils found in temperature and humid regions.

11.  Inceptisols - Slightly developed, young soils found on steep slopes and mountain ranges.

12.  Entisols - Newly-formed soils found in steep rocky lands.

§  Soil and Soil Quality

            Soil is not dirt – it is a resource. It is a main component of land resources, agriculture, and ecological sustainability. It also provides food and foundation for shelter. Soil is important in sustaining man’s existence and provides humans with the following services:

1.      Arable land for agriculture – The word “arable” comes from the Latin arabilis, which means “able to be plowed”. Thus, arable lands are plowable lands which could be used to grow crops. An important component of arable lands is soil that can sustain plant and animal life. According to statistics from the World Bank, the agricultural land in the Philippines from 2011-2015 is 41.7% of its total land area.

2.      Regulating water and filtering potential pollutants – In the water cycle, soil plays an important part in absorbing water and storing it as groundwater. Water and dissolved solutes also flow over the land or into and through the soil. The minerals and microbes in soil are also responsible for filtering, degrading, immobilizing, and detoxifying organic and inorganic materials.

3.      Nutrient cycling – Carbon, nitrogen, phosphorus, and other essential nutrients are stored, transformed, and cycled in the soil. Nitrogen in the atmosphere, for example, cannot be readily used by organisms. It must be first be converted into ammonium by nitrogen-fixing bacteria and then into nitrates by nitrifying bacteria before it can be assimilated by plants and transferred into the food chain. All of these processes occur within the soil.

4.      Foundation and support – Soil structure provides a base for plant roots. Soils along the bedrock also provide foundation and support for human shelter and structures such as houses and roads.

5.      Mineral deposits – Soils are mined for their mineral content – whether it is iron, nickel, or aluminum. These soils are called laterites. They are commonly formed in hot and wet tropical areas. Laterites are mostly rusty-red in color because of their high iron oxide content. They develop through an intensive and extended chemical weathering of the underlying parent rock, resulting in a thick accumulation of metals in the soil.

§  How Human Activities Degrade Soil Quality

            Soil is a nonrenewable, finite resource, which is generally not recoverable within a human lifespan. It is an important natural resource which is often overlooked. The increasing demands of an ever-growing population put pressure on the world’s soil resources due to intensive agriculture and fertilizer use, land conversions, and rapid urbanization.

            The Food and Agriculture Organization of the United Nations (UNFAO) define soil degradation as a change in the soil health status, resulting in a diminished capacity of the ecosystem to provide goods and services for its beneficiaries. Degraded soils cannot provide the goods and services required by its ecosystem.

1.      Soil erosion causes the loss of topsoil and nutrients in the soil. It is indeed the most visible effect of soil degradation. Soil erosion is a natural process but is often made much worse by poor management practices.

2.      Soil compaction reduces the amount of air, water, and space available to roots and soil organisms. An example of compaction is caused repeated traffic or travelling on wet soil.

3.      Desertification is the irreversible change of the land to such a state where it can no longer be recovered for its original use. It is characterized by droughts and arid conditions as a result of human activities and exploitation. Desertification is a major threat to world soils and currently affects about one-sixth of the world’s population and a quarter of the world’s land.

4.      Intensive agriculture has been increasing sing 1960’s in an effort to feed the world population more efficiently. However, this intensification of agriculture has led to the use of more and heavier machinery, deforestation, and clearing of land for cultivation. As a consequence, these activities led to the loss of organic matter, soil compaction, and damage to soil physical properties. The over-application of nutrients with fertilizers and pesticides led to contaminated soil and water supplies, leading to the further loss of soil fauna and flora which are essential parts of healthy soil.

5.      Urbanization or the increasing population has led to the conversion of land to urban centers which are generally characterized by concrete structures, roads, and pavements. These concrete structures represent a significant loss of soil, which is more or less permanent as soil as is sealed with concrete. Such sealing hinders the soil’s interaction with the atmosphere hydrosphere, and biosphere which affects nutrient cycling and other soil services.

            According to the UNFAO, 33% of global soil is moderately to highly degrade through erosion, salinization, compaction, acidification, chemical pollution, and nutrient depletion which hamper soil functions and limit food production.

§  Conservation of Soil Resource

5 Practical Ways to Conserve Soil

1.      Plant trees and groundcover – The vegetative cover acts as a barrier against erosion by wind and softens the impact of rain. Roots stabilize the soil and, at the same time, naturally and slowly loosen to soil and draw nutrients into the plants.

2.      Employ “no dig” gardening techniques – Digging can loosen compacted soil, but it can also disturb the structure and leave it more vulnerable to erosion from wind and water.

3.      Avoid compacting soil – Do not walk on soil that is soaking wet. Create dedicated paths in gardens and wilderness areas and stick to them.

4.      Follow the trail – When hiking, do not make shortcuts that will destroy plant life and enhance erosion.

5.      Control rainwater runoff – Create suitable runoff channels and drains to help prevent soil erosion.

            The effects of human activities on soil can be mitigated through sustainable management, such as increasing soil organic matter content, keeping the soil surface vegetated, avoiding excessive tillage, using nutrients wisely, promoting crop rotations, reducing erosion, and preventing soil compaction.

1.      Increasing soil organic matter – The addition of new organic matter is important in improving and maintaining soil quality. It also improves soil structure and enhances water and nutrient-holding capacity.

2.      Keeping the soil covered and vegetated – Ground cover and vegetation protects the soil. It also provides habitats for larger soil organisms and can improve water availability.

3.      Avoid excessive tillage – Tillage is done to loosen surface soil and break up soil structure. Reducing it minimizes the loss of organic matter and protects the soil surface from further erosion.

4.       Managing pests and nutrients efficiently – Efficient pest and nutrient management require regular testing and monitoring of soil conditions and pests, along with the application of only the necessary chemicals at the right time and place.

5.      Promoting crop rotation – Changing vegetation across the landscape over time allows the soil to recover, as different plants contribute in a unique way to soil structure and composition. It also increases the diversity of plants as well as other organisms in the area.

6.      Reducing erosion and preventing soil compaction – Erosion can be prevented by keeping the ground covered and vegetated, and by channeling excess surface water runoff. Soil compaction can be prevented by restricting human activities to designated pathways and areas.