Topsoil looks dark in color because it consists of different forms of rocks, minerals, plants, and animal remains that determine its nutrient content. Moreover, the quantity of minerals and nutrients in the soil determines the difference in color between the top and subsoils.
Topsoil is dark in color because it consists mainly of decomposed organic material which makes up the substance known as humus. The decomposed organic compounds are mainly carbon-based which is known for its characteristic black color and when mixed with other minerals already contained in the soil, contributes to the dark color of topsoil.
Soil is a renewable natural resource, providing the physical foundation and chemical support to life. Plant growth and development are highly dependent on the readily available nutrients within the region of soil known as topsoil.
Therefore, to understand the differences in soil types and their nutrient content, we need to look at the factors which contribute to its dark color.
What Gives Topsoil Its Dark Color?
Many features help us distinguish between different types of soils. These factors include soils texture, structure, color, and depths of the soil horizons.
Structure refers to how soil particles are held together. Texture means the coarseness or fineness of the soil particles, while horizons’ depths refer to the thickness of the individual soil profile.
Lastly, color refers to the blackness or redness, and its darkness or lightness.
Talking about the depth of the soil horizons, there is topsoil and subsoil. The subsoil is the layer just below the top surface of the soil.
It is mainly made up of sandy and clay deposits and does not contain much organic material. The subsoil is usually referred to as ‘B’ horizon.
Topsoil refers to the general top layer, about 2 to 8 inches, of soil. It is usually characterized by more fertility and is rich in organic material. It also has an active, thriving micro-fauna population that makes it deep brown or black.
This is the reason it is referred to as black earth or black soil.
See our detailed articles on Mixing mulch into soil and Using rotted wood to amend Soil which explains how they add organic material into the soil and boost microbiological activity.
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Reasons Why Topsoil is Dark
Presence of Organic Matter
Topsoil generally contains decomposing organic materials. These organic materials include leaves, stalks, plant roots, dead animals and wastes, and insects.
The decomposition of these organic matters contributes to the deposition of organic waste that eventually turns to humus. Therefore, humus is a complex, carbon-rich mixture of degrading organic compounds in polymers form.
Degrading components contain water-extractable humic acids or humates, which are dark brown or black. They also contain remains that are black and insoluble in water.
Therefore, these by-products contribute to the dark color of the topsoil because they contain a high carbon concentration.
Additionally, humus also contains fluvic acids, proteins, and lignin, although they do not impact the dark color.
Presence of Minerals Such as Iron and Manganes
The presence of minerals also contributes to the dark color of the topsoil. For instance, black soils contain either high iron or manganese minerals.
Additionally, the topsoil contains two major iron compounds; iron pyrite and iron sulfide. Similarly, high manganese concentration also impacts the black color of the soil.
Color of the parent material
The color of the topsoil is also determined by the color of the parent materials from which it was formed.
If the topsoil is formed from darker materials such as peat or muck, it will be darker. However, if it developed from sand or light-colored rocks, it will also be lighter.
Age
Topsoil’s color is also based on the age of the soil. For instance, as the age of soil, much of its organic matter breaks down, making the soils lose their dark color.
Therefore, the darker color of the topsoil may be due to its young age.
Slope
Soils on the lower grounds or depressions appear to be much darker than soils on the hills’ tops. This is because the darker topsoil on the hills is washed down to the lower grounds, leaving the hills with lighter topsoil.
Additionally, there is higher moisture on the lower land, which enhances organic matter growth that gives the topsoil its dark color. Not forgetting higher moisture slows the decaying of organic matter in the lower topsoil.
Different Types of Soils
Farmers will classify soil in different categories depending on what they intend to grow. These types have distinct features that provide numerous benefits and limitations to plants.
Therefore, identifying the type of soil is crucial in supporting the plant’s life and organic matters in the soil. Here are the major types of soils;
Sandy Soil
Sandy soil is warm, light, dry, and more acidic. It is also low in nutrients and has a high proportion of sand and little clay. Sandy soils have higher drainage and are easy to work with because they warm up quickly in springs.
However, they dry out quickly in summer and contain fewer nutrients that are easily washed away by rain.
Clay Soils
Clay soil consists of finer particles than other soil types. These particles are less than 0.002mm in size and consist of microscopic particles derived from rock chemical decomposition.
Clay soil particles stick together readily to form a sticky texture when mixed with water.
Clay soil holds a higher amount of water due to the spaces between the particles. It also expands when mixed with water and shrinks when dry.
Additionally, the organic matter in clay soil is highly compressible with high strength when dry. This is why it is majorly considered for construction as mud mortar.
Silt Soil
Silt is a type of soil with high moisture retention and fertility rate. This is because silt soil contains medium-sized particles that are well-drained and hold moisture longer. However, these fine particles can be easily compacted and washed away with the rain.
Therefore, the addition of organic matter can help to bind the loose particles into stable clumps.
Loam Soil
Loam soil combines clay, sand, and silt soils. It combines the benefits of all other soil types, including textures, water retention, air circulation, drainage, and fertility.
Loam soils are generally fertile, easy to till, and offer suitable growth requirements to plants. They are also black or dark brown due to the presence of organic matter.
What Should Good Soil Look Like?
The question is, ‘what is good soil?’ Whether you are an engineer or a farmer, you may want to see your garden soils with special features that support plant growth.
These features include good drainage, easy compactions, fertility, and easy to till. However, what does good soil mean to trees and plants?
Plants need water, nutrients, and oxygen for their growth. They also need to be protected against pests and diseases to grow and look better.
Therefore, with little knowledge, practice, and work, you can achieve good soil for your farming activities, no matter your location.
However, how do you identify good soil? Here are some of the characteristics of good soil.
Presence of soil organisms –
Plenty of living or dead organic matter in the soil symbolizes good soil. These soils organisms include spiders, ground beetles, earthworms, and centipedes.
To test for organic matters in the soils, dig a hole about six inches deep. Count how many living organisms you find in the soil chunk.
Good soil should contain more than ten critters like earthworms or centipedes. These critters help to slow the occurrence of soil pests and diseases.
Additionally, the decomposition of these organic materials binds the smaller soil particles together, increasing soil aeration.
It also improves the soil’s absorption and drainage. However, anything below three earthworms in the soil chunk indicates a bad soil unsuitable for plant growth.
Soil structure –
Soil structures refer to how the soil particles are packed together in clumps called aggregates. In an agricultural view, soil structure impacts how good or bad the soil is and is determined by how well we treat the soil.
A good soil structure is soft and crumbly, with granular aggregates that stick together even in water. On the other hand, lousy soil is characterized by blocky, platy, columnar, or structureless single-grain sands or massive clay soil.
A farmer can achieve good soil structure by adding organic matter such as compost and mulch to the bad soil. They also improve it by avoiding compaction of soil disturbance.
For that case, does tilling improve the quality of the soil?
Tilling affects good soil, both negatively and positively. This is because tilling helps a compacted, poor-structured soil to regain its right structure.
However, continuous tilling destroys the soil aggregate, hence turning the right soil into a bad state. Therefore, it is recommended to check the soil structure before massive tilling to avoid destroying it.
Soil pH
Soil pH refers to the soil’s acidity or alkalinity; a higher value indicates alkalinity, while a lower value indicates acidity. From the basic sol logarithms, pH6 is ten times more acidic than pH7, while pH5 is 100 times more acidic than pH7.
This means plants do well in soils with a pH between 6.5 to 7 because it is slightly acidic. Lower acidity promotes the growth of soil organisms and nutrients.
However, some plants grow at lower pH than 5.5, while others survive at a pH above 7.5, although the list is shorter.
Adding lime may improve the soil pH, making the soil suitable, although the changes may be negligible, especially on subsoils. Therefore, the addition of lime or sulfur can improve the quality of the topsoil.
Additionally, clay soil pH is not significantly affected by the buffering practices and needs further treatments to make it good. Therefore, it is recommended to stick on plants adapted to a specific pH to avoid plant deaths.
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Water availability and filtration
The presence of water also indicates either good or bad soil. This means water availability and how long it takes for the plant to show signs of thirst is a primary concern for farmers.
For instance, if the plants require frequent watering, it means you have bad soil. This is because good soil prevents water evaporation and drainage.
Water filtration is the process by which water reaches plant roots. Good water filtration should also prevent runoff, erosion, and allow air circulation in the soil.
Therefore, if the soil allows water to reach the plant roots effectively, it is a good soil sign.
Soil Fertility and Plant Growth
Soil fertility refers to the soil’s ability to sustain plant growth.
In other words, it refers to the soil’s ability to provide plant habitat and give sustainable and consistent high-quality yields. Fertile soil is characterized by;
- The ability to supply adequate plant nutrients and water and sustain balanced plant growth and reproduction.
- The absence of toxic substances that hinder plant growth.
Mostly, farmers treat soil averagely and assume increasing fertility since it does not show any apparent stress. However, a wise farmer needs to care for the soils because human life depends on the topsoil for survival. Therefore, the following exercises can positively soil fertility and support plant growth;
- Maintaining sufficient soil depth for good root development and water retention.
- Maintaining good internal drainage to allow adequate aeration.
- Maintaining a conducive soil pH between 5.5 and 7.0 for optimal plant growth. Other plants grow better in more acidic or alkaline soils.
- Improving plant nutrient concentrations.
- Improving the presence of microorganic matters that support plant growth and prevent soil diseases.
Final Words
Topsoil is the primary soil layer that supports plant growth due to its nutrient-richness and suitable soil pH. It is also dark or black-colored due to organic matter and minerals such as iron and manganese.
Therefore, topsoil supports plant growth due to adequate water-soluble inorganic salts and microorganisms.
The dark color is lost during erosion by rainwater as the nutrients are washed away in colloids form. This leaves the topsoil lighter with fewer nutrients.
Additionally, soil fertility can be lost through an excessive accumulation of unwanted matters, which depletes useful salts.
Improper irrigation and acidic rain also deplete soil fertility. Therefore, this fertility can be enhanced through adequate irrigation and good drainage.
