Soil horizons are the vertical appearance of different layers of the soil whose physical, chemical, biological characteristics differ from the layer above and below. We can examine the soil horizons and their characters simply by digging the earth vertically and by examining different layers of the soil formed below parallel to the ground.
Mostly soils have three major horizons (A, B, C) and some have an organic horizon (O) at the top. O (Humus or organic) horizon is composed of mostly organic matter such as decomposing leaves. The O horizon is thin in some soils, thick in others, and not present at all in others. A (Topsoil) horizon is rich in minerals with organic matter incorporated. A horizon is important for plants and other organisms to live and generally 8 to 10 inch in depth. B (Subsoil) horizon is composed of minerals that leach (moved down) from the A horizon and accumulate here. C (Parent material) horizon is composed of the inorganic material that is coming from the bedrock. R (Bed Rock) horizon is the mass of rock such as granite, basalt, quartzite, limestone or sandstone that forms the parent material through weathering.
We can also examine the texture and composition of soil of different horizons to understand that their composition and respective depths vary from location to location and their physic-chemical characteristics also vary accordingly. The formation of horizons and their characteristics are also reflective of the environmental and ecological conditions of their area.
Spade or shovel, notebook, colored pens or pencils, zip-lock soil sampling bag, measuring tape, magnifying glass.
1- Identify the locations where you wish to examine soil horizons (Presence of grasses, trees and other types of vegetation are usually a good indicator that you can dig a successful pit).
2- Using a spade or shovel dig a pit that is at least 1 meter (3 feet) deep. The pit should be wide enough that you can easily observe the different layers (horizons) of soil all the way to the bottom of the pit. Usually this means digging a pit that is as wide as it is deep.
3- Carefully observe the vertical profile and formation of soil layers above and below from each other.
4- Starting at the top of the profile and working your way down to the bottom of the profile, determine the types of soil horizons (O, A, E, B, C, R) present and the depths at which horizons are located. Sometimes the horizons do not show distinct boundaries therefore discuss as a group and decide the types and depths of horizons and mark them.
5- Take depth measurement at the top and bottom of each horizon using the measuring tape or cale starting with 0 cm at the top of the profile.
6- Make a sketch of the horizons types and depths in your field notebook.
7- Take photograph of the pit in which all horizons are clearly visible. Take some photographs of the surrounding landscape to record the ecological conditions of the location.
8- Examine each horizon in-situ for moisture, color, smell, texture, and structure and record in the field notebook.
9- Collection of soil sample from each identified horizon will help in further identifying their physico-chemical characteristics.
# You can also observe soil horizons where some digging is going on
● In your field notebook, describe characteristics of the site that could influence the soil profile. This includes features like: vegetation types (percent of coverage and heights), land use (urban, agricultural, garden, forest etc), buildings or other artificial features, surrounding landforms and water bodies, approximate slope of the site and exposure to sun and shade.
● In order to determine the moisture, color, smell, texture, and structure of soil of each horizon you can examine directly at pit or after collecting the soil from each horizon and bringing the samples to the test facility, or you can attempt both in-situ examination as well as sample examination. Describe these characteristics for each horizon in your notebook -
● Moisture - Take some soil between fingers and check whether it is dry (no feel of moisture), moist (feels damp), or wet (feels saturated);
● Color and smell – If the collected soil is dry, you will need to spray some water on it to make it wet in order to best observe the color and smell. You can know more about soil colours through our activity on Soil Colour.
● Structure - With the help of magnifying glass examine the general structure of the soil in each layer, consistency, presence or absence of rock, plant roots and animals seen etc.
● Calcareousness - Put some drops of lemon juice or mild acid on the soil, watch the effervescence in terms of Nil, Low, Medium or High. Nil effervescence means the soil is not calcareous and whereas production of effervescence indicates calcareous nature.
● Precautions - If the soil pit has been dug for the examination of soil horizons then before leaving refill the pit before leaving from the site. Take care that you have not forgotten any equipment at the site.
● Present your results in the form of numerical data as well as qualitative characteristics. Various interpretations can be done through mathematical and/or statistical calculations.
● Photographic evidences and sketches and diagrams of the studied soil horizons will be useful for further reference.
Key points learned –
This activity helps in finding and interpreting the facts like -
● What does soil profiles tells us?
● What effect does the soil type have on the amount of soil moisture it can hold?
● Observing the profile you are able to understand the newly-created soil and well developed soil.
● Explore in your area about different soil types and find the difference between soil horizons of an arid area, forest, grassland, agriculture fields etc?
● Make replica of soil horizons by collecting soil samples of different horizons and putting them in the same order and proportional depth in a glass container. Illustrative examples of soil horizon replicas are given here –
