“Soil fertility is the condition which results from the operation of Nature’s round, from the orderly revolution of the wheel of life, from the adoption and faithful execution of the first principle of agriculture –there must always be a perfect balance between the processes of growth and the processes of decay. The consequences of this condition are a living soil… The key to a fertile soil… is humus.” — Sir Albert Howard, An Agricultural Testament, 1940.
“Humus is the product of living matter, and the source of it.” — Albrecht Thaer, 18th century German agronomist.
In most middle school curricula, students learn about soil as a chapter in Science, Social Studies or Geography. This chapter usually discusses the physical structure of soil, types of soils, how soil is formed, soil profiles, and the crops supported bydifferent types of soils. Learning about life in soils is generally restricted to a brief introduction to earthworms, or stating that chemical pollution can destroy life in soils.
Interactions with middle school children, from both rural and urban schools, show that neither does this evoke a sense of wonder, nor an appreciation for the centrality of soil in our lives. While working with students from rural farming communities, it becomes particularly evident that what they learn from their textbooks does not relate to how they perceive soil in practice — as life-giving.
This article presents some activities that are designed to bring soils to life, making their study relevant and relateable to students — even those from rural farming communities1. These activities are shaped by our experience using them in a field programme for students of Class VI from an urban school in Hyderabad. Through these activities, we hope to encourage middle school students and teachers to explore humus — the ‘heart and soul’ of soil, integral to which are microorganisms — the often ‘unsung heroes’ of soil.
How is soil formed?
While most of us tend to think of soil as being static and inert, in reality, it is a complex medium which is constantly being formed. Most definitions of soil refer to it as a clay-humus complex, born of the fusion of mineral (derived from rocks) and organic substances (derived from plants and animals), which supports a large part of the living biomass on earth (refer Fig. 1).
This fusion of minerals and organic materials happens due to the:
(i) Breakdown of rocks — by changes in temperature that take place seasonally, erosion caused by water, corrosive action of plant roots, and action of the numerous acids secreted by microorganisms residing in soil,
(ii) Breakdown of leaf litter and animal material into debris — by the action of insects, and,
(iii) Further breakdown and transformation of the debris — by the action of microorganisms.
The slowly transforming organic material produced by the microbial breakdown of leaf litter and animal material forms humus (refer Fig. 2).
The amount of humus formed at any given location depends on the presence of oxygen, moisture, soil temperature, and the amount of carbohydrates and proteins in the litter being decomposed.
Why is humus so important for life on this planet?
Humus makes soil more fertile, improving plant growth and preventing disease. It does this by influencing the quality of soil in three key ways:
(i) Physical — Humus modifies the color, texture, structure, moisture-holding capacity, and aeration of soil. For example, soil rich in humus is looser in consistency, allowing both air (oxygen) and water to move through it easily to reach the roots of plants.
(ii) Chemical — Humus contains many nutrients, like nitrogen, that are essential for plant growth. It influences the solubility of certain essential soil minerals such as sulphur and phosphorous, by enabling their combination with elements, such as iron, into compounds that are more readily available for plant growth. The organic content in humus resists changes in pH, increasing the buffering capacity of soil. For example, excessive application of fertilizers or pesticides or contamination by highly acidic or alkaline waste material in soil rich in humus will not affect plant growth. The organic material in the humus will bind the hydrogen or hydroxyl ions, buffering soil and protecting plant life.
(iii) Biological — By acting as a source of energy for soil microorganisms, and a slow but continuous stream of nutrients for plant life, humus makes soil a better medium (more fertile) for the growth of higher plants.
Can we see humus?
The activity described in this section is probably something that a lot of teachers use in the context of soil. Some of the questions that we can explore through this activity include: Can we see humus? Is soil the same everywhere? Is there more humus in some soils than in others — why? What kind of soil is more likely to have more humus — soil from the playground/garden/under a tree/pots/cowshed etc.? Why and how do you know for sure?
Activity 1: Looking for humus!
Collect soil samples from a number of locations in and around your school campus. Decide on these locations with your students. These could include, e.g., open ground, under a tree’s canopy or any other shaded or heavily vegetated area, from a potted plant, a cultivated area (if available) or the school garden, a cowshed (if available) or a manure/compost sample (if available) etc. Assign a code for each location. Encourage your students to form groups of 5 – 6 members, with each group being assigned the task of collecting soil samples from a different location. Give each group a container, labeled with the appropriate location code, to collect their surface soil samples. Ask each group to make some notes about the location of their soil samples. Their observations can be recorded in Table 1.
Identify a location to collect your soil sample — from your playground, the school garden, under a tree, just outside the school gate. Take a few minutes to observe it.
- Use your hands or a small garden shovel to collect a sample of soil from some locations in and around your school campus. Clean each sample in a clean container or a zip-lock bag.
- Use the field, in the table, called ‘Brief Description’, to record the non-living and living components that you think influence the quality of the soil sample.
Eg: Is the location of the sample wet or dry? Does the soil sample feel moist to touch or does it feel sandy? Can you smell the wet mud or not?
Is the location barren? If not, what kind of vegetation do you see there(including grass, small plants etc)? is the vegetation mainly in the form of trees? , or are there other plants as well? is the vegetation cover dense or can you see many bare patches of soil? is there leaf litter on the ground? Did you collect soil samples below the leaf litter? etc.
3. Are there any animals or birds at the location? Did you see any earthworms, ants, or snails?
Once the different groups return with their soil samples, encourage them to observe their samples carefully and record their observations in Table 2.
|Characteristics||Sample 1||Sample 2||Sample n|
|Texture(loose/compact, dry or moist)|
|Water retention capacity*|
|Presence of root hairs,fungal filament|
|Presence of insects, worms etc.|
|Any other observations|
* To determine water retention capacity of the soil sample, place a small amount of the soil sample on a plate. Add a few drops of water to it and leave for 5 minutes. Look at the surface to see what has happened to the water and record your observations. For example, does the water remain on the surface of the soil sample, or has it disappeared to the bottom of the plate? Or is it neither on the top nor at the bottom of the plate?
When this activity was performed in a rural area, for example, students observed that the soil from around the cowshed, fields that were heavily mulched or where manure was used, was moister, darker, and had more insects and worms compared to soils.
Based on your analysis of each sample, discuss characteristics of the location from which it was collected. Typically, soils from more vegetated areas or under leaf litter will be darker, hold more water and smell like mud. This darker soil is rich in humus (refer Fig. 3).
When this activity was performed in a rural area, for example, students observed that the soil from around the cowshed, fields that were heavily mulched or where manure was used, was moister, darker, and had more insects and worms compared to soils from areas without vegetation or from fields with intensive chemical use. This led to the next question: since intensive use of synthetic fertilizers and pesticides destroys soil fertility, is there any way to bring humus back into these soils? Can farmers maintain soil fertility without chemicals, and still keep growing sufficient food?
Humus acts as a reserve and a stabilizer for organic life:
As a result of the formation and accumulation of humus, a part of the elements essential for organic life, especially carbon, nitrogen, phosphorus, sulfur, and potash, become locked up and removed from circulation. Since the most important of these elements — carbon, combined nitrogen, and available phosphorus — are present in nature in only limited concentrations, their transformation into an unavailable state, in the form of humus, tends to serve as a check upon plant life. On the other hand, since humus can undergo slow decomposition under certain favorable conditions, it tends to supply a slow but continuous stream of the elements essential for new plant growth.
Can we produce humus?
Humus, as we have seen, is produced as a result of the continual natural degradation of organic matter by soil micro-organisms. Fertile soils are therefore rich in organic matter, and provide a habitat for decomposing soil bacteria. While mineral layers of soil are found deeper down, its organic components remain concentrated near the surface — also called topsoil. Consequently, this is also the layer where soil bacteria accomplish their decomposition of organic matter to produce humus. To maintain soil fertility, it is critical that this topsoil be conserved. However, a variety of human activities, including deforestation or cultivation practices, like ploughing, destroy this surface organic layer. Although humus is produced naturally in forests and fields, it can also be produced by us, outside of these areas, through the process of composting. Farmers do this outside their farm plots, for example, by piling up organic matter and directing its fermentation through microbial action.
Compost, like humus, is made of decomposed organic material (animal dung, human waste, decaying organic material, such as food and garden scraps etc.). Not surprisingly, the most widely used method for composting is inspired from the natural site of humus formation — the forest floor. In his compelling treatise on humus and composting, The Agricultural Testament, Sir Albert Howard places a lot of importance on the location for the compost pit. Composting is ideally carried out away from direct sunlight and dominant wind. Organic matter must be piled in such a manner that the oxygen necessary for generating heat and the survival of humifying fungi must easily penetrate the pile. When all these conditions are met, the pile heats up (with temperatures rising to even 80˚C). This rise in temperature is caused by action of thermophilic microbes.
Typically, after about 15 days, the temperature falls and insects and worms start breaking down organic matter. At this stage, the humifying fungi begin to multiply and manufacture humus from the cellulose and lignin released from the decomposing plant matter. In about 8 weeks, this process of composting fulfills its role of cleaning and humifying organic matter (refer Fig. 4).
Activity 2: Making humus
To understand the relationship between humus and compost, involve students in setting up a compost pit, using the method described above, within the school premises. Once the compost pit is set up, encourage students to monitor and record the colour, smell, and appearance of the organic matter within it on the first day and, again, after a week. They can continue to monitor it twice a week till the first batch of compost is ready (in about 6 – 8 weeks). This compost can be used by students to set up a little vegetable patch. To learn about microbial activity, students could use a small stick to poke the pile/pit and check the temperature within.
The temperature can be monitored every day for the entire period and plotted in a graph. This graph can then be used to discuss the nature of microbial activity and its relationship with the formation of compost from vegetative and other organic material. It can also be used to explore how compost is produced by the action of microbes followed by that of insects and worms, thus setting up a diverse food web in the compost pit.
Reflection on this activity may lead to another question — why not use animal dung / human waste directly on soil to allow the natural production of humus? Why is it more efficient to apply compost? While this can be done, the process of composting produces a larger quantity of humus from an equal mass of organic matter. For example, 30 tons of manure/dung spread on 1 hectare of land will yield about 3 tons of humus, but when the same 30 tons is composted, it can produce about 10 tons of compost, containing as much as 5 – 6 tons of humus. In addition, the organic matter is pasteurized during composting, which minimizes the possibility of spreading any pathogens from animal dung or plant waste from the field/garden.
Going beyond the biology of soil
As is evident, in the process of learning about soil, there are many other skills that students develop and hone: observation, learning through and by their senses, systematic recording of observations and arriving at inferences based on observations. Thus, soil provides a versatile medium for interdisciplinary learning.
An exploration of soil can be used to understand micro- and macro-perspectives to ecology. For instance, students learn about the versatility of microorganisms — through their beneficial role as nutrient enablers in production of fermented foods such as curd, bread, idlis, tofu, tempeh etc. vs. their role as disease causing agents. At the other end of the spectrum, they also learn about the significance of humus and organic carbon in soils in the sequestration of carbon, a function that is critical in mitigating climate change.
Similarly, learning about soils is useful in introducing ideas of nutrient cycling, mineralization, and the significance of organic or chemical-free farming. Principles of surface tension, binding of water molecules to soil, the idea of interstitial spaces and capillary action can also be discussed and extended to an understanding of physics and chemistry.
Soil can thus be used to design a theme- based study in middle school across social studies, science, mathematics, languages (poems and essays) and history (use of soil in architecture, food, pottery).
Imagination is all it needs to make soil come alive!
1. These activities were part of a module titled ‘Soil’s Health is Your Health’ that was tested and used in a Government school and private rural school in Andhra Pradesh. Readers interested in accessing the entire module may contact the author for a copy. 2. Credits for the image used in the background of the article title: Soil photo. Pixabay, Pexels. URL: https://www.pexels.com/photo/grey-small-mushroom-on- brown-soil-68732/. License: CC0.
1. Radha Gopalan. Soil’s Health is Your Health. 2. Waksman, S.A. Humus, Origin, Chemical Composition and Importance in Nature. The Williams and Wilkins Company, USA. 1936. 3. Howard, A. 1940. An Agricultural Testament. First published in London, in 1940. First Indian edition published in 1996 by the Other India Press.