All Form One Agriculture topics
Read Topic 2 free on this page. The other 9 topics are ready in the Click Online Academy app.
- Topic 1Introduction to AgricultureIn the app
- Topic 2The Agricultural SoilFree preview below
- Topic 3Introduction to Horticulture ProductionIn the app
- Topic 4General Principles and Practices of Vegetable ProductionIn the app
- Topic 5Production of AmaranthIn the app
- Topic 6Production of CabbageIn the app
- Topic 7Production of OnionsIn the app
- Topic 8Production of TomatoIn the app
- Topic 9Introduction to Poultry ProductionIn the app
- Topic 10General Principles and Practices of Chicken ProductionIn the app
Section 1Key terms and concepts
Master these definitions before you begin. Examiners often award full marks for precise wording.
- Soil
- The loose, upper layer of the earth’s surface made up of weathered rock particles, organic matter, water and air. It supports plant life.
- Agricultural soil
- Soil that is used or managed for growing crops and rearing livestock. Its quality directly affects food production.
- Soil texture
- The proportion of sand, silt and clay particles in a soil. It determines how the soil feels and behaves.
- Soil structure
- The way individual soil particles are arranged and grouped together into aggregates or clumps called peds.
- Porosity
- The amount of pore space (tiny holes) between soil particles that holds air and water.
- Permeability
- The ability of soil to allow water and air to pass through it. Sandy soils have high permeability; clay soils have low.
- Soil pH
- A measure of how acidic or alkaline a soil is on a scale from 0 to 14. Most crops grow well at pH 6.0–7.5.
- Cation Exchange Capacity (CEC)
- The ability of soil to hold and exchange positively charged nutrients (calcium, potassium, magnesium) so plants can use them.
- Soil organic matter
- Decaying plant and animal remains in the soil, including humus. It improves soil fertility and structure.
- Humus
- The dark, stable part of organic matter that results from complete decomposition. It is rich in plant nutrients and helps soil retain moisture.
- Soil microorganisms
- Tiny living things in the soil that cannot be seen without a microscope (bacteria, fungi, protozoa).
- Macroorganisms
- Larger soil creatures visible to the naked eye (earthworms, termites, ants, beetles).
- Soil health
- The continued ability of soil to function as a living ecosystem that sustains plants, animals and humans.
- Leaching
- The washing away of dissolved nutrients from the top layers of soil into deeper layers where plant roots cannot reach them.
- Soil conservation
- Practices that protect the soil from erosion, loss of fertility and other forms of degradation.
Section 2The concept of agricultural soil
What is soil?
To a farmer, soil is not just “dirt.” It is a living, breathing natural resource that gives life to crops and pastures. Scientifically, soil is the topmost layer of the earth’s crust made up of:
- Mineral matter (about 45%): Sand, silt and clay from weathered rocks.
- Organic matter (about 5%): Decomposed plants and animals, plus living organisms.
- Water (about 25%): Held in the pore spaces.
- Air (about 25%): Also held in pore spaces, giving roots and soil organisms oxygen.
What makes soil “agricultural soil”?
Not all soils are suitable for farming. Agricultural soil has the right physical, chemical and biological properties to support crop growth and animal pasture. A good agricultural soil can:
- Anchor plant roots firmly
- Supply essential nutrients
- Hold and release water for plant use
- Allow excess water to drain (avoid waterlogging)
- Provide a habitat for helpful soil organisms
- Resist erosion and degradation
Importance of agricultural soil
- Foundation of all crop production — without fertile soil there is no food
- Provides nutrients, water and air for plants
- Supports livestock through pasture and fodder crops
- Stores and purifies water that later feeds wells and streams
- Home to organisms that recycle nutrients and improve the soil itself
A handful of soil is a living city
One teaspoon of healthy soil can contain more living organisms than there are people on Earth. Bacteria, fungi, earthworms and countless other creatures work together to turn dead leaves into plant food — nature’s own recycling system.
Section 3Physical properties of soil
Physical properties are characteristics we can see, touch and measure without changing the soil’s chemical make-up. They largely determine how soil handles water, air and root growth.
3.1 Soil texture
Soil texture is the relative proportions of sand, silt and clay.
| Particle | Size (diameter) | Feel when rubbed | Properties |
|---|---|---|---|
| Sand | 2.0 – 0.02 mm | Gritty | Large pores; drains quickly; low nutrient holding |
| Silt | 0.02 – 0.002 mm | Smooth / silky | Medium pores; holds some water and nutrients |
| Clay | < 0.002 mm | Sticky when wet | Tiny pores; holds water and nutrients well; drains slowly |
Loam — a balanced mixture of sand, silt and clay — is the best texture for most crops because it combines good drainage with good water and nutrient holding capacity.
3.2 Soil structure
Structure is how particles are arranged into aggregates (peds). Good structure has crumbly, porous aggregates that allow roots, water and air to move freely. Poor structure may be hard and compacted or powdery.
3.3 Porosity and permeability
- Porosity — the amount of pore space that holds air and water.
- Permeability — how easily water and air pass through the soil. Sandy soils = high permeability; clay soils = low permeability.
Section 4Chemical properties of soil
4.1 Soil pH
Soil pH measures acidity or alkalinity on a scale from 0 to 14:
- pH < 7 = acidic
- pH = 7 = neutral
- pH > 7 = alkaline
Most crops grow best at pH 6.0–7.5. Outside this range, many nutrients become unavailable to plants.
Correcting pH:
- Too acidic → add lime (calcium carbonate)
- Too alkaline → add organic matter or sulphur
4.2 Cation Exchange Capacity (CEC)
CEC is the soil’s ability to hold and exchange positively charged nutrients (Ca²⁺, K⁺, Mg²⁺). Clay and organic matter have high CEC; sandy soils have low CEC.
4.3 Essential nutrients
- Macronutrients: Nitrogen (N), Phosphorus (P), Potassium (K) — needed in large amounts
- Micronutrients: Iron, zinc, copper, manganese, boron — needed in small amounts
4.4 Salinity and leaching
Leaching washes dissolved nutrients from the topsoil into deeper layers. Salinity (excess salts) can harm plants; managed by good drainage and salt-tolerant crops.
Section 5Biological properties of soil
A handful of healthy soil contains billions of living organisms — the secret behind soil fertility.
5.1 Macroorganisms (visible)
- Earthworms — burrow, mix and aerate soil; their casts are rich in nutrients; a sign of healthy soil
- Termites and ants — decompose dead wood and plant matter; tunnels improve water infiltration
- Beetles, centipedes, millipedes — shred and break down organic matter
5.2 Microorganisms (microscopic)
- Bacteria — most numerous; some decompose organic matter; others fix nitrogen (e.g. Rhizobium in legume root nodules)
- Fungi — break down tough plant material; mycorrhizal fungi help roots absorb water and phosphorus
- Actinomycetes — produce the “earthy” smell; help make humus
- Protozoa and nematodes — some release nutrients; others may be pests
5.3 Organic matter and humus
When leaves, roots and dead organisms fall to the ground they are decomposed by microorganisms. This process:
- Releases nutrients (N, P) back into the soil
- Produces humus — a dark, spongy material that holds water and nutrients and binds soil into crumbs
- Improves both sandy soils (holds moisture) and clay soils (opens up spaces)
5.4 Role of biological activity in agriculture
- Decomposition of organic materials to release nutrients
- Nitrogen fixation by bacteria in legumes reduces need for nitrogen fertiliser
- Improvement of soil structure through earthworm activity and microbial substances
- Suppression of soil-borne diseases by beneficial organisms
- Breakdown of chemical residues (bioremediation)
Earthworms are living ploughs
An earthworm can eat its own weight in soil every day. In one year, the earthworms in a hectare of healthy farmland can move several tonnes of soil — mixing, aerating and fertilising it for free.
Enjoying this topic? Your progress, practice questions and the remaining 9 topics are waiting in the app.
Continue in the appSection 6Soil health
What is soil health?
Soil health (or soil quality) is the capacity of a soil to function as a living ecosystem. A healthy soil can sustain crop production year after year without being degraded. It is not just about chemical fertility — it includes physical condition and biological life.
Indicators of a healthy soil
- Good, crumbly structure (not hard clods or powder)
- Dark colour and earthy smell (organic matter + active microbes)
- High water infiltration (soaks in quickly yet holds moisture)
- Earthworm counts of at least 5–10 per spade-full
- Steady crop growth without frequent fertiliser
- Resistance to erosion
- Neutral or slightly acidic pH
Practices that maintain and improve soil health
- Add organic matter regularly — compost, manure, crop residues
- Practice crop rotation — e.g. maize → beans → sweet potatoes
- Include legumes — beans, cowpeas, groundnuts add nitrogen via root bacteria
- Mulch the soil — conserves moisture, moderates temperature, suppresses weeds
- Reduce tillage — excessive ploughing destroys structure and kills earthworms
- Control erosion — terraces, contour ploughing, grass strips, windbreaks
- Test soil and correct imbalances — apply fertiliser and lime only as needed
- Avoid overgrazing — too many animals compact soil and remove vegetation
- Promote diverse soil life — avoid excessive chemical pesticides
Section 7Chapter summary
Key take-aways
- Agricultural soil is the living upper layer of the earth that supports crop and animal production. It is made of minerals (~45%), organic matter (~5%), water (~25%) and air (~25%).
- Physical properties — texture (sand, silt, clay, loam), structure, porosity, permeability, colour — determine how soil handles water, air and roots. Loam is best for farming.
- Chemical properties — pH (ideal 6–7.5), CEC, macronutrients (N, P, K) and micronutrients. Excess salinity harms plants; leaching washes nutrients away.
- Biological properties — macroorganisms (earthworms, termites) and microorganisms (bacteria, fungi) decompose organic matter, form humus, fix nitrogen and improve structure.
- Soil health is the soil’s continued ability to function as a productive ecosystem. Maintain it by adding organic matter, rotating crops, mulching, reducing tillage and controlling erosion.
Section 8End of topic questions
Test Yourself
1. Define agricultural soil and list its four main components.
2. State two reasons why loam soil is preferred for crop production over pure sand or pure clay.
3. Explain how a farmer can tell if a soil is too acidic and what can be done to correct it.
4. Describe the role of any three soil organisms in maintaining soil fertility.
5. What is the difference between a physical property and a chemical property of soil? Give one example of each.
6. Why is soil colour used as an indicator of soil quality? Give two examples of what different colours may mean.
7. List four practices a farmer can use to maintain soil health on his or her farm.
8. Fill in the blanks: The relative amounts of sand, silt and clay is called soil ________; the dark decomposed organic matter is ________; the measure of acidity is soil ________; washing away of nutrients is called ________.
9. A farmer’s field is very hard, cracks when dry, water ponds after rain, crops are stunted and yellow, and earthworms are rare. The farmer plants only maize every year without manure.
(a) What texture do you suspect? Give two reasons.
(b) What is the likely problem with biological properties?
(c) Suggest three changes to improve soil health.
(d) If pH is 4.8, what does this mean and how can it be corrected?
Section 9Suggested answers
Answers to end-of-topic questions
1.
Show answer
Agricultural soil is soil managed for growing crops and rearing livestock. Its four main components are: mineral matter (~45%), organic matter (~5%), water (~25%) and air (~25%).
2.
Show answer
Loam has a balanced mixture of sand, silt and clay, so it drains well (unlike pure clay) yet holds enough water and nutrients (unlike pure sand). It also allows easy root penetration.
3.
Show answer
Test with litmus or pH paper/strips. If pH is below about 6, the soil is too acidic. Correct by adding lime (calcium carbonate).
4.
Show answer
Earthworms — mix and aerate soil; casts are nutrient-rich.
Bacteria (e.g. Rhizobium) — fix nitrogen from the air into forms plants can use.
Fungi (mycorrhizal) — help roots absorb water and phosphorus; break down tough organic matter.
5.
Show answer
Physical property — can be seen/touched without changing chemistry (e.g. texture, structure, porosity).
Chemical property — involves chemical composition or reactions (e.g. pH, CEC, nutrient content).
6.
Show answer
Colour reflects organic matter and mineral content. Dark/black soil often indicates high organic matter and good fertility. Red/yellow may indicate iron oxides and good drainage. Grey/blue may indicate waterlogging and poor aeration.
7.
Show answer
Any four of: add organic matter (compost/manure); crop rotation; include legumes; mulching; reduce tillage; control erosion (terraces, contour ploughing); soil testing; avoid overgrazing; promote soil life (limit chemical pesticides).
8.
Show answer
texture; humus; pH; leaching.
9.
Show answer
(a) Clay texture — hard when dry, cracks, water ponds (poor drainage).
(b) Low biological activity — few earthworms indicate poor organic matter and microbial life.
(c) Add manure/compost; rotate with legumes; reduce continuous maize monoculture; mulch; improve drainage.
(d) pH 4.8 = strongly acidic; many nutrients unavailable. Correct by applying lime.
Ready for Topic 3: Introduction to Horticulture Production?
Continue Form One Agriculture in the Click Online Academy app, with all 10 topics in your pocket.
Click here to continue learning on our appClick Online Academy · Agriculture Form One · Topic One: The Agricultural Soil
Competences and topics follow the Form One schemes of work. Pale background for easy printing.