Soil and its Properties

# Soil and its Properties ## Objectifs et prérequis By the end of this chapter, students will be able to: define soil and its components; describe the physical properties of soil including texture and structure; explain soil depth and its significance; analyse the chemical properties of soil and their impact on fertility; conduct a practical test to determine soil texture; evaluate soil fertility based on physical and chemical indicators; and apply knowledge to real-world agricultural and environmental contexts in Cameroon and Africa. Prerequisites: Basic understanding of rocks and minerals (from Physical Geography), knowledge of weathering processes, familiarity with the concept of ecosystems, and ability to interpret simple data tables and graphs. ## Introduction Soil is the uppermost layer of the Earth’s crust, formed through the weathering of parent rock and the accumulation of organic matter over time. It is a dynamic, living system that supports plant growth, regulates water flow, stores carbon, and provides habitat for countless organisms. In Cameroon, soil varies significantly across agro-ecological zones—from the sandy, leached soils of the coastal plains to the fertile volcanic soils of the Western Highlands and the lateritic soils of the northern savannas. Understanding soil properties is essential for sustainable agriculture, land management, and environmental conservation, particularly in a country where over 70% of the population depends on farming for livelihood. This chapter systematically explores soil as a natural resource, beginning with its definition and progressing through its physical, structural, depth-related, and chemical characteristics, culminating in an assessment of fertility. ## I. Lesson 1: The soil ### Context Soil is not merely dirt; it is a complex mixture of mineral particles, organic matter, water, and air, formed over long periods through the interaction of five factors: parent material, climate, topography, organisms, and time. This lesson establishes the foundational concept of soil as a natural body with distinct horizons and functions. ### Definitions and Key Facts > Soil is the unconsolidated mineral or organic material on the immediate surface of the Earth that serves as a natural medium for the growth of land plants. > The soil profile consists of horizontal layers called horizons: O (organic), A (topsoil), B (subsoil), C (parent material), and R (bedrock). > In Cameroon, the O horizon is often thin in savanna zones due to rapid decomposition and frequent burning, while in forest zones (e.g., South-West and East regions), it can be several centimetres thick due to high litter input. > The A horizon is typically darkest due to humus accumulation and is the most biologically active layer. > Parent material in Cameroon includes granite and gneiss in the West, basalt in the Volcanic Line (e.g., Mount Cameroon area), sandstone in the North, and alluvial deposits along rivers like the Sanaga and Benue. ### Vocabulary to Remember - Parent material - Soil profile - Horizons (O, A, B, C, R) - Humus - Weathering - Leaching ### Example Exam Question with Plan **Question:** Describe the soil profile and explain how the characteristics of each horizon influence agricultural productivity in the Cameroonian context. **Plan:** I. Definition of soil profile and its horizons II. Characteristics and functions of the O horizon (organic matter, decomposition) III. Characteristics and functions of the A horizon (topsoil, nutrient availability, root penetration) IV. Characteristics and functions of the B horizon (subsoil, accumulation of clay, iron, aluminium; implications for drainage and root growth) V. Characteristics and functions of the C and R horizons (parent material and bedrock; influence on soil depth and mineral supply) VI. Application to Cameroonian soils: e.g., thin O horizon in northern savannas limiting fertility; deep A horizon in Western Highlands supporting intensive farming; lateritic B horizon in the North reducing nutrient availability ## II. Lesson 2: Physical properties of the soil ### Context Physical properties determine how soil behaves under various conditions—its ability to retain water, allow air movement, resist erosion, and support root penetration. These properties are primarily influenced by soil texture and structure. ### Definitions and Key Facts > Soil texture refers to the relative proportions of sand, silt, and clay particles in the soil, determined by particle size. > Sand particles range from 0.05 to 2.0 mm in diameter; silt from 0.002 to 0.05 mm; clay is less than 0.002 mm. > Texture is permanent and does not change with management, unlike structure. > In Cameroon, sandy soils dominate the coastal plains (e.g., Littoral and South-West regions) due to marine sedimentation and quartz-rich parent material; silty-loam soils are common in the Adamawa Plateau; clayey soils are prevalent in the volcanic soils of the West and North-West regions. > The textural triangle is used to classify soil into 12 classes based on the percentage of sand, silt, and clay. > Sandy soils drain quickly but retain little water or nutrients; clayey soils hold water and nutrients well but drain poorly and are prone to compaction; loamy soils (balanced mix) are ideal for agriculture due to good drainage, water retention, and aeration. ### Vocabulary to Remember - Sand - Silt - Clay - Textural triangle - Soil texture classes (sand, loamy sand, sandy loam, loam, silt loam, silt, sandy clay loam, clay loam, silty clay loam, sandy clay, silty clay, clay) - Drainage - Water holding capacity - Aeration ### Example Exam Question with Plan **Question:** Using the textural triangle, classify a soil sample containing 40% sand, 40% silt, and 20% clay. Explain its suitability for maize cultivation in the Cameroonian context. **Plan:** I. Plot the percentages on the textural triangle: 40% sand, 40% silt, 20% clay II. Identify the resulting class: Loam (since it falls within the loam zone) III. Describe properties of loam: balanced drainage, good water retention, adequate aeration, ease of tillage IV. Relate to maize requirements: maize needs well-drained soil with moderate water holding capacity and good root penetration V. Conclude: Loam is highly suitable for maize; examples from Western Highlands (e.g., Bamenda, Bafoussam) where loamy volcanic soils support high yields VI. Note limitations: if compacted or low in organic matter, even loam may underperform; recommend organic matter addition ## III. Practical work 1: Determining soil texture ### Context This hands-on activity allows students to estimate soil texture using the feel method, a practical skill essential for farmers and agronomists. It reinforces theoretical knowledge from Lesson 2 through direct observation and tactile analysis. ### Procedure and Key Facts > The feel method involves moistening a small soil sample and rubbing it between the fingers to assess grittiness (sand), silkiness (silt), and stickiness (clay). > Steps: > 1. Take ~25g of air-dried soil, remove stones and organic debris. > 2. Add water dropwise until the soil is pliable (like putty). > 3. Form a ball: if it won’t form a ball → sandy; if it forms a ball but crumbles when tapped → loamy sand/sandy loam; if it forms a strong ball → proceed. > 4. Form a ribbon by pushing soil between thumb and forefinger: > - No ribbon → sandy > - Ribbon < 2.5 cm → loamy > - Ribbon 2.5–5 cm → clay loam > - Ribbon > 5 cm → clay > 5. Assess feel: gritty = sand; silky = silt; sticky = clay. > In Cameroon, this method is widely used by extension officers in the North and Far North regions to advise farmers on soil management without laboratory access. > Accuracy improves with practice; laboratory methods (hydrometer or pipette analysis) are more precise but require equipment. ### Vocabulary to Remember - Feel method - Ribbon test - Pliability - Grittiness - Silkiness - Stickiness - Air-dried soil - Field texture estimation ### Example Exam Question with Plan **Question:** Describe the step-by-step procedure for determining soil texture using the feel method. Explain how the results can inform farming decisions in the Adamawa Region of Cameroon. **Plan:** I. Step 1: Sample preparation (drying, debris removal) II. Step 2: Moistening to putty consistency III. Step 3: Ball formation test IV. Step 4: Ribbon formation and measurement V. Step 5: Sensory assessment (grittiness, silkiness, stickiness) VI. Interpretation: link ribbon length and feel to textural class VII. Application to Adamawa Region: e.g., if ribbon is 3 cm and silky → clay loam → good water retention but risk of compaction → recommend reduced tillage, cover cropping, organic matter addition VIII. Contrast: if sandy → recommend irrigation, frequent light fertilisation, windbreaks ## IV. Lesson 3: Soil structure ### Context While texture refers to particle size, structure refers to how those particles are arranged into aggregates or peds. Structure influences porosity, permeability, root growth, and resistance to erosion—making it a dynamic property that can be improved or degraded by management. ### Definitions and Key Facts > Soil structure is the arrangement of soil particles into aggregates (peds) separated by pores of varying sizes. > Primary structure types: granular (crumb-like, common in topsoil), blocky (angular or subangular, common in subsoil), prismatic (vertical columns, typical in clayey soils), platy (flat, horizontal layers, often indicative of compaction), and single-grained (no aggregation, typical of sands). > Granular structure promotes good aeration, water infiltration, and root penetration—ideal for agriculture. > Platy structure restricts downward water and root movement, often caused by heavy machinery or livestock trampling. > In Cameroon, granular structure is prevalent in the A horizon of forest soils (e.g., South-West and East regions) due to high organic matter and biological activity (earthworms, roots). > Blocky and prismatic structures dominate B horizons in clayey soils of the West and North-West (e.g., Bamenda highlands), reflecting shrink-swell behaviour of montmorillonite clays. > Soil structure is influenced by: organic matter content, biological activity, freezing-thawing cycles, wetting-drying cycles, and human activities (tillage, compaction). > Good structure increases total porosity (typically 40–60% in healthy soils), with a balance of macropores (for drainage and air) and micropores (for water retention). ### Vocabulary to Remember - Soil structure - Peds (aggregates) - Granular structure - Blocky structure - Prismatic structure - Platy structure - Single-grained structure - Porosity - Macropores - Micropores - Aggregate stability ### Example Exam Question with Plan **Question:** Explain how soil structure affects water movement and root growth, and describe two agricultural practices that can improve soil structure in the volcanic soils of Cameroon’s Western Highlands. **Plan:** I. Define soil structure and its role in pore space formation II. Link granular structure to high macroporosity → rapid infiltration, good aeration, deep root penetration III. Link platy/compact structure to low macroporosity → poor drainage, waterlogging, restricted roots, surface runoff IV. Practice 1: Addition of organic matter (compost, manure, crop residues) → binds particles into stable aggregates, increases biological activity V. Practice 2: Reduced tillage or conservation agriculture → minimises disruption of aggregates, prevents compaction, maintains pore continuity VI. Application to Western Highlands: volcanic soils are naturally rich in minerals but prone to structural degradation under intensive farming; recommend zero-till maize with mucuna cover crop (as promoted by IRAD in Bambui) VII. Expected outcome: improved infiltration, reduced erosion on slopes, higher yields ## V. Lesson 4: SOIL depth ### Context Soil depth—the vertical extent of usable soil above bedrock or a restrictive layer—directly affects the volume of soil available for root growth, water storage, and nutrient retention. It is a critical factor in determining land suitability for agriculture. ### Definitions and Key Facts > Soil depth is measured from the surface to the depth where root penetration is severely limited by bedrock, hardpan, high clay content (e.g., laterite), or water table. > Classes: very shallow (<25 cm), shallow (25–50 cm), moderately deep (50–90 cm), deep (90–150 cm), very deep (>150 cm). > In Cameroon, soil depth varies greatly: > - Very shallow: rocky outcrops in the Mandara Mountains (Far North) and Adamawa Plateau, where bedrock is near surface > - Shallow: lateritic crusts in the North (e.g., Garoua region), where iron-aluminium hardpan forms at 30–60 cm depth > - Moderately deep: sandy coastal plains (Littoral), where depth is limited by water table or coarse texture > - Deep to very deep: volcanic soils of the West (e.g., around Mount Cameroon, Bamboutos) and alluvial valleys (e.g., Benue, Sanaga), where depths exceed 150 cm > Deep soils allow extensive root systems, greater water storage (reducing drought vulnerability), and larger nutrient reservoirs. > Shallow soils limit root growth, increase drought stress, and are prone to erosion due to thin protective layer. > In the Far North, shallow lateritic soils restrict millet and sorghum root development, making crops more vulnerable to terminal drought. ### Vocabulary to Remember - Soil depth - Bedrock - Hardpan - Laterite - Water table - Root penetration depth - Very shallow - Shallow - Moderately deep - Deep - Very deep ### Example Exam Question with Plan **Question:** Define soil depth and explain its influence on agricultural productivity, using two contrasting examples from different regions of Cameroon. **Plan:** I. Definition of soil depth and limiting factors (bedrock, hardpan, water table) II. Classification of depth classes III. Example 1: Very shallow soil in the Mandara Mountains (Far North) - Bedrock at <25 cm depth - Limits root growth of sorghum and millet - Low water holding capacity → high drought risk - Leads to low yields and land degradation IV. Example 2: Very deep soil in the Benue Valley (Adamawa/North) - Alluvial deposits >150 cm deep - Supports deep-rooted crops like cassava and yam - High water storage → buffers dry spells - Enables intensive cultivation and fallow shortening V. Conclusion: soil depth is a key determinant of land suitability; deep soils are more resilient and productive ## VI. Lesson 5: Chemical properties of soil ### Context Chemical properties govern nutrient availability, pH balance, cation exchange capacity, and the soil’s ability to resist toxic element buildup. These properties are dynamic and can be managed through fertilisation, liming, and organic amendments. ### Definitions and Key Facts > Soil pH measures the acidity or alkalinity of the soil solution on a scale of 0–14, with 7 being neutral. > Most crops prefer pH 5.5–7.0; outside this range, nutrient availability declines. > In Cameroon: > - Forest soils (South-West, East) tend to be acidic (pH 4.5–5.5) due to high rainfall and leaching of bases > - Savannah soils (North, Far North) are often slightly acidic to neutral (pH 5.5–6.5) > - Volcanic soils (West, North-West) can be moderately acidic (pH 5.0–6.0) but are rich in weatherable minerals > - Some alluvial soils (e.g., Logone floodplains) may be alkaline (pH >7.5) due to calcium carbonate accumulation > Cation Exchange Capacity (CEC) is the soil’s ability to hold and exchange positively charged ions (cations: Ca²⁺, Mg²⁺, K⁺, Na⁺, NH₄⁺). > High CEC = greater nutrient retention; low CEC = prone to leaching. > Clay and organic matter contribute most to CEC; sand contributes little. > In Cameroon, clayey volcanic soils have high CEC (20–40 meq/100g); sandy coastal soils have low CEC (<5 meq/100g). > Essential plant nutrients: macronutrients (N, P, K, Ca, Mg, S) and micronutrients (Fe, Mn, Zn, Cu, B, Mo, Cl). > Nutrient availability is pH-dependent: e.g., phosphorus is least available below pH 5.5 and above pH 7.5; iron, manganese, zinc become toxic in acidic soils; molybdenum deficient in acidic soils. > Salinity (excess soluble salts) is rare in Cameroon but can occur in irrigated areas with poor drainage (e.g., some rice schemes in the North). ### Vocabulary to Remember - Soil pH - Acidity - Alkalinity - Cation Exchange Capacity (CEC) - Cations (Ca²⁺, Mg²⁺, K⁺, Na⁺, NH₄⁺) - Anions - Macronutrients - Micronutrients - Nutrient availability - Leaching - Salinity - Liming ### Example Exam Question with Plan **Question:** Explain how soil pH affects nutrient availability in Cameroonian soils, and recommend two management practices to correct excessive acidity in the soils of the South-West Region. **Plan:** I. Define soil pH and its scale II. Explain pH effect on nutrient solubility: - Low pH (<5.5): Al, Mn, Fe soluble → toxicity; P, Mo, Ca, Mg less available - High pH (>7.5): Fe, Zn, Mn, Cu insoluble → deficiency; P may precipitate III. Application to South-West Region: high rainfall → leaching → acidic soils (pH 4.5–5.5) IV. Consequence: phosphorus fixation, aluminium toxicity limiting maize and soybean growth V. Practice 1: Application of agricultural lime (CaCO₃) → raises pH, reduces Al toxicity, increases P availability VI. Practice 2: Use of organic manure or compost → buffers pH, adds nutrients, improves CEC VII. Note: lime requirement depends on soil texture and current pH; recommend soil testing before application VIII. Expected outcome: improved nutrient uptake, higher yields, reduced need for excessive fertilisers ## VII. Lesson 6: Soil fertility ### Context Soil fertility is the capacity of soil to supply essential nutrients in adequate amounts and proportions for plant growth, without toxicities or deficiencies. It integrates physical, chemical, and biological properties and is the ultimate measure of soil quality for agriculture. ### Definitions and Key Facts > Soil fertility depends on: > - Nutrient content (N, P, K, etc.) > - Nutrient availability (influenced by pH, CEC, organic matter) > - Soil structure and texture (affecting root growth and water relations) > - Biological activity (microorganisms, earthworms) > - Absence of harmful substances (salts, heavy metals, pollutants) > Fertile soils are typically dark (high humus), well-structured, deep, and have balanced pH and high CEC. > In Cameroon: > - High fertility: volcanic soils of the West (e.g., Bamenda, Bafoussam) and North-West (e.g., Kumbo) — rich in weatherable minerals, high CEC, good structure, moderate depth > - Moderate fertility: alluvial soils of the Benue and Sanaga valleys — good texture and depth, but may require N and P fertilisation > - Low fertility: sandy coastal soils (Littoral) — low CEC, low water holding capacity, prone to leaching > - Low fertility: lateritic soils of the North — high P fixation, low CEC, shallow depth, hardpan > Soil fertility is not static: it can be depleted by continuous cropping without replenishment (e.g., maize monoculture in the West leading to soil acidification and nutrient mining) or enhanced through fallowing, crop rotation, legume intercropping, agroforestry, and organic or inorganic fertilisation. > The concept of nutrient balance is critical: applying only N-P-K without addressing micronutrients or organic matter can lead to imbalances (e.g., zinc deficiency in high-phosphorus soils). > In the Far North, declining fertility due to overgrazing and short fallows has led to reduced millet yields; interventions include zaï pits and farmer-managed natural regeneration (FMNR). ### Vocabulary to Remember - Soil fertility - Nutrient content - Nutrient availability - Organic matter - Cation Exchange Capacity (CEC) - pH - Soil structure - Soil texture - Biological activity - Nutrient balance - Fallowing - Crop rotation - Legume intercropping - Agroforestry - Zaï pits - Farmer-managed natural regeneration (FMNR) ### Example Exam Question with Plan **Question:** Define soil fertility and analyse the factors that contribute to low fertility in the sandy soils of Cameroon’s Littoral Region, proposing three sustainable improvement strategies. **Plan:** I. Definition of soil fertility as the soil’s capacity to support healthy plant growth II. Factors causing low fertility in Littoral sandy soils: - Low clay and organic matter → very low CEC (<5 meq/100g) → poor nutrient retention - Large pore spaces → rapid drainage → low water holding capacity → drought stress - Low nutrient reserves → reliance on external inputs - Prone to leaching of nitrates and potassium - Often low in organic matter due to high decomposition rates and low biomass return III. Strategy 1: Addition of organic matter (compost, manure, green manure) → increases CEC, water holding capacity, nutrient supply, and biological activity IV. Strategy 2: Use of cover crops (e.g., mucuna, cowpea) during fallow → protects soil, fixes nitrogen, adds biomass V. Strategy 3: Application of balanced fertilisers (NPK + micronutrients like Zn, B) based on soil test → addresses deficiencies without causing imbalances VI. Additional: agroforestry with nitrogen-fixing trees (e.g., Faidherbia albida) → improves microclimate, adds litter, deep nutrient cycling VII. Expected outcome: improved soil structure, increased yields, reduced input dependency over time ## Synthèse du chapitre - Soil is a natural body composed of mineral particles, organic matter, water, and air, organised into horizons (O, A, B, C, R). - Physical properties (texture and structure) determine water retention, aeration, root penetration, and erosion resistance. - Soil texture (sand, silt, clay proportions) is innate and assessed via the textural triangle or feel method. - Soil structure (granular, blocky, prismatic, platy, single-grained) is dynamic and influenced by management. - Soil depth limits root growth and water storage; deep soils are more productive and resilient. - Chemical properties (pH, CEC, nutrient availability) govern nutrient solubility and retention; pH affects macro- and micronutrient uptake. - Soil fertility integrates physical, chemical, and biological factors; it can be degraded or improved through management practices. - In Cameroon, soil variability reflects geology, climate, and land use: volcanic soils (West) are highly fertile; sandy coastal (Littoral) and lateritic (North) soils are fragile and require careful management. - Sustainable agriculture depends on understanding and managing soil properties to maintain productivity and prevent degradation. ## Glossaire - **Soil profile**: The vertical section of soil showing its distinct horizons (O, A, B, C, R). - **Soil texture**: The relative proportion of sand, silt, and clay particles in the soil. - **Soil structure**: The arrangement of soil particles into aggregates (peds) and the pore spaces between them. - **Cation Exchange Capacity (CEC)**: The soil’s capacity to hold and exchange positively charged ions (cations). - **Soil pH**: A measure of the acidity or alkalinity of the soil solution, ranging from 0 to 14. - **Humus**: The stable, dark organic matter formed from the decomposition of plant and animal residues. - **Laterite**: A soil layer rich in iron and aluminium oxides, formed under intense tropical weathering, often forming a hardpan. - **Leaching**: The downward movement of water-soluble nutrients (e.g., nitrates, potassium) through the soil profile. - **Granular structure**: A soil structure type resembling small crumbs, favourable for root growth and aeration. - **Platy structure**: A soil structure type with horizontal, plate-like aggregates, often indicating compaction. - **Fertility**: The capacity of soil to supply essential nutrients in adequate amounts and proportions for plant growth. - **Zaï pits**: Small planting pits dug in degraded soils to concentrate water, organic matter, and nutrients, used in the Sahelian zones of Cameroon.