Soil Science and Management

Introduction to Soil in Landscape Horticulture

Soil is the foundation of any sustainable landscape installation. For the horticultural apprentice, mastering soil science is not a theoretical option: it determines plant survival, irrigation system efficiency, landscape structure stability, and compliance with Canadian environmental standards. This chapter covers all the knowledge required by the Red Seal exam regarding soil classification, analysis, amendment, and management.

Classification and Physical Properties of Soil

The Three Mineral Fractions: Sand, Silt, Clay

Mineral soil is composed of particles classified by their diameter. The textural triangle of the United States Department of Agriculture (USDA), also used in Canada, defines twelve textural classes. The particle size limits are:

Fraction	Diameter (mm)	Main Characteristics
Sand	0.05 to 2.0	Rapid drainage, low water and nutrient retention, high macroporosity
Silt	0.002 to 0.05	Moderate water retention, susceptible to compaction, silky texture
Clay	< 0.002	High water and nutrient retention, slow drainage, high cohesion, swelling/shrinking

Stokes' Law allows you to determine the sedimentation velocity of particles suspended in water: v = (2 × r² × (ρp − ρe) × g) / (9 × η), where r is the particle radius, ρp its density, ρe the density of water, g gravitational acceleration (9.81 m/s²), and η the viscosity of water. This law is the basis of the hydrometer method for particle size analysis.

Soil Structure

Structure describes the arrangement of particles into aggregates. Types include:

Granular structure: ideal for surface soils, porous spherical aggregates

Blocky structure: angular aggregates, typical of clayey subsoils

Prismatic and columnar structure: vertical aggregates, often associated with drainage problems

Platy structure: horizontal layers, common in compacted soils

A stable granular structure promotes total porosity, air and water circulation, and root penetration. Aggregate stability depends on organic matter, living roots, mycorrhizal fungi, and earthworms.

Porosity and Bulk Density

Total porosity (Pt) is calculated: Pt (%) = (1 − (Da / Dp)) × 100, where Da is bulk density (g/cm³) and Dp is particle density (approximately 2.65 g/cm³ for minerals).

Soil Type	Bulk Density (g/cm³)	Total Porosity (%)
Sand	1.5 to 1.7	35 to 43
Silt	1.3 to 1.5	43 to 50
Clay	1.0 to 1.3	50 to 60
Organic soil	0.3 to 0.8	60 to 85

A bulk density above 1.6 g/cm³ in a loamy soil indicates severe compaction. Compaction reduces macroporosity, limits aeration (less than 10% oxygen required for root respiration) and increases root penetration resistance beyond 2 MPa.

Soil Water: Matric Potential and Hydraulic Constants

Soil water is held by capillary and adsorption forces. Total water potential (Ψt) is the sum of matric potential (Ψm), gravitational potential (Ψg), and osmotic potential (Ψo). It is measured in kilopascals (kPa) or bars (1 bar = 100 kPa).

Fundamental hydraulic constants:

Field capacity (FC): water content after gravitational drainage (Ψm = −33 kPa)

Permanent wilting point (PWP): water content at which plants can no longer extract water (Ψm = −1500 kPa)

Available water: the difference between FC and PWP

Texture	Available Water (cm water per cm soil)
Sand	0.05 to 0.10
Sandy loam	0.10 to 0.15
Loam	0.15 to 0.20
Clay loam	0.15 to 0.22
Clay	0.12 to 0.18

The water retention curve (relationship between Ψm and volumetric water content θ) is essential for sizing irrigation systems. Water tension is measured with a tensiometer (0 to −80 kPa) or with electrical resistance blocks (gypsum blocks).

Chemical Properties of Soil

Soil pH

pH measures the activity of hydrogen ions (H⁺) in the soil solution. The scale ranges from 0 to 14, with 7 being neutral. The optimal pH for most ornamental plants is between 6.0 and 7.0, where nutrient availability is at its maximum.

pH	Interpretation	Consequences
< 5.5	Very acidic	Al³⁺ and Mn²⁺ toxicity, P, Ca, Mg, Mo deficiency
5.5 – 6.5	Acidic to slightly acidic	Optimal availability of most nutrients
6.5 – 7.5	Neutral	Good microbial activity, P available
> 7.5	Alkaline	Fe, Mn, Zn, Cu deficiency (iron chlorosis)

The buffering capacity of soil depends on clay and organic matter content. Clayey and organic soils resist pH variations better than sandy soils.

Cation Exchange Capacity (CEC)

CEC is the total quantity of exchangeable cations (Ca²⁺, Mg²⁺, K⁺, Na⁺, NH₄⁺, H⁺, Al³⁺) that a soil can retain on its negative exchange sites. It is expressed in centimoles of charge per kilogram (cmol(+)/kg) or in milliequivalents per 100 g (meq/100 g).

Constituent	Typical CEC (cmol(+)/kg)
Sand	1 to 5
Silt	5 to 15
Kaolinite clay	3 to 15
Illite clay	20 to 40
Montmorillonite clay	80 to 150
Organic matter	150 to 300

Base saturation percentage (BSP) is calculated: BSP (%) = (Ca²⁺ + Mg²⁺ + K⁺ + Na⁺) / CEC × 100. A BSP above 80% indicates a soil well supplied with exchangeable bases.

Organic Matter and the Carbon Cycle

Soil organic matter (SOM) includes decomposing plant and animal residues, humus, and microbial biomass. It plays a major role in:

Water retention (up to 20 times its weight in water)

CEC (30 to 70% of total CEC in mineral soils)

Soil structuring (binding agents)

Supply of nitrogen, phosphorus, and sulfur through mineralization

The carbon/nitrogen (C/N) ratio controls the decomposition rate:

Material	C/N Ratio
Poultry manure	5 – 8
Cattle manure	15 – 20
Mature compost	12 – 15
Dead leaves	40 – 80
Straw	80 – 100
Sawdust	200 – 500

A C/N ratio above 30 causes nitrogen immobilization (nitrogen drawdown): microorganisms immobilize mineral nitrogen from the soil to decompose organic matter, depriving plants of this element.

Salinity and Exchangeable Sodium

Electrical conductivity (EC) of the soil solution measures salinity. It is expressed in deciSiemens per meter (dS/m) or mmhos/cm (1 dS/m = 1 mmhos/cm).

EC (dS/m)	Salinity Level	Effect on Plants
0 – 2	Low	No effect
2 – 4	Moderate	Sensitive plants affected
4 – 8	High	Many plants affected
> 8	Very high	Only tolerant plants survive

Exchangeable sodium is measured by the SAR (sodium adsorption ratio): SAR = Na⁺ / √((Ca²⁺ + Mg²⁺) / 2), with concentrations in mmol/L. An SAR above 13 indicates a risk of clay dispersion and structure degradation.

Soil Analysis and Interpretation

Sampling Procedure

For a representative sample:

53.Divide the area into homogeneous units (soil type, fertilization history, drainage)

54.Collect 10 to 15 cores per unit, at a depth of 15 to 20 cm (0 to 15 cm for lawns)

55.Use a clean auger or soil probe, avoiding abnormal areas (compost piles, old fire pits)

56.Mix the cores in a clean plastic bucket

57.Take approximately 500 g of the mixture, air-dry it (never oven-dry)

58.Label the bag with the date, site, depth, and analysis requested

Accredited Canadian laboratories use methods from the Canadian Standards Association (CSA) and the Canadian Council of Ministers of the Environment (CCME) for contaminated soil analysis. For agronomic analyses, methods from the American Society of Agronomy are commonly used.

Interpreting Results

Fertility levels are classified according to the laboratory's fertility index:

Element	Very Low	Low	Medium	High	Very High
Phosphorus (P, mg/kg)	< 10	10 – 20	21 – 40	41 – 60	> 60
Potassium (K, mg/kg)	< 60	60 – 120	121 – 200	201 – 300	> 300
Nitrate nitrogen (NO₃⁻, mg/kg)	< 10	10 – 20	21 – 40	41 – 60	> 60
Organic matter (%)	< 2	2 – 3	3 – 5	5 – 8	> 8

pH is measured in water (1:1 or 1:2 ratio) or in a 0.01 M CaCl₂ solution (pH CaCl₂ = water pH − 0.5 to 0.8 unit).

Amendments and Fertilization

Mineral Amendments: Lime and Gypsum

Agricultural lime (CaCO₃, Ca(OH)₂, CaO, CaMg(CO₃)₂) is used to raise the pH of acidic soils. The neutralizing value (NV) is expressed as calcium carbonate equivalent (CCE). The lime application rate is calculated:

Rate (t/ha) = (target pH − current pH) × buffer factor × depth (m) × bulk density

Gypsum (CaSO₄·2H₂O) does not affect pH but provides calcium and improves the structure of sodic soils by replacing exchangeable sodium.

Organic Amendments

Amendment	Dry Matter (%)	N (%)	P₂O₅ (%)	K₂O (%)	C/N
Leaf compost	50 – 60	1.0 – 2.0	0.3 – 0.5	0.5 – 1.0	15 – 25
Composted cattle manure	40 – 50	1.5 – 2.5	1.0 – 1.5	2.0 – 3.0	15 – 20
Sphagnum peat moss	25 – 35	0.5 – 1.0	0.05 – 0.1	0.05 – 0.1	40 – 60
Composted bark	50 – 60	0.5 – 1.5	0.1 – 0.3	0.1 – 0.3	80 – 150

The CAN/BNQ 0413-200 standard from the Bureau de normalisation du Québec (BNQ) — although provincial, it is recognized across Canada — classifies composts into categories P (putrescible) and C (compost) based on maturity and heavy metal content. For the Red Seal exam, remember that mature compost must have a C/N ratio below 20 and a stable temperature.

Mineral Fertilization: Calculating Application Rates

Fertilizers are identified by their N-P-K formula (nitrogen, phosphorus, potassium). The application rate is calculated:

Fertilizer quantity (kg) = (Element rate (kg/ha) × 100) / Percentage of element in fertilizer

Example: To apply 50 kg N/ha with 34-0-0 (ammonium nitrate):

Quantity = (50 × 100) / 34 = 147 kg/ha

For an area of 250 m² (0.025 ha):

147 × 0.025 = 3.68 kg of fertilizer

Conversion to g/m²: 147 kg/ha = 14.7 g/m² (1 kg/ha = 0.1 g/m²).

Slow-Release and Controlled-Release Fertilizers

Slow-release fertilizers (sulfur-coated, IBDU) and controlled-release fertilizers (polymer resin, Osmocote®) release nutrients by diffusion through a membrane. Their release rate depends on soil temperature (Q₁₀ ≈ 2: the rate doubles for every 10 °C increase). The stated release duration (3, 6, 9, 12 months) is valid at 21 °C; at 10 °C, it is approximately doubled.

Water Management and Irrigation

Calculating Water Requirements

Reference evapotranspiration (ET₀) is calculated using the Penman-Monteith method (FAO-56). Actual evapotranspiration (ETc) = ET₀ × Kc, where Kc is the crop coefficient.

Vegetation Type	Kc
Lawn (cool season)	0.8 – 1.0
Lawn (warm season)	0.6 – 0.8
Shrubs	0.5 – 0.7
Trees	0.4 – 0.6
Annual flower beds	0.8 – 1.0

The irrigation dose (D, in mm) is calculated: D = (FC − PWP) × root depth (mm) × leaching fraction. Frequency depends on available water reserve and daily ETc.

Example: Loam with FC = 30%, PWP = 15%, root depth = 300 mm, leaching fraction = 10%:

D = (0.30 − 0.15) × 300 × 1.10 = 49.5 mm

Irrigation Efficiency

Irrigation Type	Typical Efficiency (%)
Drip irrigation	85 – 95
Sprinkler (adequate pressure)	65 – 75
Sprinkler (strong wind)	40 – 50
Furrow	50 – 60

The precipitation rate of a sprinkler is calculated: Rate (mm/h) = Flow rate (L/min) × 60 / (Coverage area (m²) × 1). To avoid runoff, the precipitation rate must not exceed the soil's infiltration rate:

Texture	Infiltration Rate (mm/h)
Sand	25 – 50
Sandy loam	15 – 25
Loam	8 – 15
Clay loam	3 – 8
Clay	1 – 3

Drainage and Stormwater Management

Subsurface Drainage

Agricultural drainage using perforated pipes (100 mm drains) spaced 5 to 15 m apart depending on soil texture and water table depth. The typical depth is 0.6 to 1.2 m. The minimum slope is 0.1% (1 mm/m) to prevent sedimentation.

Calculating drain spacing (Hooghoudt's equation) is complex; for the exam, remember that:

Clay soils: closer spacing (5 – 8 m)

Sandy soils: wider spacing (10 – 15 m)

Deeper drains increase efficiency but increase cost

Sustainable Stormwater Management

The Canadian Standards Association (CSA) National Guide and Best Management Practices (BMPs) recommend:

Retention ponds: volume calculated for the design storm (typically 1:5 years for residential areas)

Rain gardens: sized to infiltrate the first 25 mm of rainfall; surface area = 10 to 20% of the contributing impervious surface

Dry wells: for soils with an infiltration rate above 15 mm/h

Drainage trenches: filled with gravel, with geotextile to prevent clogging

CSA A100-14 (Residential buildings) and CSA B184 (Stormwater management systems) govern these practices at the national level.

Soil Conservation and Erosion Prevention

Water and Wind Erosion

The Universal Soil Loss Equation (USLE): A = R × K × LS × C × P, where:

A = soil loss (t/ha/year)

R = rainfall erosivity factor

K = soil erodibility factor

LS = topographic factor (length and slope)

C = vegetation cover factor

P = conservation practice factor

Factor	Typical Value
R (Canada, southern Ontario)	100 – 150
K (loam)	0.32
K (clay)	0.25
K (sand)	0.15
C (bare soil)	1.0
C (dense lawn)	0.01
C (wood mulch, 5 cm)	0.05

The soil loss tolerance in Canada is 6 to 11 t/ha/year depending on soil type and depth.

Control Measures

Geotextiles: classified according to CAN/BNQ 3660-950 (geotextiles and geomembranes) — classified by tensile strength, permeability, and filtration opening size

Mulch: reduces erosion by 80 to 95%; minimum thickness of 5 to 7 cm

Berms and ditches: maximum slope of 3:1 (H:V) for stability

Vegetation establishment: fast-growing grasses (annual ryegrass) stabilize soil in 2 to 3 weeks

Canadian Standards and Regulations

Soil Quality Standards

The Canadian Council of Ministers of the Environment (CCME) has established the Canadian Soil Quality Guidelines (CSQG) for contaminated sites. Thresholds are classified by land use:

Land Use	Generic Threshold (mg/kg)
Residential/parkland	Varies by contaminant
Commercial	Varies by contaminant
Industrial	Varies by contaminant

For heavy metals (e.g., lead): residential use = 140 mg/kg, commercial = 600 mg/kg, industrial = 1000 mg/kg (indicative values, to be verified by province).

Canadian Electrical Code, Part I

For the installation of electrical irrigation systems, the Canadian Electrical Code, Part I (CSA C22.1 standard) applies. Key points:

Rule 8-200: electrical load calculation — the load of an irrigation system is calculated at 100% of the rated power of the motors

Rule 68-054: buried conductors must be at a minimum depth of 600 mm below pedestrian surfaces

Rule 68-056: conductors must be protected by rigid conduit when buried at less than 600 mm

Rule 26-700: motors must have overload protection (thermal relays)

CSA B149.1 — Natural Gas and Propane Code

For irrigation or greenhouse heating systems using propane:

Article 3.2.1: piping must be supported at maximum intervals of 2.4 m for 25 mm pipes

Article 6.3.1: buried piping must have a minimum cover of 300 mm

Article 5.10.1: appliances must be vented in accordance with manufacturer's specifications

CSA Standards for Materials

CSA A23.1: concrete for landscape structures (curbs, retaining walls)

CSA G40.20: steel for retaining structures

CSA O80: pressure-treated wood for exterior structures — treatment classes (UC1 to UC4B) determine use:

Class	Use	Minimum Retention (kg/m³)
UC1	Interior, dry	4.0
UC2	Interior, damp	4.0
UC3A	Exterior, above ground	4.0
UC3B	Exterior, ground contact	6.4
UC4A	Ground contact or freshwater	9.6
UC4B	Ground contact or water, high-risk areas	12.8

Common Pitfalls to Avoid

145.Confusing field capacity and wilting point: FC is at −33 kPa, PWP at −1500 kPa. Available water is the difference between the two, not the total water content.

146.Forgetting the factor of 100 in fertilizer calculations: the formula (rate × 100) / % is the most common source of error. Always check your units (kg/ha vs g/m²).

147.Neglecting the C/N ratio: an amendment with C/N > 30 causes nitrogen immobilization. Compost carbon-rich materials before incorporation.

148.Confusing lime and gypsum: lime raises pH, gypsum does not change it. Use gypsum for sodic soils, lime for acidic soils.

149.Sampling only the surface: for trees and shrubs, sample at 20 – 30 cm depth. For lawns, 0 – 15 cm is sufficient.

150.Ignoring compaction: a bulk density > 1.6 g/cm³ in a loam blocks root growth. Aeration (core cultivation) is necessary before any amendment.

151.Calculating irrigation without accounting for efficiency: gross dose = net dose / efficiency. At 70% efficiency, you need 43% more water.

152.Forgetting Canadian Electrical Code requirements: buried irrigation cables must meet the minimum depths of Rule 68-054.

153.Confusing wood treatment classes: UC3A is for wood above ground, UC4A for ground contact. Use UC4A for fence posts.

154.Neglecting SAR: a soil with SAR > 13 requires gypsum, not lime, even if the pH is high.

Summary

The textural triangle classifies soils by their proportions of sand, silt, and clay; texture determines drainage, water retention, and CEC.

Granular structure is optimal; compaction (Da > 1.6 g/cm³) destroys porosity and limits root growth.

Available water = field capacity (−33 kPa) − permanent wilting point (−1500 kPa). The tensiometer guides irrigation between 0 and −80 kPa.

Optimal pH is 6.0 to 7.0; lime raises pH, gypsum corrects sodium without changing pH.

CEC depends on clay and organic matter; it determines the soil's ability to retain cationic nutrients.

Sampling must be representative: 10 – 15 cores per homogeneous zone, depth adapted to the crop.

Fertilization calculations: quantity (kg) = (rate (kg/ha) × 100) / % element; convert to g/m² by dividing by 10.

Irrigation: dose (mm) = (FC − PWP) × root depth × leaching fraction; adjust for system efficiency.

Canadian standards: Canadian Electrical Code (Rules 8-200, 68-054, 68-056), CSA B149.1 for gas, CSA O80 for treated wood, CCME for contaminated soils.

Erosion prevention: mulch (5 – 7 cm), geotextiles, rapid vegetation establishment, respecting maximum 3:1 slopes.

Mastery of soil science distinguishes the competent landscape horticulturist from the mere executor. Every decision — plant selection, amendment calculations, irrigation system sizing, drainage design — relies on a rigorous understanding of the physical, chemical, and biological properties of soil. For the Red Seal exam, practice solving fertilization and irrigation calculations without a calculator, and memorize the reference values from the textural triangle table and hydraulic constants.

Soil Science and Management