Soil Consistency and Soil Temperature

B. Sc. Agriculture (Hons.) Ist. Semester (Six Deam Commitee of ICAR)

1.1 Definition

Soil consistency is the degree of cohesion and adhesion (or resistance to deformation) of soil mass under varying moisture conditions.
It describes how soil behaves when it is wet, moist, or dry, and how much force is required to break or deform it.

1.2 Explanation

Cohesion: Attraction between similar soil particles (mainly clay).
Adhesion: Attraction between soil particles and water molecules.
The combined effect determines soil’s plasticity, stickiness, and friability.

1.3 Stages of Consistency (According to Soil Moisture Conditions)

Moisture Condition	Soil Consistency Behavior	Common Term
Wet soil	Becomes sticky and plastic	Plastic limit and liquid limit observed
Moist soil	Friable — easily crumbles under light pressure	Optimum for tillage
Dry soil	Hard and brittle — resists crushing	Dry consistency

1.4 Atterberg Limits (Important for Fine-Textured Soils)

Proposed by Atterberg (1911) — define the limits of consistency based on soil moisture content.

Limit	Definition	Typical Range
Shrinkage Limit (SL)	Lowest water content at which further drying does not reduce soil volume.	10–20%
Plastic Limit (PL)	Moisture content at which soil begins to crumble when rolled into threads (3 mm).	20–30%
Liquid Limit (LL)	Moisture content at which soil changes from plastic to liquid state (flows under its own weight).	30–50%

Plasticity Index (PI) = LL – PL

1.5 Importance of Soil Consistency

Aspect	Importance
Tillage operations	Determines proper soil moisture for plowing (friable stage).
Soil classification	Helps identify fine-textured or clayey soils.
Engineering uses	Used to estimate load-bearing and shrink-swell behavior.
Irrigation management	Prevents soil crusting or compaction.
Root penetration	Loose and friable soil favors root growth.

1.6 Typical Values of Consistency Limits

2.1 Definition

Soil temperature is the measure of the heat content of the soil — it represents the degree of warmth or coldness of the soil at a given time and depth.
It affects seed germination, root growth, microbial activity, and nutrient availability.

2.2 Sources of Soil Heat

Source	Description
1. Solar radiation	Primary and major source of soil heat.
2. Decomposition of organic matter	Microbial activity releases heat.
3. Chemical and biological reactions	Exothermic reactions in soil.
4. Heat from rainfall or irrigation water	Transfers heat to upper layers.
5. Heat from earth’s interior (geothermal)	Minor contribution, affects deep soils.

2.3 Factors Affecting Soil Temperature

Factor	Effect
1. Soil color	Dark soils absorb more heat → higher temperature.
2. Soil moisture	Moist soils heat and cool slowly (high specific heat).
3. Texture and structure	Sandy soils warm faster than clayey soils.
4. Organic matter	Increases water retention → lowers temperature slightly.
5. Vegetation cover	Shading reduces temperature fluctuations.
6. Aspect and slope	South-facing slopes (in India) receive more sunlight.
7. Depth	Temperature decreases and fluctuates less with depth.
8. Season and climate	Higher in summer, lower in winter.

2.4 Measurement of Soil Temperature

Instrument	Use / Depth
Soil thermometer / Thermocouple	Measures temperature at surface and different depths (5, 10, 20 cm).
Soil thermograph	Records continuous temperature variation.
Infrared sensors	Measure surface soil temperature remotely.

2.5 Importance of Soil Temperature

Process	Effect of Temperature
Seed germination	Optimum temperature required (e.g., wheat: 20–25°C; rice: 25–35°C).
Root growth	Roots develop best at moderate (20–30°C) temperatures.
Microbial activity	Increases with temperature up to ~35–40°C.
Nutrient availability	Enhanced by increased biological and chemical activity.
Organic matter decomposition	Faster at higher temperatures.
Soil moisture loss	Higher temperature → higher evaporation.

2.6 Soil Temperature Profile

2.7 Methods to Regulate Soil Temperature

Practice	Purpose
Mulching (organic or plastic)	Conserves moisture and moderates temperature.
Irrigation	Cools soil during hot periods.
Drainage	Improves aeration and reduces heat loss in wet soils.
Vegetation / Cover crops	Reduces temperature extremes.
Tillage	Exposes soil → increases heating and drying.

2.8 Diagram Suggestion (for Notes)

Title: Soil Temperature Variation with Depth and Time
Draw a simple graph:

X-axis: Time (Day/Night or Summer/Winter)
Y-axis: Temperature (°C)
Curves:
- Surface layer → high fluctuation
- 10 cm → moderate fluctuation
- 30 cm → nearly constant line

Parameter	Soil Consistency	Soil Temperature
Definition	Cohesion & adhesion of soil particles under varying moisture	Degree of warmth or heat in soil
Main factors	Clay content, moisture, organic matter	Solar radiation, color, moisture, texture
Measurement	Atterberg limits (LL, PL, SL)	Thermometer, thermograph
Importance	Affects tillage, structure, root penetration	Affects germination, microbial activity, nutrient release
Ideal condition	Friable at field capacity	20–30°C for most crops

Quick Links