🎒 Soil-Plant-Atmosphere Continuum

This lesson builds core elective concepts in BSc Agriculture with practical applications and exam-oriented clarity.

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Soil-Plant-Atmosphere Continuum

The Soil-Plant-Atmosphere Continuum (SPAC) describes the continuous pathway of water movement from soil through the plant to the atmosphere. This concept is fundamental to crop simulation models that calculate water uptake, transpiration, and crop water stress.

The SPAC Concept

Water moves along a gradient of decreasing water potential:

• Soil — highest water potential (-0.01 to -1.5 MPa)
• Root — intermediate (-0.5 to -2.0 MPa)
• Leaf — lower (-1.0 to -3.0 MPa)
• Atmosphere — lowest (up to -100 MPa at low humidity)

The driving force is the vapour pressure deficit (VPD) between the leaf interior and surrounding air, which pulls water upward through the continuous liquid column in xylem vessels.

Water Uptake Models

Crop simulation models calculate root water uptake using different approaches:

Approach	Description	Used In
Tipping bucket	Water drains from upper to lower layers based on field capacity	DSSAT
Richards equation	Physics-based flow using soil hydraulic conductivity	SWIM (APSIM)
Root density weighted	Uptake proportional to root length density in each soil layer	Most models
Demand-supply	Daily transpiration demand met by available water across root zone	WOFOST, InfoCrop

Key factors affecting water uptake:

• Root depth — increases during vegetative growth, typically reaching 60-150 cm
• Soil texture — clay soils hold more water but release it less readily than loams
• Available water capacity — water held between field capacity and permanent wilting point

Nutrient Dynamics

Models simulate nutrient cycling, particularly nitrogen:

• Mineralisation — conversion of organic N to inorganic forms (NH4+, NO3-)
• Nitrification — NH4+ converted to NO3- by soil bacteria
• Denitrification — NO3- lost as N2O/N2 gas under waterlogged conditions
• Leaching — NO3- moves downward with percolating water beyond root zone
• Crop uptake — absorption of NH4+ and NO3- by roots, driven by demand and availability

The nitrogen balance in models ensures that:

• N supply = fertiliser + organic matter + fixation + atmospheric deposition
• N demand = crop uptake + losses (leaching + denitrification + volatilisation)

Evapotranspiration Modeling

Evapotranspiration (ET) is the combined loss of water through soil evaporation and plant transpiration:

• Reference ET (ETo) — calculated using FAO Penman-Monteith equation based on temperature, humidity, wind speed, and radiation
• Crop ET (ETc) — ETo multiplied by a crop coefficient (Kc) that varies with growth stage
• Actual ET (ETa) — adjusted for soil water availability; ETa < ETc under drought stress

Crop Coefficient (Kc) Values

Growth Stage	Kc (Rice)	Kc (Wheat)
Initial	1.05	0.40
Mid-season	1.20	1.15
Late season	0.90	0.30

Importance in Crop Simulation

The SPAC framework allows models to simulate water stress effects on crop growth — reduced photosynthesis, accelerated senescence, and yield loss — providing a mechanistic basis for irrigation scheduling and drought impact assessment.

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Summary Cheat Sheet

Topic	Key takeaway
Main focus	Soil-Plant-Atmosphere Continuum — water uptake models, nutrient dynamics, evapotranspiration modeling, and SPAC concept in crop simulation.
Section context	Revise this lesson with the rest of System Simulation and Agro-Advisory for stronger conceptual continuity.