Soil–Water–Plant Relationships

Where crops get their water. Learn soil texture and structure, bulk density and porosity, gravimetric and volumetric moisture, soil water potential and the retention curve, saturation, field capacity, permanent wilting point, available water and management allowed depletion, infiltration and hydraulic conductivity, root zones, evapotranspiration basics, soil salinity and measuring soil moisture (gravimetric, tensiometer, TDR/capacitance sensors) — with Python for available water and irrigation interval.

Soil is the crop's water reservoir. How much water it can hold between field capacity (after drainage) and wilting point (plants can no longer extract water) decides how often to irrigate.

Understanding soil–water relationships is the basis of irrigation scheduling, drainage design and efficient water use.

An analogy

Soil is like a sponge in a sink: when soaked it drips (gravity drainage) until it reaches field capacity; plants then suck water out until what remains is held so tightly they can't pull it (wilting point). The water between is the plant's 'bank balance'.

Soil physical properties

Texture (sand, silt, clay proportions — USDA textural triangle), structure (aggregation), bulk density (1.1–1.7 g/cm³; compaction raises it and restricts roots), porosity (~40–55%). Clay soils hold more water but drain slowly; sandy soils drain fast and hold little.

Soil water constants

Key moisture levels:

Infiltration and movement

Infiltration rate starts high and decreases to a steady basic rate (sand > 30 mm/h, clay < 5 mm/h). Water moves by Darcy's law with unsaturated hydraulic conductivity K(θ) that drops sharply as soil dries (Richards' equation). Infiltration governs choice of irrigation method and application rate to avoid runoff.

Measurement and salinity

Measure moisture by gravimetric oven drying (reference), tensiometers (suction), TDR/FDR/capacitance probes, neutron probes, and satellite soil moisture (SMAP, Sentinel-1). Salinity (ECe) reduces water availability via osmotic potential; leaching requirement LR = EC_w/(5EC_e − EC_w) guides extra irrigation. Sodic soils need gypsum and drainage.

Formulas

Worked example: Volumetric moisture

Given: w = 0.18, ρ_b = 1.5

Find: θ.

Answer: 0.27

Worked example: Available water

Given: FC 0.30, PWP 0.12, root depth 0.9 m

Find: TAW.

Answer: 162 mm

Worked example: Irrigation depth

Given: θ now 0.22, FC 0.30, root 0.6 m

Find: Net depth to refill.

Answer: 48 mm

Available water and irrigation interval

# Soil-water-plant: available water, bulk density and moisture
FC, PWP = 0.32, 0.15          # volumetric water content at field capacity and wilting point
root_m = 0.6
TAW = (FC - PWP) * root_m * 1000          # mm total available water
p = 0.5                                    # allowable depletion fraction
RAW = p * TAW
print(f"total available water {TAW:.0f} mm, readily available {RAW:.0f} mm")
# Gravimetric -> volumetric
w, rho_b = 0.20, 1.40                      # gravimetric moisture, bulk density g/cm3
theta = w * rho_b
porosity = 1 - rho_b / 2.65
print(f"volumetric moisture {theta:.2f}, porosity {porosity:.2f}, saturation {theta/porosity:.0%}")
# Days between irrigations if crop uses 6 mm/day
print(f"irrigation interval at ETc 6 mm/day: {RAW/6:.1f} days")
# total available water 102 mm, readily available 51 mm
# volumetric moisture 0.28, porosity 0.47, saturation 59%
# irrigation interval at ETc 6 mm/day: 8.5 days

A loam with 0.6 m roots stores ~102 mm of available water; allowing 50% depletion before stress gives ~51 mm, i.e. irrigate about every 8 days in peak demand. Sandy soils hold less and need more frequent, smaller irrigations.

Common mistakes

Applications

Key takeaways

Going further