The Garden Ledger | Carla Kay

The Wisdom of Dry and Wet

In Redwood City, my grandmother taught me that a tomato plant tells you everything you need to know by the way it stands at dawn. Drooping? It's thirsty. Perky? It's content. But she didn't have equations.

Today we marry her intuition to the mathematics of Martinus Theodorus van Genuchten, whose inverted sigmoid curve maps the invisible dance between soil particles and water molecules. This is not merely calculation—it's translation.

When Anchi Le speaks of survival margins in colony kitchens, and Arthur Ibay measures the Van Genuchten retention curves, I hear the same truth: resilience is quantified care.

Source: van Genuchten–Gupta model (Wikidata Q41804881) • soil moisture property (Wikidata Q889507)

Available Water Capacity Calculator

This tool translates soil texture into the liters of drinkable water your plants can access between field capacity and wilting point. Input your conditions; receive your stewardship mandate.

θ(wet) − θ(dry) = Available Water

Where:
θ(wet) = Field Capacity (vol%)
θ(dry) = Permanent Wilting Point (vol%)

Derived from Van Genuchten soil water retention model

Soil Texture Class:

Root Zone Depth (cm):

Bed Area (m²):

The Working Example

My raised bed is 2 meters × 2 meters (4 m²), filled with loam amended with compost, rooting 30 cm deep. The Van Genuchten parameters for loam give us:

Field Capacity θ(wet): 27.4 vol%
Permanent Wilting Point θ(dry): 12.8 vol%
Available Water: 14.6 vol%

Volume of soil: 4 m² × 0.30 m = 1.2 m³ = 1,200 liters
Available water: 1,200 × 0.146 = 175.2 liters

That is the total reservoir your tomatoes, basil, and collards share. Divide by daily evapotranspiration, and you know the hours until thirst.