Soil Water Retention, Energy Concepts, Soil Water Potential, Retention Curves (pF) and Hysteresis

Historical evolution of the tensiometer:

• Burton E. Livingston (1908) first invented the porous porcelain cup auto-irrigator device, developing the foundational porous-cup tensiometer concept to measure the drying power of soil and plant water relations.
• Willard Gardner and his colleagues in 1922 developed the field capillary tensiometer.
• L. A. Richards (1928, 1941) refined tensiometer engineering and introduced pressure plate apparatus for laboratory soil moisture retention curves.
  NTA officially credited B. E. Livingston as the original proposer of the tensiometer design.

Instrumentation for matric potential measurement:

• Tensiometers have a narrow operating range restricted to $0\text{ to }0.85\text{ bar}$ ($0-85\text{ kPa}$), breaking cavitational suction in drier soils.
• Resistance blocks (gypsum blocks) are insensitive at high wetness ($\lt 0.5\text{ bar}$).
• Pressure plate / pressure membrane apparatus (Richards, 1941) operates across the widest range from near-saturation ($0.1\text{ bar}$) up to the wilting point and beyond ($15\text{ to }100\text{ bars}$), making it the laboratory benchmark for establishing moisture characteristic curves.

The concept of $\text{pF}$ was introduced by R. K. Schofield (1935):

• Definition: $\text{pF}$ is defined as the common logarithm (base 10) of the soil water suction or tension head expressed in centimeters of a water column: $\text{pF} = \log_{10}(h)$, where $h$ is the hydraulic suction head in $\text{cm}$.
• Calculation: Here, tension head $h = 100\text{ cm}$. Therefore, $\text{pF} = \log_{10}(100) = 2.0$.
• Reference points on $\text{pF}$ scale: Free water / saturation: $\text{pF } 0$ ($1\text{ cm}$); Field capacity: $\text{pF } 2.53$ ($340\text{ cm}$ or $1/3\text{ bar}$); Permanent wilting point: $\text{pF } 4.18$ ($15,000\text{ cm}$ or $15\text{ bars}$); Hygroscopic coefficient: $\text{pF } 4.50$ ($31,000\text{ cm}$ or $31\text{ bars}$); Oven dry: $\text{pF } 7.0$ ($10^7\text{ cm}$).