6. Qualitative description of soil wetness

(1) Maximum retentive capacity: soil water content when all pores are filled with water, or totally saturated.

(2) Field capacity (or field water holding capacity): maximum soil water content after the drain of macropore water due to gravity gradient (~ -33 Kpa).

(3) Permanent wilting point (or, wilting coefficient, wilting percentage): the soil water potential (content) at or below which plant roots cannot absorb water any more (~ -1500 Kpa).

(4) Hygroscopic coefficient: the soil water content of air-dried soil after reaching equilibrium with water vapor in the air of 98% relative humidity. Clay colloids are the main agents for holding this water.

(5) Factors influencing thresholds above: Texture, SOM, bulk density, and salinity