This constant represents the percentage water content below which permanent wilting will occur in many plants.  This value is referred to as the wilting point.¹ 

This value is the upper bound on a range defined to be the range of water content that most likely will cause a plant to die.  Since we're using the upper bound in this constant, this means to be safe, one should use a greater amount of water per unit volume of soil than this constant implies.

Usage

In soil science, hydrology and agricultural sciences, water content has an important role for groundwater recharge, agriculture, and soil chemistry. Many recent scientific research efforts have aimed toward a predictive-understanding of water content over space and time.

Observations have revealed generally that spatial variance in water content tends to increase as overall wetness increases in semiarid regions, to decrease as overall wetness increases in humid regions, and to peak under intermediate wetness conditions in temperate regions .

1. ^ http://en.wikipedia.org/wiki/Water_content#Earth_and_agricultural_sciences