Do we need to fit threshold tension to soil type (sand, light soil, heavy soil, soil-less media)?

- Yes we do. The question is why. Soil water tension is a measure of suction, it

is the force by which the soil particles bind water and quantitavely is

expressed in units of Paskal (i.e. kPa – kilo Paskal). The force is negative and

absolute and therefore it is independent of the soil type. A -30 kPa is the same

potential energy in sand, light or heavy soil types. It can be argued therefore

that the plant need to invest the same amount of energy to extract water from

all soil types and threshold tension does not have to be adjusted. However,

Soil texture varies in different soil types and volumetric water holding

capacity varies according to soil type. A 20% available volumetric water

content in sand has a much lower water tension (ca. -10 kPa) than a heavy soil

water tension (ca. -80 kPa). To supply the same amount of water needed for a

plant in sand as in heavy soil we need to maintain a lower water tension in

sand.