Usually percent depletion of available water content of soil ( soil water content at field capacity minus permanent wilting point) is used to execute differential intensity of drought stress....

1) Determine the exact water holding capacity (WHC) for the select soil per pot.

2) after sowing keep the soil filed capacity in a range of 90-95% WHC.

3) let's suppose that we want to have 6 water levels in your experiment.

4) Start immediately to create the different levels in the next time when you irrigate the plants to see a strong effect at the end of the experiment.

8) Weight the control pots to estimate the evaporated water.

- For instance, when the evaporated water was 200 ml, the irrigation would be 200, 170, 140, 110, 80, 50 ml to the treatments 100, 85, 70, 55, 40, 25 % WHC, respectively.

- Two or three weeks later (it depends on the studied species and its growth rate), the plants will need an additional amount of water to compensate for transpiration

- To keep the difference fixed between the treatments, calculate the different levels according to the evaporated water. then sum to it the amount of water that you would expect to compensate transpiration

E.g., when the pots needs to be irrigated with 75 ml for evaporation + 50 ml for transpiration, it represented 125 ml for 100% WHC. Then, the water add for the different levels will be as the following:

100 % WHC = 125

85% WHC = 96

70% WHC = 79

55% WHC =62

40% WHC = 45

25% WHC = 28

please see the attached excel file that includes a simple example of the calculations for one-day irrigation.

Great answer Heba Elsalahy Another term used for WHC when talking about potting media is Air Filled Porosity. I put together a technical paper describing how to measure it. Technical Report Air Filled Porosity of Potting Media

Dear Dr. Simons,

Thank you very much for this interesting information about Air Filled of Potting Media.

Heba Elsalahy Jacob Simons Thank you very much for your help and interesting information.