Dear Buhroy,

Field capacity is the content of water on a mass or volume basis remaining in a soil, 1or 2 days (depending on soil type, sandy soil reaches faster compared to clayey soil) after completely saturated with water and free drainage is negligible. To maintain the soil at field capacity the bags can be weighted and the loss in weight can be replaced with water. Or else the soil moisture content can be measured through soil moisture probes (Delta T) and the requisite amount of water can be added to bring back the moisture level to FC depending on the soil type. 

Thanks

K.B.Hebbar

field capacity is easy, as that is (sort of) the maximum it will hold, but 50% is undefined without a clear picture of what you mean by 0% water? Can you work out the weight of a bag of soil at 100% and 0%? Then 50% will be half way between the two values. Maintaining 50% through the expt will be impossible using weight, as the weight of the plant will influence both weighing the bag, and how much water you need to add. You will need to use a soil moisture probe that you calibrate b4 you start growing anything.

Thank you K.B. Hebbar and Gayle Somerville for your kind response....

The different type of soil have different field capacity, The 100 % field capacity is the point that soil capacity to hold maximum water. And the wilting point is the water holding capacity of soil which plants can not absorb. Between wilting point and the maximum field capacity the water present in soil that plant can absorb.

To give water stress means limiting water availability of plant to absorb. for that you have to monitor the soil moisture content for particular type of soil to irrigate the crop to maintain the 50% field capacity. Refer diagram for the field capacity of different kind of soil.

SAND DUNES

H. Tsoar, in Encyclopedia of Soils in the Environment, 2005