Field Capacity :
The maximum quantity of water which a soil can retain against the force of gravity is known as field capacity.
The quantity of water greater than the field capacity simply passes away.
Permanent wilting Point :
Permanent wilting point is the moisture content at which the moisture is no longer available in sufficient quantity so that the plants can sustain.
Even though the soil contains some moisture but it was so held by the soil that roots of plants cannot uptake it and results in wilting of plant.
Available water :
The difference between two moisture contents ( field capacity and permanent wilting point ) is known as available water.
It is the moisture content which is available for plants .