WHC stands for Water Holding Capacity, which indicates how well the media holds water. Water Holding Capacity is the ability of a certain soil texture to physically hold water against the force of gravity. The WHC also determines how much irrigation water can be applied at one time to match the infiltration rate and avoid applying more water than field capacity.
Knowing the WHC of your growing medium is important because some hydroponic systems will be more compatible with lower WHCs and others will be more compatible with higher ones.
AFP means Air Filled Porosity and is the proportion of volume that is filled with air. A low AFP means the medium can’t provide the plants with oxygen very well, putting them at risk for drowning and rotting.
CEC, or Cation Exchange Capacity, refers to the medium’s ability to capture positive ions like potassium, calcium, and magnesium.
In hydroponic systems, a low CEC allows you to have better control over the nutrients your plants receive.
Growing mediums with a higher CEC, like coconut fiber, may require specially tailored nutrient solution in order to achieve the right balance.