I was amazed to look thru several Dictionaries (online and in book form) and discover the word "irrigant" does not -according to them- exist.
I would propose that the word "irrigant" be launched and used.
A proposed usage would be: Water is probably the world's most used irrigant."
At least one definition would be "an application of one thing to another that encourages growth."
"Once the coastal farmer did away with salt water as an irrigant, and used fresh water, his crops began to grow."
"My wife is such an irrigant, always coming up with ideas that stretch my mind and cause me to grow."