Is irrigation the artificial of application of water to the land or soil

I believe both is the answer to your question. In the definition of this word both land and soil are correct.

The definition of irrigation is:

"Irrigation is the artificial process of applying controlled amounts of water to land to assist in production of crops. Irrigation helps to grow agricultural crops, maintain landscapes, and revegetate disturbed soils in dry areas and during periods of less than average rainfall. "

the function makes a strait line horizontally with no x intercept but a y intercept at 9.