​1) The soil is important for the plants because that is where they get their nutrients  and food to help them grow  and healthier plants or crops. Water conservation is important  to manage the water as a sustainable  resources and to protect the water environment  for human needs for everyday living. 
2) The reason why people and farmers plants cover crops is because it helps reduce heavy water flows when it rains and lead  to dirty water and ruin the soil nutrients. The roots on the plants impact together the soil leading to a somewhat clean water. Prevent to loss the nutrients  and to keep  the soil in one place to hold the nutrients. 
They help the Hawaii Ahupua'a by reducing the soil erosion going to other place like river or beach. 
3) Three career  in agriculture that I know are plant pathology, soil scientist  and conservation  specialist 

Attachments area

This is a water shed model it tells you what will happen if it got heavy rain where would all the soil and other stuff will end up after the heavy rain.