They stop growing a full amount of low value subsistence crops needed to survive and start growing cash crops on some portion or on all of the land. Those cash crops have a higher value.

An example - say you have 4 acres of land and have a family of 4.

In the old world, say you needed one acre per person to grow enough food to the next crop harvest. This would be something like corn or potatoes that can keep. So all your land goes to growing food to survive and you cant make any money.

In the new world, with irrigation, you can do much more - say for the sake of argument, 4 times the crop, in the same space. Now, you only need 1/4 of an acre per person or an acre for everyone. So you grow vegetables that sell for 10 times as much on the 3/4s of land you have that you no longer need to use to survive.

Or even better, you grow high veg on the entire piece of land for income and use the cash to buy your corn and potatoes or whatever as you need them.

Just as all other commercial farmers do across the world.

In other words, solar allows them to become small business owners.