Cane Farmer

function top_row()
  turtle.up()
  turtle.up()
  while turtle.detectUp()==false do
    turtle.forward()
    turtle.turnLeft()
    turtle.dig()
    turtle.turnLeft()
    turtle.turnLeft()
    turtle.dig()
    turtle.turnRight()
  end
  turtle.turnLeft()
  turtle.turnLeft()
  turtle.forward()
end

function bottom_row()
  turtle.down()
  while turtle.detectUp()==false do
    turtle.forward()
    turtle.turnLeft()
    turtle.dig()
    turtle.turnLeft()
    turtle.turnLeft()
    turtle.dig()
    turtle.turnRight()
  end
    turtle.turnLeft()
    turtle.turnLeft()
    turtle.forward()
    turtle.down()
end

while true do
  top_row()
  bottom_row()
  turtle.detectDown()
  while false do
    turtle.down()
  end
  turtle.select(1)
  turtle.dropDown()
  turtle.select(2)
  turtle.dropDown()
  turtle.select(3)
  turtle.dropDown()
  turtle.select(4)
  turtle.dropDown()
end