harvest

 doLeft = true

function harvest(length, width)
 for i=1,width,1 do
  harvestLength(length)
  if width ~= i then
   if doLeft then
    left()
   else
    right()
   end
  end
  doLeft = not doLeft
 end

 if doLeft then
  turtle.turnLeft()
  for i=2,width,1 do
   turtle.forward()
  end
  turtle.turnLeft()
 else
  for i=1,length,1 do
   turtle.back()
  end
  turtle.turnRight()
  for i=1,width,1 do
   turtle.forward()
  end
  turtle.turnLeft()
 end
end

function harvestLength(length)
 thing = length - 1
 for i=1, thing,1 do
  turtle.digDown()
  replant()
  turtle.forward()
 end
 turtle.digDown()
 replant()
end

function replant()
 turtle.select(1)
 turtle.placeDown()
end

function left()
 turtle.turnLeft()
 turtle.forward()
 turtle.turnLeft()
end

function right()
 turtle.turnRight()
 turtle.forward()
 turtle.turnRight()
end

harvest(8,4)