Original 1.12.2 farm (one observer, three bonemeal filled dispensers, four dust)...
1.13.1 fix - just two dust, a repeater and a sticky piston added to front of existing farm.
(Move the light-blocking slab at the front up to the piston to let light in so crop can be planted. This will be temporarily lowered to harvest crops)
Well done on showing the fix. :>:
For readers new(ish) to RS, however, a bit more explanation would likely be appreciated.
The first pic [636710550649955972.png] shows the 1.12 version:
A block of tilled land hydrated by a water source [probably directly behind it in the pic] with dispenser loaded with bonemeal on three sides and a top slab on the fourth side.
The trench in front gives room for the player to fit under the top slab.
The observer over the tilled land (looking down at it) fires when a crop is planted, triggering the bone meal to grow the plant.
Because the tilled land is at too low a light level, the crop pops out – just slowly enough that the bonemeal has time to work.
The first pic [636710550744803447.png] shows the 1.13 version:
The game code was changed so crops cannot be planted a very low light levels: this required moving the light blocking slab up to allow light onto the tilled land.
Harvesting is now accomplished by the sticky piston lowering the slab which darkens the tilled land causing the crop to "pop out".
Either model can be fitted with a hopper (beneath the tilled land) and chest to assist collection. [Tilled land is not a full block allowing a hopper to 'suck' items on tilled land.]
WARNING: I have an extemely "grindy" playstyle; YMMV — if this doesn't seem fun to you, mine what you can from it & bin the rest.