Leeloo: Leeloo Dallas mul-ti-farm.
Korben Dallas: Yeah.
Korben Dallas: Yeah, multifarm, she knows it's a multifarm. Leeloo Dallas. This is my wife.
Korben Dallas: We're newlyweds. Just met. You know how it is. We bumped into each other, sparks happen…
Korben Dallas: Yes, she knows it's a multifarm. Anyway, we're in love.
MultiFarms are multi-block structures that are built using special Farm Blocks and building blocks (stones/bricks).
Pre v3.1 The smallest multi-farm is 11×11 while the largest is 17×17
(2 x (width of central structure + 1) + width of central structure)
Post v3.1 The smallest multi-farm is 21×21 while the largest is 35×35
(2 x (width of central structure x 3) + width of central structure)1)
Post v4.0 The size of the farm can be modified in common.cfg by changing the I:size variable. By default this is set to 2.
(2 x (width of central structure x <size>) + width of central structure)
By default the smallest farm size is 15×15 while the largest is 25×25.
Multi-Farms also support specialized Farm Blocks to add functionality, the Farm Gearbox being the only one required (as it's the only way to supply power to the multi-farm). There is also a Farm Valve, Farm Hatch and Farm Control. See Farm Structure for more information.
The GUI is divided into seven areas:
I - Water Input Slot
Place Water Buckets/Cans/Capsules in here in order to fill the Water Tank (II).
II - Water Tank
This shows how much water the MultiFarm currently has. It can be filled through (I) or through Farm Valve blocks connected into Buildcraft Waterproof Pipes.
III - Current Configuration and Circuit Board Socket
This area shows the current configuration of the MultiFarm. It has a socket in the center for you to place (or remove, using a Soldering Iron) Circuit Boards in order to place Electron Tubes that will alter the MultiFarm's behaviour. See Configuring the Farm for more information.
IV - Fertilizer Slot
Most farm configurations will need Fertilizer for operation. Place Fertilizer in here or pipe it into the MultiFarm using Farm Hatch blocks connected into Buildcraft Transport Pipes.
V - Soil Inventory
This is where you will place the necessary soil for the farm to either lay down the farmland (if it's a Managed Farm) or to replace decayed soil blocks. Items can be piped in through Buildcraft Transport Pipes connected into Farm Hatch blocks.
VI - Germlings Inventory
This is where you will place the germlings (Saplings, Seeds, Cactus, etc.) for use in the farm. Items can be piped in through Buildcraft Transport Pipes connected into Farm Hatch blocks.
VII - Output Inventory
Any waste or harvested resources end here. Farm Hatches will automatically eject them into inventories or Buildcraft Transport Pipes directly below them.
Due to soil erosion caused by overharvesting on an industrial scale, farms need to be supplied with fertilizer to replenish their soil's mineral content. The amount required varies for different types of farms (see below). Though compost and mulch can both replace fertilizer anywhere else, only phosphorous fertilizer ground from apatite gems satisfies these needs of a farm. A single unit of fertilizer equates to 2.000 mU2) on default settings.
Most farms will also require a steady supply of water to keep crops and soil hydrated.
If, for any reason, you want to modify the farm's behaviour (for example, to change the crops it will harvest), you can do so by soldering Electron Tubes into Circuit Boards with a Soldering Iron.
More information can be found at Farm Configuration.