When you middle click on farmland you get a dirt block despite the fact that farmland is an item as well.
- Furrow a dirt block and middle click it
You will get a dirt block
- Use the following command
You will have a farmland block
The following is based on decompiled version of Minecraft 1.9 using MCP 9.24 beta. All method and class names are the names used in the decompiled version.
The reason why this happens is because the net.minecraft.block.BlockFarmland.func_185473_a(World, BlockPos, IBlockState) method returns a dirt item stack.