chapter 1
wet plowed tile = by using a watering can on a plowed tile uses 0.1litres of water
by water tile in landscaping or by irrigation cannals (see chapter 2)
this boosts the output of your crops by 50%
fertilized ground = using ash/slag on plowed tiles
by fertilize ground in landscape menu
output boosted by 50%
these can be combined so you will get a double harvest at the end, a good reward for hard working farmers.
chapter 2:
irrigation:
rivers naturaly provide some tiles of wet soil when you plow them.
this could be extended by the use of irrigation cannals and the screw of archimedes able to spread irrigation to far beyond the rivers.
around rivers the wet soil should extend to about max 30 tiles.
irrigation canals should provide around 15 tiles of wet soil.
disadvantage of irrigation:
carts possibly cant cros it (up to players to decide)
screw need to be operatered several times to rewet to soil after the harvest.