Green Manure is a crop is growable in all types of farm. It does not produce Food but instead restores Fertility to the farm's soil. It takes 4 months to reach maturity and can survive for 4 months without Water.
In the early game before Fertilizer is readily available, Green Manure represents the only method for maintenance of Fertility levels. When planting in alternating cycles with Potatoes, 100% Fertility can be maintained indefinitely unless allowed to wither due to extended drought with irrigation.