How is zakat paid on leased agricultural land when its rent is paid in crops that are later sold?

If the land is cultivated under a muzara'ah (sharecropping) agreement between the questioner (landowner) and the worker, such that the worker receives a known share of the crop, then each of them must pay zakat (definition) on his share of the grains if it reaches the nisab (definition) (threshold) and is among the items upon which zakat is obligatory. The amount of zakat is one-tenth (1/10) if the land is irrigated without cost, and one-half of one-tenth (1/20) if it is irrigated at a cost.