When a tank is cycled, there is no detectable level of ammonia or nitrite. Ammonia is made constantly by the fish but it is turned into nitrite then nitrate almost as soon as it's produced and the levels in the water are so low they can't be detected. If either ammonia or nitrite are detectable it means there are not enough bacteria; or in other words the tank is not cycled, either because it is not yet completed cycling or because something has killed some bacteria.
Most aquarium plants take up ammonia rather than nitrate because they have to expend energy turning nitrate back into ammonia to use it. If there is insufficient ammonia for the plants' needs they will use nitrate.
Most aquarium plants take up ammonia rather than nitrate because they have to expend energy turning nitrate back into ammonia to use it. If there is insufficient ammonia for the plants' needs they will use nitrate.