This is most likely the result of over watering or the soil having too heavy a water retention quality. If roots are not able to breathe oxygen, many symptoms of nutrient deficiency surface.

okay thanks, but is it ok even if the soil on top is really dry ?

Melbel, don't be afraid to water less, as long as you check the plant often. The plant leaves will droop when it's really really thirsty, and it will perk up within a couple of hours of watering. Containers are tricky because they have very specific needs for watering because they are small and have crazy potting soil. As summer heat comes, more frequent waterings will be needed. But during the spring when evaporation is minimal....