Pansies and violas are very hardy little plants but here are some potential problems and their solutions.
Nutrient Deficiencies:
Boron deficiency - Boron is one of the many trace nutrients plants need but in such
minute quantity it's almost impossible to measure. Most fertilizers contain trace nutrients along with the main three but not all of them. A boron deficiency is fairly easy to diagnose. The newly developing leaves will be much smaller and thicker than average and appear very 'cabbage like' (curled). The easiest solution for this is to look for a fertilizer that does contain trace nutrients and apply as directed. If this is not an option for you, a simple home remeady is 'Mule Team' laundry detergent. Add a pinch of detergent per gallon of water and apply to the affected areas. One dose is plenty. Don't be discouraged if you don't see an immediate change. Pansies usually will take a few weeks to recover from this problem.
Iron deficency - This is the most common problem I have run into while growing pansies and 
violas. These little plants just love iron! If you have an iron deficiency in your plants, the leaves will start to turn yellow on the outside edges and slowly move inwards. This usually is first noticable on the youngest leaves and then spreads to the rest of the foliage. The best solution when faced with this problem is to sprinkel some granular ironite in your flowerbed. I use about a tablespoon per square foot. You should see a noticable change in your plants within a week or two.
magnesium deficiency can be recognised by yellowing of the leaves. However, the yellowing will began on the newly matured leaves, not the new growth, and then spread to the rest of the plant. The best cure for this particular problem is Epsom Salts. Mix about a teaspoon of Epsom Salts per gallon of water and apply.
