There ya go....
Potassium
Potassium serves to aid in the process of photosynthesis, nutrient uptake, respiration, and is a key to sturdy stems and disease resistance. Having an adequate supply for the plant early in it?s life stage will keep your seedlings from falling over.
Potassium deficiencies are identified by necrosis on the margins of larger fan leaves. Necrotic patches can be seen on the leaves as well. Leaves will eventually turn yellow, brown and die off. Most of the plants that show signs of a Potassium deficiency are going to be the tallest and best looking plants you have.
Again, feeding with a good all-around fertilizer will clear it up as well.
You can also use wood ashes and water them into your soil to fix this. You'll need to replace your soil's holding capacity of Potassium, and have a surplus before it will become available to the roots again.