Nitrogen burn or depletion...

[Christian J. Brennan]

Hello!

In our outdoor garden my wife and I noticed some leaves on our plants 
were turning yellow.  Starting from the tip of the plants, they are 
crispy brown, and loosing clorophyll from the outer edges and working its
way inward.  The root structures are healthy-a mauve color, almost 
white.  This leads me to think since the roots aren't getting burned,  
the yellowing of the leaves cannot be caused by a nitrogen burn, and 
perhaps it nitrogen depletion...

Can anyone confirm this?  As well, we are trying to grow things without 
chemicals, can anyone recommend a good natural nitrogen supplement?

Thanks for your time.

Respectfully,
Jim.