mediterranean type nitrogen fixing plants

Hi
 
With the advent of summer some of my established plants, especially Lavender and Barleria, are showing signs of, what I believe, is nitrogen deficiency. That is, their leaves are yellowing and, although the growth doesn't seem to be affected, it is unsightly.
 
Two questions:
 
Is there any other possible explanation for the yellowing?
I am looking for a long-term low-maintenance solution. I know there are plants that fix nitrogen in the soil and my intention is to companion plant the ailing plants with these. Have any members got any suggestions for waterwise plants that could fulfil this role and would also survive acid sandy soil and hot salty winds?  
 
Regards
Charl
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