Very interesting question. Many times , it strikes to me that if any fertilizer is applied as folk at.. how much benefit , it will evoke to plants .since in foliar spray, nutrient is already in nano size..However , please find some interesting manuscripts..
We have tested bio-synthesized nano-calcium on arid crops like pearl millet and cluster bean. We found that 40 ppm concentration is the most effective. But we have not tried on any trees.