Plants need more water in the heat, but not necessarily more food. Fresh water on a hot day is good for both the garden and the gardener.
The idea is simple. Plant an extra row of vegetables, and donate the harvest to your local food bank or shelter.
Conversion for US Gentle Readers
It is often easiest to make calculations in milliliter (ml) or grams (g) per liter. Unfortunately is it also often easiest to actually mix nutrients by the gallon.
If you have the application rate in liters, and you want to calculate how much to add for a gallon, select the first option. If you have the application rate in gallons, and you want to know how much to add per liter select the second option.