There is actually no optimum general quantity. Each species needs, each soils characteristics and each valley’s climate are different. However, it is impossible to communicate a manual for each soil type, each species and each climate. This advise works worldwide on each species, each soil and in each climate. Suppose you have a Tara tree (Ceasalpinia Spinosa), clay soil (that has a better water fixing capacity than sand) and moderate circumstances (temperatures do not rise above +35⁰C). Then you can use less water. Suppose you have a Tara tree (Ceasalpinia Spinosa), sandy soil (that has a little water fixing capacity) and hot circumstances (temperatures above +45⁰C). Then you need more water. Our advise of 60 liters is therefore a safe average advise. So in general people start with the advise and investigate their own local optimum.