Once children begin to read they acquire new vocabulary most naturally as the indirect result of reading difficult, challenging material  The major alternative -- direct studying of new vocabulary items -- is less natural; most people are not so inclined  From age six to age thirteen, children learn thousands of new words a year, mostly through reading; direct studying accounts for less than one-tenth of all new words learned
So it is bound to be inefficient for young students to learn new vocabulary by direct studying
Children from age six to age thirteen spend vastly much more time reading than they spend directly studying new vocabulary.