…the probability of acquiring an unknown word incidentally through reading is only about 15% (Swanborn & de Glopper, 1999), which means the word would need to be encountered eight times to be learned with high probability. The probability of learning any word at a first encounter is lower for younger readers, for more difficult texts, and probably for students who have had no training in deriving meanings for unknown words (Fukkink & de Glopper, 1998; Kuhn & Stahl, 1998). Thus, incidental vocabulary learning is not a reliable procedure for promoting vocabulary growth.
Carlo, M., August,D., McLauglin, B., Snow, C., Dressler, C., Lippman, D., Lively, T., & White, C. (2004). C losing the gap: Addressing the vocabulary needs of English -language learners in bilingual and main stream classrooms. Reading Research Quarterly, 39, no 2 p. 188 – 215
In other words, don't just assume wide reading will engender vocabulary learning. On the other hand, even if you teach three words a day explicitly, that's only 600 a year, far too few for a student with weak vocabulary to catch up. So the best currently known approach is to do both: explicit instruction and frequent encounters with new words during reading. With vocabulary, once you are behind, it is simply very difficult to catch up.
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