Knowledge builds upon knowledge, skill upon skill. And the skill of skills reading well, which is the key to further learning, is not just a mechanical skill. Reading well isn't just knowing how to sound out words and perform rapid eye movements, or look for the main idea. It entails having a wide vocabulary so that one can understand newspapers, serious books, and training manuals for new skills. And a wide vocabulary means having correspondingly wide knowledge.
Words stand for things.
To possess a wide vocabulary without wide knowledge is a thing that has never existed and never will. In the early grades, breadth of knowledge is ultimately the key to depth. Broad, well-chosen knowledge in the academic disciplines is the one thing needful in effective early schooling. Apologists for the current state of public schools continue to blame the persistent achievement gap between ethnic and racial groups on social conditions or on shortcomings in the innate abilities of some groups. But the proof that such social and psychological determinism is false is the fact that the achievement gap between social and racial groups has been greatly reduced in France and other democracies.