Japanese dependence on Korean agriculture

Why did Japan become dependent on Korean agriculture so quickly after trade opened?

Formal trade probably exposed and expanded an older unofficial maritime exchange that had few practical alternatives.

Opening formal trade likely made an existing exchange visible rather than creating dependence from nothing. Korean produce had few practical overland routes toward China, while the sea offered a direct path across the Korea Strait. Unofficial traffic could therefore precede the treaties that made volumes legible to officials and historians. The answer is explicitly an inference from geography, but it explains why dependence could appear to grow so quickly once trade became formal.