More than 40 years have passed since Justus von Liebig first developed his theory of the laws of plant nutrition. It took more than twenty years of continuous work and a series of scientific struggles before, in the last decade of his life, the doctrine finally gained general recognition and found solid ground among the farmers of Germany. The unwavering and tireless support from his friend Th. Reuning in Dresden, who outlived Liebig by only a few years, contributed significantly to achieving this final goal. A record of the collaborative work of these two men is captured in the correspondence that we now present to the public. This book is a reprint of the long out-of-print original edition from 1884.