Farming and Gardening for Health or Disease

It was Francis Bacon who first observed that any species of plants impoverished the soil of the particular elements which they needed, but not necessarily of those required by other species. This true observation might have put subsequent investigators on the right path had their general knowledge of scientific law been less fragmentary. As it was, many ingenious guesses were made in the course of the seventeenth and eighteenth centuries as to the nurture and growth of plants, some near the truth, some wide of the mark. Confusedly it began to be recognized that plants draw their food from several sources and that water, earth, air, and sunlight all contribute. Priestley's discovery of oxygen towards the end of the eighteenth century opened up a new vista and the principles of plant assimilation soon came to be firmly established, by which is meant the fact that under the influence of light the green leaves absorb carbon-dioxide, break it up, retaining the carbon and emitting the oxygen (hence their purifying effect on the atmosphere) -- what is more delicious than the air of the forest, garden, or field? -- while without light, i.e. during the night-time, plants reverse the process and emit carbon-dioxide. Though the investigation of the parallel processes of root respiration, i.e. the use made by the roots of the oxygen available from the soil-air or the soil-solution, did not follow until a good deal later, yet the foundations of knowledge about the life of plants were at least thus laid on sound lines.

THE ORIGIN OF ARTIFICIAL MANURES