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6 THE INTRUSION OF SCIENCE I T 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 FERTILIZERS It was at this juncture that a special direction was given to investigation by Liebig. Liebig is counted the pioneer of agricultural chemistry. His Chemistry in Its Application to Agriculture, contributed to the British Association in 1840, was the starting point of this new science. His inquiries into general organic chemistry were so vast and so illuminating that scientists and farmers alike naturally yielded to the influence 69 of his teaching. His views throughout his life remained those of a chemist and he vigorously combated the so-called humus theory, which attributed the nourishment of plants to the presence of humus. At that time the soil in general and the humus in it were looked on as mere collections of material without organic growth of their own; there was no conception of their living nature and no knowledge whatever of fungous or bacterial organisms, of which humus is the habitat. Liebig had no difficulty in disproving the role of humus when presented in this faulty way as dead matter almost insoluble in water. He substituted for it a correct appreciation of the chemical and mineral contents of the soil and of the part these constituents play in plant nourishment. This was a great advance, but it was not noticed at the time that only a fraction of the facts had been dealt with. To a certain extent this narrowness was corrected when Darwin in 1882 published The Formation of Vegetable Mould Through the Action of Worms, with Obse7Vflr tions of Their Habits, a book founded on prolonged and acute observation of natural life. The effect of this study was to draw attention to the extraordinary cumulative result of a physical turnover of soil particles by natural agents, particularly earthworms. It was a salutary return to the observation of the life of the soil and has the supreme merit of grasping the gearing together of the soil itself and of the creatures who inhabit it. Darwin's book, based as it is on a sort of experimental nature study, established once for all this principle of interlocked life and, from this point of view, remains a landmark in the investigation of the soil. Meanwhile Pasteur had started the world along the path of appreciati .ng the marvellous existence of the microbial populations traceable throughout the life of the universe, unseen by our eyes but discoverable to the microscope. The effect of his investigations has been immense; enormous new fields of science have been opened up. The application of this knowledge to agriculture was only gradual. Many years slipped by before it was realized that the plants and animals, whose life histories are based ultimately on living protoplasm, have their counterparts in vast families and groups of miscroscopic flora and fauna in the very earth on which we tread. It thus came about that the chemical aspects of the soil for...


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