572 TRANSACTIONS OF SUB-SECTION B. 



has this association of nitre with the fertility of soils become that in 1675 

 John Evelyn writes : ' I firmly believe that where saltpetre can be ob- 

 tained in plenty we should not need to find other composts to ameliorate 

 our ground ' ; and Henshaw, of University College, one of the first members 

 of the Royal Society, also writes about saltpetre : ' I am convinced indeed that 

 the salt which is found in vegetables and animals is but the nitre which is 

 so universally diffused through all the elements (and must therefore make the 

 chief ingredient in their nutriment, and by consequence all their generation,) 

 a little altered from its first complexion.' • 



But these promising beginnings of the theory of plant nutrition came to no 

 fruition ; the Oxford movement in the seventeenth century was but the false 

 dawn of science. At its close the human mind, which had looked out of doors 

 for some relief from the fierce religious controversy with which it had been so 

 long engrossed, turned indoors again and went to sleep for another century. 

 Mayow's work was forgotten, and it was not until Priestley and Lavoisier, De 

 Saussure. and others, about the beginning of the nineteenth century, arrived at 

 a sound idea of what the air is and does that it became possible to build afresh 

 a sound theory of the nutrition of the plant. At this time the atten- 

 tion of those who thought about the soil was chiefly fixed upon the 

 humus. It was obvious that any rich soils, such as old gardens and 

 the valuable alluvial lands, contained large quantities of organic matter, 

 and it became somewhat natural to associate the excellence of these fat, 

 unctuous soils with the organic matter they contained. It was recognised that 

 the main part of a plant consisted of carbon, so that the deduction seemed obvious 

 that the soils rich in carbon yielded those fatty, oily substances which we now 

 call humus to the plant, and that their richness depended upon how much of 

 such material they had at their disposal. But by about 1840 it had been definitely 

 settled what the plant is composed of and whence it derives its nutriment — the 

 carbon compounds which constitute nine-tenths of the dry weight from the air, 

 the nitrogen, and the ash from the soil. Little as he had contributed to the 

 discovery, Liebig's brilliant expositions and the weight of his authority had 

 driven this broad theory of plant nutrition home to men's minds ; a science of 

 agricultural chemistry had been founded, and such questions as the function of 

 the soil with regard to the plant could be studied with some prospect of success. 

 By this time also methods of analysis had been so far improved that some 

 quantitative idea could be obtained as to what is present in soil and plant, and, 

 naturally enough, the first theory to be framed was that the soil's fertility was 

 determined by its content of those materials which are taken from it by the crop. 

 As the supply of air from which the plant derives its carbonaceous substance is 

 unlimited, the extent of growth would seem to depend upon the supply available 

 of the other constituents which have to be provided by the soil. It was 

 Daubeny, Professor of Botany and Rural Economy at Oxford, and the real 

 founder of a science of agriculture in this country, who first pointed out the 

 enormous difference between the amount of plant food in the soil and that taken 

 out by the crop. In a paper published in the ' Philosophical Transactions ' in 

 1845. being the Bakerian Lecture for that year, Daubeny described a long series 

 of experiments that he had carried out in the Botanic Garden at Oxford, wherein 

 he cultivated various plants, some grown continuously on the same plot and others 

 in a. rotation. Afterwards he compared the amount of plant food removed by the 

 crops with that remaining in the soil. Daubeny obtained the results with 

 which we are now familiar, that anv normal soil contains the material for 

 from fifty to a hundred field crops. If, then, the growth of the plant depends 

 upon the amount of this material it can get from the soil, why is that growth so 

 limited, and why should it be increased by the supply of manure, which only 

 adds a trifle to the vast stores of plant food already in the soil? For example, 

 a turnip crop will only take awav about 30 lb. per acre of phosphoric acid from a 

 soil which may contain about 3,000 lb. an acre: vet, unless to the soil about 50 lb. 

 of phosphoric acid in the shape of manure is added, hardly any turnips at all will 

 be grown. Daubeny then arrived at the idea of a distinction between the active 

 and dormant plant food in the soil. The chief stock of these materials, he con- 

 cluded, wa,s combined in the soil in some form that kept it from the plant, and 

 only a small proportion from time to time became soluble and available for food. 



