NATURE 



[September f, 1910 



169 lb. 3 oz., whereas the soil, when redried, had lost 

 ■but 2 oz., though the surface had been carefully protected 

 meantime with a cover of tin. Van Helmont concluded 

 .that he had demonstrated a transformation of water into 

 the material of the tree. Boyle repeated these experi- 

 ments, growing pumpkins and cucumbers in weighed 

 earth, and obtaining similar results, except when his 

 gardener lost the figures, an experience that has been 

 repeated. Boyle also distilled his pumpkins, &c., and 

 obtained therefrom various tars and oils, charcoal and 

 ash, from which he concluded that a real transmutation 

 had been effected, " that salt, spirit, earth, and even oil 

 (though that be thought of all bodies the most opposite 

 to water) may be produced out of water." 



There were not, however, wanting among Boyle's con- 

 temporaries men who pointed out that spring water used 

 for the growing plants in these experiments contained 

 abundance of dissolved material, but in the then state of 

 chemistry the discussion as to the origin of the carbon- 

 aceous material in the plant could only be verbal. Boyle 

 himself does not appear to have given any consideration 

 to the part played by the soil in the nutrition of plants, 

 but among his contemporaries experiment was not lack- 

 ing. Some instinct seems to have led them to regard 

 nitre as one of the sources of fertility, and we find that 

 Sir Kenelm Digby, at Gresham College in 1660, at a 

 meeting of the Society for Promoting Philosophical Know- 

 ledge by Experiment, in a lecture on the vegetation of 

 plants, describes an experiment in which he watered young 

 barley plants with a weak solution of nitre, and found 

 how their growth was promoted thereby ; and John 

 iV'Iayow, that brilliant Oxford man whose early death cost 

 so much to the young science of chemistry, went even 

 further, for, after discussing the growth of nitre in soils, 

 .he pointed out that it must be this salt which feeds the 

 plant, because none is to be extracted from soils in which 

 plants are growing. So general 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 obtained 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 Roj'al 

 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 Priestly 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 attention 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 com- 

 pounds which constitute nine-tenths of the dry weight 

 from the air, the nitrogen, and the ash from the soil. 

 LitJie 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 



NO. 2132, VOL'. 84] 



with regard to the plant could be studied with some pro- 

 spect 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 fertiUty 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 con- 

 stituents which have to be provided by the soil. It was 

 Daubeny, Professor of Botany and Rural Economy at 

 O.xford, and the real founder of a science of agriculture 

 in this country, who first pointed out the enormous ditler- 

 cnce 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, 

 wherein he cultivated various plants, some grown con- 

 tinuously 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 

 any 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 Umited, 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 away about 

 30 lb. per acre of phosphoric acid from a soil which may 

 contain about 3000 lb. an acre; yet, 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 concluded, was 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. He took a further step, and 

 attempted to determine the proportion of the plant food 

 which can be regarded as active. He argued that since 

 plants only take in materials in a dissolved form, and as 

 the great natural solvent is water percolating through the 

 soil more or less charged with carbon dioxide, therefore 

 in water charged with carbon dioxide he would find a 

 solvent which would extract out of a soil just that material 

 which can be regarded as active and available for the 

 plant. In this way he attacked his Botanic Garden soils, 

 and compared the materials so dissolved with the amount 

 taken away by his crops. The results, however, were in- 

 conclusive, and did not hold out much hope that the 

 fertility of the soil can be measured by the amount of 

 available plant food so determined. Daubeny's paper was 

 forgotten ; but exactly the same line of argument was 

 revived again about twenty years ago, and all over the 

 world investigators began to try to measure the fertility 

 of the soil by determining as " available " plant food the 

 phosphoric acid and potash that could be extracted by 

 some weak acid. A large number of different acids were 

 tried, and although a dilute solution of citric acid is at 

 present the most generally accepted solvent, I am still of 

 opinion that we shall come back to the water charged 

 with carbon dioxide as the only solvent of its kind for 

 which any justification can be found. Whatever solvent, 

 however, is employed to extract from the soil its available 

 plant food, the results fail to determine the fertility of the 

 soil, because we are measuring but one of the factors in 

 plant production, and that often a comparatively minor 

 one. In fact, some investigators — Whitney and his 

 colleagues in the American Department of Agriculture — 

 have gone so far as to suppose that the actual amount of 

 plant food in the soil is a matter of indifference. They 

 argue that as a plant feeds upon the soil water, and as 

 that soil water must be equally saturated with, say, phos- 

 phoric acid, whether the soil contains 1000 or 3000 lb. 

 per acre of the comparatively insoluble calcium and iron 

 salts of phosphoric acid which occur in the soil, the plant 

 must be under equal conditions as regards phosphoric acid, 

 whatever the soil in which it may be grown. This argu- 



