260 



KNOWLEDGE. 



[November 1, 1895. 



Moreover, bones are buried by the castings and brought 

 ■within reach of the roots of plants. 



The earthworm is thus seen to be one of the best 

 examples which show how "great effects from little causes 

 spring." This unpleasant-looking and slimy animal, before 

 the days of Darwin, was looked upon as an entirely useless 

 creature, except as a bait for fish and as food for birds. 



THE NITROGEN OF THE AIR AS A PLANT 

 FOOD. 



By George McGowan, Ph.D. 



IN the whole range of agricultural science there is no 

 problem whose solution has presented greater 

 charms, and, it may be added, greater difficulties 

 than the following one — Are plants capable of 

 feeding direrthi upon the nitrogen of the air, or are 

 they not ; or, to put the query in other words, can plants 

 assimilate fir,- nitrogen, or must their nitrogen be pre- 

 sented to them as food entirely in the form of a nitrogenous 

 compound such as ammonia or nitric acid '? The recently 

 published volume for the current year of the TnuiMctii'nti 

 of the Hiijhland and AiiricuUnral Societij contains a most 

 interesting paper on this subject by Dr. A. P. Aitken, the 

 well-known authority on agricultural chemistry, and we 

 think that we cannot do better than bring a resume of this 

 before the readers of Kno-\\xedge, since it covers within 

 short compass the main points of the whole subject. 



Apart from the purely mineral constituents (phosphates, 

 potash, etc.) which plants extract from the soil, they are 

 built up from the four elements— carbon, hydrogen, 

 oxygen and nitrogen, the source of the hydrogen and 

 oxygen being water, and the source of the carbon the 

 carbonic acid of the air. But what about the source 

 or sources of the nitrogen ? In the year 1840, Liebig 

 brought out his famous treatise on Oi-ijanic Chemistry in its 

 Application to Aijriculture ayid rhysioloe/ij. the publication 

 of which, as Dr. Aitken justly remarks, forms the greatest 

 epoch in the history of scientific agriculture. In this 

 memorable work Liebig gives it as his definite opinion 

 that the one source from which plants derive their 

 nitrogen is the ammonia of the air (or its products of 

 oxidation, nitrous and nitric acids). " He dismissed from 

 his mind the idea that plants could take any of their 

 nitrogenous matter from the free nitrogen of the air, 

 because he knew that nitrogen was the most indifferent 

 among the elements (argon and helium being then un- 

 known), and naturally imagined that if plants could make 

 use of free nitrogen, they would not exhibit, as they did, 

 such an avidity for nitrogen in the form of ammonia 

 salts." 



But this was a case in which, as we now know, Liebig 

 was mistaken. The first man to make a definite experi- 

 mental investigation of the subject, such as was fitted to 

 produce results of any value, was the well-known French 

 chemist Boussingault (1802-188G), a contemporary of 

 Liebig, and one of the most eminent among agricultural 

 chemists. His experiments had been in progress for some 

 years prior to the publication of Liebig's great work. lie 

 grew plants in an artificial soil containing no nitrogenous 

 matter whatever, the whole being enclosed in a small 

 chamber into which no air was allowed to pass that had 

 not been previously deprived of every trace of the ammonia 

 or other nitrogenous compound that it contained. Under 

 those circumstances, if the plants were to absorb nitrogen, 

 that nitrogen must be the free nitrogen of the atmosphere. 

 These conditions of culture were, however, such that the 



plants hardly grew at all in the ordinary sense of the word, 

 the whole produce weighing only two or three times as 

 much as the original seeds. As a result, Boussingault 

 found in the ultimate crop — whether of oats, beans, cress, 

 or lupines — no more nitrogen than was originally present 

 in the seeds when they were planted. Notwithstanding the 

 unnatural conditions of growth, however, he was eventually 

 so satisfied with the results of his experiments as to have 

 no hesitation in concluding that plants could not absorb 

 free nitrogen from the air, and this opinion, coming as it 

 did from such a distinguished authority, was for long 

 received as correct. 



In the year 1840, the late M. Georges Ville, director of 

 the Agricultural Experiment Station at Vincennes, near 

 Paris, began a long and laborious research on the same 

 subject, being of opinion that Boussingault's results were 

 not to be relied upon, inasmuch as the conditions of growth 

 were unnatural, the plants experimented upon having 

 never really entered into an independent state of existence. 

 " He therefore managed his plants in such a manner as to 

 enable them to attain a good normal growth. They were 

 kept under cover, but well ventilated . The soil afforded them 

 was abundant, and contained a certain definite amount 

 of nitrate of soda, and the roots had plenty of room to in- 

 crease, and they were also provided with good drainage and 

 ventilation. The result was that his plants grew to be ten, 

 twenty, fifty, or one hundred times or more the weight of 

 the seed, and in their substance they contained more nitrogen 

 than was contained in tlie seed and the soil tor/ether" (the 

 italics are ours). Since no nitrogenous matter other than 

 the known amount of nitrate of soda in the soil and the 

 free nitrogen of the air had access, M. Ville came to the 

 conclusion that plants, or at least certain plants, could and 

 did assimilate free nitrogen. 



Ville's deductions, however, were by no means accepted 

 as correct, being in absolute contradiction to those of 

 Boussingault. The discussion over the point became 

 very warm, and ultimately the French Academy, of which 

 Boussingault was a distinguished member, accepted Ville's 

 ofi'er to appoint a commission to examine his apparatus 

 and superintend his experiments. This commission 

 reported that M. Ville's conclusions were justified by 

 the results he had obtained, but the members composing 

 it were not satisfied that the plants had not been supplied 

 with ammonia as an impurity in the distilled water used 

 for watering them. M. Ville ended his experiments in 

 1857, and in that year Messrs. Gilbert, Lawes, and Pugh 

 undertook a repetition of Boussingault s experiments at 

 Eothamstead, being careful to conduct these in such a 

 manner as to meet any possible objection that might be 

 taken to any part of Boussingault's procedure. Their 

 results entirely confirmed those of the famous French 

 chemist. Notwithstanding all this, M. Ville still main- 

 tained the accuracy of his own results, and published in 

 1867 a new and extended edition of his researches, in 

 which he reviewed and criticized the work both of 

 Boussingault and of the English experimenters already 

 cited. He maintained that in these the plants sufl'ered 

 from want of ventilation, that they were usually sown at 

 the wrong season of the year, that the quantity of soil used 

 was inadequate, and that — owing to this last circumstance 

 — the mineral manurial matter was too concentrated, and 

 so interfered with the development of the plants. He 

 further pointed out the futility of comparing plants of 

 immature growth with fully developed ones, and also 

 gave some experimental proof for the important statement 

 that " plants do not begin to assimilate the free nitrogen 

 of the air until they have attained a stage of development 

 in which they have acquired at least ten times the weight 



