498 



NATURE 



{_Scpt. 23, 1880 



Nor was tliis curious re ult in any way accidental. It is quite 

 consistent willi agricultural experience that the growth and re- 

 moval of a highly nitrogenous leguminous crop should leave the 

 land in high condition for the growth of a gramineous corn crop, 

 whicli characteristically requires nitrogenous manuring ; and the 

 determinations of nitrogen in numerous samples of the soil taken 

 from the two separate portions of the field, after the removal of 

 the barley and the clover respectively, concurred in showing con- 

 siderably more nitrogen, especially in the first nine inches of 

 depth, in the samples from the portion where the clover had been 

 grown, than in those from the portion whence the barley had 

 been taken. Here then the surface soil at any rate had been 

 considerably enriched in nitrogen by the growth and removal of 

 a very highly nitrogenous crop. 



Lastly, clover ha; now been grown for twenty-seven years in 

 succession, on a small plot of garden ground which had been 

 under ordinary garden cultivation for probably two or three 

 centuries. In the foui-th year after the commencement of the 

 experiment, the soil was found to contain, in its upper layers, 

 about four times as much nitrogen as the farm-arable-land sur- 

 rounding it ; and it would doubtless be correspondingly rich in 

 other constituents. It is estimated that an amount of nitrogen 

 has been removed in the clover crops grown, corresponding to an 

 average of not far short of 200/. per acre per annum ; or about 

 ten times as much as in the cereal crops, and several times as 

 much as in any of the other crops, growing on ordinary arable 

 land ; and, although the yield continues to be very large, there 

 has been a marked decline over the second half of the period 

 compared with the first. Of course, calculations of the produce 

 of a few square yards into quantities per acre can only be 

 approximately correct. But there can at any- rate be no doubt 

 whatever that the amount of nitrogen annually removed has been 

 very great ; and very far beyond what it would be possible to 

 attain on ordinary arable land ; where, indeed, we have not 

 succeeded in getting even a moderate growth of clover for more 

 than a very few years in succession. 



One other illustration should be given of the amounts of 

 nitrogen removed from a given area of land by different descrip- 

 tions of crop, namely, of the results obtained when plants of the 

 gramineous, the leguminous, and other families, are growing 

 together, as in the mixed herbage of grass-land. 



It is necessary here to remind you that gramineous crops 

 grown separately on arable land, such as wheat, barley, or oats, 

 contain a comparatively small percentage of nitrogen, and assi- 

 milate a comparatively small amount of it over a given area. 

 Yet nitrogenous manures have generally a very striking effect in 

 increasing the growth of such crops. The highly nitrogenous 

 leguminous crops (such as beans and clover), on the other hand, 

 yield, as has been seen, very much more nitrogen over a given 

 area, and yet they are by no means characteristically benefited 

 by direct nitrogenous manuring ; whilst, as has been shown, 

 their growth is considerably increased, and they yield consider- 

 ably more nitrogen over a given area under the influence of 

 purely mineral manures, and especially of potass manures. 

 Bearing these facts in mind, the following results, obtained on 

 the mixed herb.age of grass land, will be seen to be quite 

 consistent. 



A plot of such mixed herbage, left entirely unmanured, gave 

 over twenty years an average of 33 lbs. of nitrogen per acre per 

 annum. Over the same ] eriod another plot, which received 

 annually a complex mineral manure, including potass, during the 

 first six years, but excluding it during the last fourteen years, 

 yielded 463 lbs. of nitrogen ; whilst another, which received 

 the mixed mineral manure, including potass, every year of the 

 twenty, yielded 55 6 lbs. of nitrogen p-r acre per annum. With- 

 out manure there was some decline of yield in the later years ; 

 witli tlie partial mineral manuring there %\'as a greater decline ; 

 but with the complete mineral manuring throughout the whole 

 period, there was even some increase in the yield of nitrogen in 

 the later years. 



Now, the herbage growing without manure comprised about 

 fifty species, representing about twenty natural families ; that 

 gro\^'ing with the limited supply of potass comprised fewer 

 species, but a larger amount of the produce, especially in the 

 earlier years, consisted of leguminous species, and the yield of 

 nitrogen was greater. Lastly, the plot receiving potass every 

 year yielded still more leguminous herbage, and, accordingly, 

 still more nitrogen. 



'I he most striking points brought out by the foregoing illustra- 

 tions are the following : — 



I. Without nitrogenous^manure, the gramineous crops annu- 



ally yielded, for many years in succession, much more nitrogen 

 over a given area tlian is accounted for by the amount of com- 

 bined nitrogen annually coming down in the measured aqueous 

 deposits from the atmosphere. 



2. The root crops yielded more nitrogen than the cereal crops, 

 and the leguminous crops very much more still. 



3. Tn all cases — whether of cereal crops, root crops, legu- 

 minous crops, or a rotation of zxo^%—the decline in the annual 

 yield of nitrogen, when none was supplied, was very great. 



How are these results to be explained ? Whence comes the 

 nitrogen ? and especially whence comes the much larger amount 

 taken up by plants of the leguminous and some other families, 

 than by the graminerc ? And lastly, what is the significance ot 

 the great decline in the yield of nitrogen in] all the crops when 

 none is supplied in the manure ? 



Many explanations have been offered. It has been assumed 

 that the comliined nitrogen annually coming down from the 

 atmosphere is very much larger than we have estimated it, and 

 tliat it is sufficient for all the requirements of annual growth. 

 It has been supposed that "broad-leaved plants" have the 

 power of taking up nitrogen in some form from the atmosphere, 

 in a degree, or in a manner not possessed by the narrow-leaved 

 gramincK. It has been argued that, in the last stages of the 

 decomposition of organic matter in the soil, hydrogen is evolved, 

 and that this nascent hydrogen combines with the free nitrogen 

 of the atmosphere, and so forms ammonia. It has been sug- 

 gested that ozone may be evolved in the oxidation of organic 

 matter in the soil, and that, uniting with free nitrogen, nitric 

 acid would be produced. Lastly, it has by some been concluded 

 that plants assimilate the free nitrogen of the atmosphere, and 

 that some de-criptions are able to do this in a greater degree than 

 others. 



We have discussed these various points on more than one occa- 

 sion ; and we have given our reasons for concluding that none 

 of the explanations enumerated can be taken as accounting for 

 the facts of growth. 



Confining attention here tothe question of the assimilation of free 

 nitrogen by plants, it is obvious that, if this were established, most 

 of our difficulties would vanish. This question has been the subject 

 of a great deal of experimental inquiry, from the time that Bous- 

 singault entered upon it, about the year 1S37, nearly up to the 

 present time. About twenty years ago it was elaborately investi- 

 gated at Rothamsted. In publishing the results of that inquiry 

 tliose of others relating to it were fully discussed ; and although 

 the recorded evidence is admittedly very conflicting, we then 

 came to the conclusion, and still adhere to it, that the balance of 

 the direct experimental evidence on the point is decidedly against 

 the supposition of the assimilation of free nitrogen by plants. 

 Indeed, the strongest argument we know of in its favour is, that 

 some such explanation is wanted. 



Not only is the balance of direct experimental evidence 

 against the assumption that plants assimilate free or uncom- 

 bined nitrogen, but it seems to us that the balance of existing 

 indirect evidence is also in favour of another explanation of our 

 difliculties. 



I have asked what is the significance of the gradual decline of 

 produce of all the different crops when continuously grown with- 

 out nitrogenous manure? It cannot be that, in growing! the 

 same crop year after year on the same land, there is any residue 

 left in the soil that is injurious to the subsequent growth of the 

 same description of crop ; for (excepting the beans) more of 

 each description of crop has been grown year after year on the 

 same land than the average yield of the country at large under 

 ordinary rotation, and ordinary treatment — p-ovided only that 

 suitable soil conditions were supplied. Nor can the diminishing 

 produce, and the diminishing yield of nitrogen, be accounted for 

 on the supposition that there was a deficient supply of available 

 mineral constituents in the soil. For, it has been shown that the 

 cereals yielded little more, and declined nearly as much as with- 

 out manure, when a complex mineral manure was used, such as 

 was proved to be adequate when available nitrogen was also 

 supplied. So far as the root crops are concerned the yield of 

 nitrogen, though it declined very much, was greater at first, and 

 on the average, than in the case of the cereals. As to the 

 leguminofoe, which require so much nitrogen from somewhere, it 

 is to he observed that on ordinary arable land the yield has not 

 been maintained under any conditions of manuring ; and the 

 decline was nearly as marked' with mineral manures as without 

 any manure. Compared with the growth of the leguminosa; on 

 arable land, the remarkable result with the garden clover would 

 seem clearly to indicate that the question was one of soil, and 



