SCIENCE.-SUPPLEMENT. 



FRIDAY, JANUARY 14, 1887. 



on the enrichment of the soil by 

 the cultivation of 'enriching 

 crops: 



It is an observation almost as old as agricul- 

 ture, — certainly much older than the earliest lit- 

 erature of agriculture, — that certain crops appear 

 to increase the fertility of the soil upon which 

 they are grown ; or, to state the case more accu- 

 rately, they exert a favorable influence upon the 

 growth of the succeeding crop. Red clover is the 

 typical example of such a crop ; and the use of 

 this plant as a means of renovating poor or ex- 

 hausted soils is co-extensive with improved agri- 

 culture. Other crops, on the contrary, have an 

 opposite effect, and are denominated exhausting, 

 as, for example, the cereals. 



But while the facts just recounted are suffi- 

 ciently well known, their cause or causes are by 

 no means so well made out. The first attempts at 

 explanation natvirally assumed that the exhaust- 

 ing crops took more from the soil than the enrich- 

 ing crops, or, what amounts to the same thing, 

 that the latter were the medium of conveying ma- 

 terials from the atmosphere to the soil. The en- 

 riching crops were also supposed to improve the 

 soil by facilitating the direct acquisition of mate- 

 rial by the soil from the air, accomplishing this by 

 shading the soil, by the mechanical action of their 

 roots, and also, in case of root-crops, for exam- 

 ple, by the tillage necessary for their cultivation. 



Thaer and his school, to whom we owe these 

 attempts at explanation, considered the humus of 

 the soil to be the real food of the plants, and the 

 mineral matters to be unessential, and naturally 

 found support for their hypotheses in the great 

 increase in the organic matter or humus of the 

 soil consequent upon the growth of such a crop as 

 clover, for example. As the progress of investi- 

 gation brought about a better understanding of 

 the laws of vegetable nutrition and the sources of 

 plant-food, these views as to the action of enrich- 

 ing crops were gradually modified ; but they con- 

 tinued, and still continue, to follow the general 

 lines laid down by Thaer. We now know that 

 the plant obtains from the soil its mineral ingredi- 

 ents and its nitrogen, while the bulk of its 'or- 

 ganic' matter is assimilated by its leaves. It is 

 plainly impossible that a crop should enrich the 

 soil in mineral matters. All crops enrich the soil 



in carbon to some extent, since their roots and 

 stubble remain in the soil ; but this carbon ap- 

 pears to be of no direct ixse to the plant. There 

 remains only the nitrogen, and the modem theo- 

 ries of the action of enriching crops are based on 

 the belief that they somehow increase the store of 

 nitrogen in the soil. Indeed, if we substitute ni- 

 trogen for humus in Thaer's hypotheses, we have 

 very nearly the views of recent authors. 



Before proceeding to discuss these views, how- 

 ever, it will be well to inquire whether this sup- 

 posed enrichment of the soil is a fact. The bene- 

 fits of a judicious rotation of crops are undoubted, 

 but they are susceptible of a variety of explana- 

 tions. A crop like clover, for example, may pro- 

 mote the growth of a succeeding grain-crop in a 

 variety of ways, having no relation to the stock of 

 nitrogen in the soil. Only careful scientific ex- 

 periments can decide whether such crops actually 

 enrich the soil in nitrogen. Unfortunately, but 

 few experiments upon this subject have as yet 

 been made, and some of those reported are of 

 doubtful value. Considerable interest, therefore, 

 attaches to the experiments made by Strecker in 

 the year 1883-84 at Gottingen, an account of 

 which has recently been published,' along with a 

 very complete review of the literature of the sub- 

 ject. 



Strecker experimented upon plants and soils in 

 pots, lupines serving to represent the legumes, and 

 oats the cereals. But one of the vegetation ex- 

 periments of 1883 succeeded ; viz., one with lu- 

 pines in unmanured sand. From the data given, 

 it appears that the soil and roots remaming in the 

 pot contained only about 40 per cent of the ni- 

 trogen originally present in the sand, or introduced 

 in the seed or in the rain to which the pots were 

 exposed. On the other hand, the amount thus re- 

 moved from the soil was only about 39 per cent of 

 the total quantity found in the aerial portions of 

 the plants : the remaining 61 per cent, therefore, 

 must either have been assimilated directly from 

 the atmosphere or been absorbed from it by the 

 soil. Six pots without plants were also exposed 

 during the summer ; and these showed, without ' 

 exception, a considerable loss of nitrogen, which, 

 as there was no drainage from the pots, must 

 have passed off into the air. Two of the pots 

 contained unmanured sand with 0.0015 per cent 

 of nitrogen ; and the variations in these were evi- 

 dently within the limits of analytical error and of 

 1 Journ. f. landw., xxxiv. 1. 



