ROTATIONS OF CR0P3. 



347 



•eed that the phospliates are principally Jo- 

 posited. 



From these numbers it is evident, that on 

 §uch a field, ii" by the ^adiial decomposition 

 of its soil it could furaish but UOO lb.s. r)f alka- 

 line salts, aiid 200 lbs. of liiiic and magnesia 

 siilts each year, we would g^row upon it but 

 half the proper croj) of l)ect, tor a full crops 

 would require 3f)0 ll)s. of alkaline salts, and 

 al.HO only a hali' cro|> of peas, for tlie full (^01)8 

 would re(]uire 371 Ib^i. of lime iuid magnesia 

 salts. The continuous culture of either i)hmt 

 would, therefore, lie most un])rotitable and 

 injurious, but if w;» cultivated beet one year 

 and peas the other, the soil would have two 

 years to jirepare tlje materiuls which cadi 

 crop would recjuire to take up in one. There 

 would be available 400 lbs. of each kind of 

 8;ilt3, and thus so far from exhaustion, there 

 should be a surplus steadily increasing tlie 

 fertility and augmenting the produce of the 

 soil. 



As it is seen in the above lal)le tiiat the 

 quantity of alkiUme and earthy salts taken up 

 by the corn crop (wheat) is so much less than 

 required for the other kinds of plants, and 

 that the principal demand of the corn crop on 

 the soil is for silica, of which wo may consid- 

 er it certain that no soil is in danger of behig 

 exhausted, it might appear natural to con- 

 clude that the corn crop should be at least 

 deti-iraental to the ground, while it is well 

 known to practical agriculturists that wliite 

 or coi-n crops are ainong the most exliausling. 

 Their injurious action on the soil is, however, 

 not so much duo to the inorganic materials 

 they take up as to the nitrogen, for which 



element they are altogether dependent on tho 

 soil, while other kinds of plants act upon the 

 atmospliere, absorbing nitrogen, and actually 

 serving rather to enrich the soil upon which 

 they grow, than in any degree to impoverish 

 it. This is in fact what constitutes the re- 

 markable relation between the white cropit 

 and green crops as members of a rotation. 

 The former exhau.sting the soil of nitrogen, 

 the other fixing in tlio soil nitrogen derived 

 from the air, and thus preparing for the nu- 

 trition of the corn crops that may succeed it. 



The coni[)lete illustration of tho principle 

 is due to Boussinq:ault, who has established 

 it as well by experiments on individual plants 

 in the laboratory as by the operations of an 

 extensive farm. Thus on grov\ing com in 

 artificial soil deprived of nitiogen, it was found 

 that the j>lant, when arrived at its full matu- 

 rity, contained only the nitrogen that had ori- 

 ginally existed in the seed. On the other 

 hand, on growing peas in the same way, tho 

 quantity of nitrogen in the mature plant was 

 found to be much greater tliaii had been in 

 the seed, and for this there was no other 

 source than the atmosphere. The foUovving 

 tables will show how fully this result is bomo 

 out on the large scale. 



In a three years' cultivation of two sue 

 cessive ci'ops of wheat manured and then h 

 year of fallow, the produce was 3,318 kilo- 

 grammes of wheat, and 7,.'500 kilos, straw per 

 hectare, from 30,000 kilos, of manure. Now 

 tiiking these dry, the following table shows 

 their composition and the relation of their 

 constituents : 



It is here quite evident that the crop con- 

 tained only the nitrogen of the manure, as the 

 difference 4-6 is so slight as to be within the 

 unavoidable errors of e.xperiment in such 

 cases. On the other hand, the carbon of the 

 crop is nearly trebh^ that of the manure, 

 Terifying in an admirable manner, the at- 

 mospheric origin of tho carbon of plants, to 

 which I have already alluded. Hydrogen 

 and oxygen were also gained abundantly. 



and almost exactly in the proportions to form 

 water. 



In contradistinction to this corn culturo 

 may be placed the results of the continued 

 growth of Lucem for five years, followed 

 by a crop of wheat, all at the expense of 

 44.000 kilos, of farm-yard manure per hec- 

 tare, put out on the land at the commence- 

 ment of the period. Those results were pub- 

 lished by M. Crud, an eminent agiiculturist. 



(731) 



