ROTATION OP CROPS. 



replant old orchards with the same trees, or to j 

 replace old quick-hedges with young quick 

 plants, yet an old orchard or the site of an old 

 hedge-row are proverbial for their fertility, 

 when planted with other crops. There is con- 

 siderable importance I think to be attached to 

 this mode of accounting for the facts of the 

 case, but it is by no means so complete an ex- 

 planation as is desirable. There are some 

 soils, for instance, which would seem (if this 

 were the sole cause of the phenomenon) to defy 

 all the excretory powers of the plant. Some of 

 the newly-enclosed lands of the United States 

 of America, for instance, have produced excel- 

 lent wheat crops for even 20 years without in- 

 terruption. Some of the alluvial soils of the 

 lower portion of the valley of the Thames have 

 3'ielded alternate crops of wheat and beans 

 from time immemorial ; and by the addition of 

 manure, the potato grounds near London yield 

 abundant crops for a series of years. There 

 are other observations too, of a similar kind, 

 which will readily be remembered by the in- 

 telligent farmer, which do not seem to assimi- 

 late entirely with this mode of removing the 

 difficulties of the case. 



The other way of explaining the reluctance 

 with which a crop follows another of the same 

 description is, by supposing that each kind of 

 plant has some peculiar and essential ingredi- 

 ent which it absorbs from, and in a great degree 

 exhausts the soil, and that it is, therefore, only 

 after a lapse of some time, when that ingredi- 

 ent or those ingredients are restored by the ap- 

 plication of manure, or by other modes, that 

 the same plants can be again profitably culti- 

 vated. To a great extent this theory is not 

 only a very plausible but a very probable and 

 reasonable explanation of the difficulty. Thus 

 the farmer is well aware that certain soils on 

 which red clover formerly grew very succ'ess- 

 fully once in 4 years, will now only yield any 

 profitable degree of produce of the same plant 

 once in 8 or once in 12 years. The excretory 

 powers of the plant in this instance, therefore, 

 are useless in explanation of the difficulty; for 

 according to that theory, the excretory matters 

 which long were successfully dissipated or ab- 

 sorbed by other plants in the course of 4 years, 

 should do so in our age just as well as in a for- 

 mer period. But if we admit what has been 

 not only sometimes, but very often clearly 

 proved to be the case, that the soils which are 

 thus reluctant to produce red clover, are now 

 totally exhausted of sulphate of lime (gypsum) 

 that, moreover, every fair average crop of 

 this valuable grass contains from 100 to 200 

 pounds weight per acre of this salt and that 

 by dressing the land with this manure, in 

 almost exactly the same proportion and quantity 

 as that which is contained in the clover, that 

 then the land will again grow the very same 

 crop once in 4 years ; when these and other 

 similar facts are proved, the very strong pro- 

 bable conclusion to which we must arrive is 

 apparent, viz., that the clover had gradually 

 exhausted the land of an essential ingredient 

 which only needed to be restored to it, to enable 

 the clover again to flourish with its wonted 

 vigour. And this is not a solitary instance: 

 thus, marine plants will only grow successfully 

 958 



ROTATION OF CROPS. 



in inland situations, where common salt is 

 added to the soil. The sun-flower and the nettle 

 need in an equal degree the assistance of salt- 

 petre. The presence in the soil of phosphate 

 of lime (the earthy salt of bones) is equally 

 essential to the vigorous growth of almost all 

 the grain crops. 



Then, again, there are other facts of a differ- 

 ent nature well known to the farmer, which 

 appear to lead us to the same conclusion; fo 

 instance, every cultivator is aware that t> 

 cutting his crops green, his land is not near 

 so much exhausted as when the same cro} 

 are allowed to ripen their seeds. And if, in 

 explanation of this observation, it can be shown 

 that the plant, when ripe, contains a larger pro- 

 portion of any peculiar saline or earthy ingre- 

 dient than it does when in a growing, unripe 

 state, this will further tend to establish the 

 truth of the last-named theory that it is the 

 abstraction from the soil by the plant of some 

 peculiar substance, which thus exhausts and 

 indisposes it to support the same crop. Now 

 this, according to chemical investigations, 

 seems, at least in many instances, to be the case. 

 Thus, M. Saussure, in his chemical researches, 

 has shown by the results of his analysis, that 

 the ashes of the plants of peas (Pisvm suti- 

 riwm), when green and in flower, contain only 

 17-25 per cent, of phosphate of lime, but that, 

 when ripe, they yield 22 per cent. And, again, 

 that the ashes of plants of vetches (Firiajfafa), 

 which yielded only 13-5 per cent, of the same 

 salt when in flower, contain 17-75 per cent, 

 when they are ripe. The same result was ob- 

 tained from other plants : the Solidago vulgaris, 

 for instance, which yielded 8-5 per cent, of 

 phosphate of lime when first in flower, con- 

 tained 11 per cent, when ripe. The turnsole 

 (Helianthus annuus), which afforded only 6 per 

 cent, when flowering, contained 22-5 per cent, 

 when ripe. The wheat plant, which held 10-75 

 per cent, in flower, contained 11-75 when ripe. 

 The ashes of the straw of wheat were found to 

 yield only 6-2 per cent, of this essentially pre- 

 sent salt, but its seeds held 44-5, and its bran 

 46-5 per cent, of it. M. Vauquelin obtained a 

 result somewhat similar in his examination of 

 the ashes of the oat plant; the seeds affording 

 him 39-3 per cent, of phosphate of lime, but 

 when he burnt the whole plant, seed and stalk 

 together, he then found only 15 per cent. 



The evidence, therefore, in favour of the ab- 

 sorbent theory, is certainly rather stronger than 

 that in support of the excretory mode of ex- 

 plaining the phenomenon. Yet, in all proba- 

 bility, both causes may contribute to produce 

 the effect. Davy, the chief of modern che- 

 mists, adopted the former mode of explaining 

 the reluctance with which a crop grows for 2 

 years successively on the same land. (Lec- 

 tures, p. 357.) Changes of all kind seem, in 

 truth, ever to be grateful to vegetation change 

 of soil, of seed, of the course of cropping, of 

 manure, &c. "Peas and beans," said Davy, 

 " in all instances, seem well adapted to prepare 

 the ground for wheat; and in some rich lands, 

 as in the alluvial soil of the Parret, and at the 

 foot of the South Downs, in Sussex, they are 

 raised in alternate crops for years together. 

 Peas and beans contain a small quantity of a 



