ROTATION OF CROPS. 



ROTATION OF CROPS. 



matter analogous to albumen (hard white of 

 egg) ; but it seems that the nitrogen which 

 forms a constituent part of this matter is de- 

 rived from the atmosphere. The dry bean leaf, 

 when burnt, yields a smell approaching to that 

 of decomposing animal matter; and in its de- 

 cay in the soil may furnish principles capable 

 of becoming a part of the gluten in wheat. 



" Though the general composition of plants 

 is very analogous, yet the specific difference in 

 the products of many of them, and other well 

 ascertained facts, prove that they must derive 

 different materials from the soil; and though 

 the vegetables, having the smallest systems of 

 leaves, will proportionately most exhaust the 

 soil of common nutritive matter, yet particular 

 vegetables, when their produce is carried off, 

 will require peculiar principles to be supplied 

 to the land in which they grow. Strawberries 

 and potatoes at first produce luxuriantly in 

 virgin mould, recently turned up from pasture, 

 but in a few years they degenerate and require 

 a f: fsh soil ; and the organization of these 

 plants is such as to be constantly producing 

 the migration of their layers. Thus, the straw- 

 berry, by its long shoots, is continually en- 

 deavouring to occupy a new soil ; and the 

 fibrous radicles of the potato produce bulbs at 

 a considerable distance from the parent plant. 

 The most remarkable instance of the powers 

 of the plant to exhaust the soil of certain prin- 

 ciples necessary to its growth, is found in cer- 

 tain fungi. Mushrooms are said never to rise 

 in two successive seasons on the same spot ; 

 and the production of the phenomena called 

 fairy rings, has been ascribed by Dr. Wollaston 

 to the power of the peculiar fungus which 

 forms it, to exhaust the soil of the nutriment 

 necessary for the growth of the species. The 

 consequence is that the ring annually extends, 

 for no seeds will grow where their parents 

 grew before them, and the interior part of the 

 circle has been exhausted by preceding crops ; 

 but where the fungus has died, nourishment is 

 supplied for grass, which usually rises within 

 the circle, coarse, and of a dark-green colour." 

 " When cattle," adds Davy, " are fed upon land 

 not benefited by their manure, the effect is 

 always an exhaustion of the soil : this is parti- 

 <Sularly the case where carrying-horses are 

 fcept on estates ; they consume the pasture 

 during the night, and drop the greatest part of 

 their manure during their labour in the day- 

 time. The exportation of corn from a country, 

 unless some articles capable of becoming ma- 

 nure are introduced in compensation, must 

 ultimately tend to exhaust the soil. Some of 

 the spots, now desert sands in northern Africa 

 and Asia Minor, were anciently fertile ; Sicily 

 was the granary of Italy, and the quantity of 

 corn carried off from it by the Romans is pro- 

 bably a chief cause of its present sterility." 



The same theory is also supported by M. 

 Liebig: in his excellent work on Organic Che- 

 mistry, p. 158, he remarks, "It is evident that 

 two plants growing beside each other will mu- 

 tually injure one another, if they withdraw the 

 same food from the soil. Hence, it is not sur- 

 prising that the Matricaria chamomilla and Spar- 

 thnn *>-t:j>rirhnn impede the growth of grain, when 

 it is considered that both yield from 7 to 7-43 



per cent, of ashes, which contain six-tenths of 

 carbonate of potash. The darnel and the Eri- 

 geron acre blossom and bear fruit at the same 

 time as the wheat ; so that, when growing min- 

 gled with it, they will partake of the compo- 

 nent parts of the soil, and, in proportion to the 

 vigour of their growth, that of the corn must 

 decrease; for what one receives the others are 

 deprived of. Plants will, on the contrary, thrive 

 beside each other, either when the substances 

 necessary for their growth, which they extract 

 from the soil, are of different kinds, or when, 

 they themselves are not in the same stages of 

 developement at the same time. On a soil, for 

 example, which contains potash, both wheat 

 and tobacco may be reared in succession, be- 

 cause the latter plant does not require phos- 

 phates, salts which are invariably present in 

 wheat, but requires only alkalies and food con- 

 taining nitrogen. According to the analysis of 

 Posselt and Reimann, 1000 parts of the leaves 

 of the tobacco plant contain 16 parts of phos- 

 phate of lime, 8-8 parts of silica, and no mag- 

 Mfsi.i; whilst an equal quantity of wheat-straw 

 contains 47-3 parts; and the same quantity of 

 the grain of wheat 99-45 parts of phosphates." 



The late George Sinclair took a similar view 

 of the cause of the exhaustion of soils. "If," 

 he says, "a plant impoverishes a soil in pro- 

 portion to the weight of vegetable matter it pro- 

 duces on a given space of ground, the follow- 

 ing will be the order in which the under-men- 

 tioned plants exhaust the ground, being the 

 proportion they bear to each other with respect 

 to weight of produce : 



Mangel-wurzel - 



Cabbages - - - - - 



White turnip - 



Potatoes - 



Kohl-rabi (bulb-stalked cabbage) 



Swedish turnip - 



Carrots - 



But when we take the weight of nutritive 

 matter which a plant affords from a given 

 space of ground, the results are very different, 

 and will be found to agree with the daily ex- 

 perience in the garden and the farm. 



The following figures represent the propor- 

 tion in which they stand to each other with re- 

 spect to the weight of nutritive matter per 

 acre, and in exhausting the land : 



Potatoes - 



Mangel-wurzel 

 Carrots 

 Kohl-rabi - 

 Swedish turnip 

 Common turnip 



Change of crops also prevents very mate- 

 rially the increase of the predatory grub and 

 insects which also more or less prey upon the 

 farmer's crops. The parent of the English 

 wire-worm, for instance, which is the larva of 

 a small beetle, the Elater segetis, may be seen 

 in the summer months depositing its eggs on 

 lays or meadows abounding with the cereal 

 grasses; for instinct teaches it to place its eggs 

 where the young wire-worm will meet with its 

 natural food, which are the cereal grasses. 

 Change of crops, therefore, not only checks the 

 deposit of the eggs, but, by removing the natu- 

 ral food of the young vermin, it materially pre- 

 959 



