EOTATION OF CROPS. 



15 



lowing land, or deep-digging, ridging, 

 and trenching, are said to be the sweet- 

 ening the soil, because the excrementitious 

 matter becomes washed out by the rains, 

 decomposed by the action of light and air, 

 or buried beyond the reach of the roots, 

 and may remain so until decomposed 

 or completely changed by some unseen 

 and as yet imperfectly understood cause. 



The alternation of crops becomes also 

 necessary, as a safeguard against the at- 

 tacks of insect enemies. Thus, some of 

 the insects which are most injurious to 

 the Brassica tribe, for example, by de- 

 positing their eggs in the soil, when the 

 period of their own brief existence ter- 

 minates, secure by this means a numerous 

 progeny to commit their baneful depre- 

 dations on the succeeding crop; whereas, 

 if a different kind of plant were substi- 

 tuted, it is, in many cases, certain that 

 they would die of starvation, rather than 

 feed on food of a character different from 

 that destined for them by nature. Take, 

 for example, a plant of their own natural 

 order, the black mustard, (Sinapis nigra,) 

 which has been recommended to be sown 

 on ground infected with the larvse of in- 

 sects feeding on the cabbage tribe : the 

 roots of the mustard being too acrid for 

 them, they have actually died of starva- 

 tion. 



As a restorative or compensation to 

 the soil for a continued cropping with the 

 same species of crops, certain materials, 

 forming in themselves the inorganic con- 

 stituents of plants, have been recommend- 

 ed. Indeed, chemically speaking, one piece 

 of ground may possibly/ be made to pro- 

 duce the same species of crop ad infini- 

 tum. To carry out, however, these ideas, 

 it will be necessary to ascertain the mat- 

 ter abstracted from the soil by such crops, 

 and then to add to them, at each sowing 

 or planting, an equivalent, and something 

 more, of the ingredients of the same na- 

 ture as that of which the ground has been 

 robbed by the preceding crop. This is, 

 however, only meeting the subject mid- 

 way, if even so much. 



To ascertain correctly what is the food 

 of plants, we must first ascertain what 

 they themselves are composed of; for 

 whatever elements constitute their struc- 

 ture, these elements are their true food ; 

 therefore it is the plant more than the" 

 soil whose component parts should be 



determined. Chemists have laboured for 

 years in determining the qualities of soils 

 and manures, while they have by far too 

 much neglected the analysis of the plants 

 themselves, which is the first and most 

 important consideration. 



It is a pretty generally received opinion 

 amongst many cultivators that each 

 species of plant requires a distinct species 

 of food to be presented to it from the 

 soil ; but vegetable physiologists have 

 shown that the organs of one plant derive 

 their food from substances which concur 

 in the true nutrition of plants generally; 

 or, that is to say, plants of the most 

 opposite characters and properties, as 

 articles of food or vehicles of poison, will 

 not only exist, but flourish in the same 

 flower-pot of earth or of manure — a cir- 

 cumstance opposed to the theory that 

 each species requires a different element 

 of food. 



M. Boussingault favoured the opinion 

 that there was no absolute necessity for 

 a rotation of crops " when dung and 

 labour can be readily procured. Never- 

 theless," he says, "there are certain 

 plants which cannot be reproduced upon 

 the same soil advantageously, except at 

 intervals more or less remote. The cause 

 of this exigence on the part of certain 

 plants is still obscure, and the hypo- 

 theses propounded for clearing it up are 

 far from satisfactory. One of the marked 

 advantages of alternate culture is the 

 periodic cultivation of plants which im- 

 prove the soil. In this way a sort of 

 compensation is made for exhaustion. 

 The main thing to be secured, in the rota- 

 tion of crops, is such a system as shall 

 enable the husbandman to attain the 

 greatest amount of vegetable production 

 with the least manure, and in the shortest 

 possible time. This system can alone be 

 realised by employing, in the course of 

 rotation, those plants which draw largely 

 from the atmosphere. The best plan. of 

 rotation in theory is that in which the 

 quantity of organic matter obtained most 

 exceeds the quantity of organic matter 

 introduced into the soil in the shape of 

 manure. This does not hold in practice. 

 It is less the surplus amount of organic 

 matter over that contained in the manure, 

 than the value of the same matter, which 

 concerns the cultivator. The excess re- 

 quired, and the form in which it should 



