ALTERNATION OF CROPS. 



57 



ength, by continued contact with the air, 

 which tillage is the means of procuring, its 

 decay are effected; and at the commence- 

 ment of spring it has become converted, 

 either in whole or in part, into a substance 

 which supplies the place of humus, by being 

 a constant source of carbonic acid. 



The quickness with which this decay of 

 the excrements of plants proceeds depends 

 on the composition of the soil, and on its 

 greater or less porosity. It will take place 

 very quickly in a calcareous soil: for the 

 power of organic excrements to attract oxy- 

 gen and to putrefy is increased by contact 

 with the alkaline constituents, and by the 

 general porous nature of such kinds of soil, 

 which freely permit the access of air. But 

 it requires a longer time in heavy soils con- 

 sisting of loam or clay. 



The same plants can be cultivated with 

 advantage on one soil after the second. year, 

 but in others not until the fifth or ninth, 

 merely on account of the change and de- 

 struction of the excrements, which have an 

 injurious influence on the plants being com- 

 pleted in the one, in the second year ; in the 

 others, not until the ninth. 



Jn some neighbourhoods clover will not 

 thrive till the sixth year, in others not till the 

 twelfth ; flax in the second or third year. 

 All this depends on the chemical nature of 

 the soil, for it has been found by experience 

 that in those districts where the intervals at 

 which the same plants can be cultivated 

 with advantage are very long, the time can- 

 not be shortened even by the use of the most 

 powerful manures. The destruction of the 

 peculiar excrements of one crop must have 

 taken place before a new crop can be pro- 

 duced. 



Flax, peas, clover, and even potatoes, are 

 plants the excrements of which, in argilla- 

 ceous soils, require the longest time for their 

 conversion into humus; but it is evident 

 that the use of alkalies and burnt lime, or 

 even small quantities of ashes which have 

 not been lixiviated, must enable a soil to 

 permit the cultivation of the same plants in 

 a much shorter time. 



A soil lying fallow owes its earlier fer- 

 tility, in part, to the destruction or conver- 

 sion into humus of the excrements contained 

 in it, which is effected during the fallow 

 season, at the same time that the land is 

 exposed to a farther disintegration. 



In the soils in the neighbourhood of the 

 Rhine and Nile, which contain much pot- 

 ash, and where crops can be obtained in close 

 succession from the same field, the fallowing 

 of the land is superseded by the inundation ; 

 the irrigation of meadows effects the same 

 purpose. It is because the water of rivers 

 and streams contains oxygen in solution that 

 it effects the most complete and rapid putre- 

 faction of the excrements contained in the 

 soil which it penetrates, and in which it is 

 continually renewed. If it was the water 

 alone which produced this effect, marshy 

 meadows should be most fertile. Hence it 



is not sufficient in irrigating meadows to 

 ccnvert them into marshes, by covering for 

 several months their surface with water, 

 which is not renewed; for the advantage of 

 irrigation consists principally in supplying 

 oxygen to the roots of plants. The quantity 

 of water necessary for this purpose is very 

 small, so that it is sufficient to cover the 

 meadow with a very thin layer, if this be 

 frequently renewed. 



The cultivation of meadows forms one of 

 the most important branches of rural eco- 

 nomy. It contributes materially to the pros- 

 perity of the agriculturist by increasing his 

 stock of cattle, and consequently by furnish- 

 ing him with manure, which may be applied 

 to the augmentation of his crops. Indeed, 

 th great progress which has been made in 

 Germany in the improvement of cattle is 

 mainly attributable to the attention which is 

 devoted in that country to the culture of 

 meadows. The environs of Siegin, in Nas- 

 sau, are particularly famed in this respect, 

 and every year a large number of young 

 farmers repair to it, for the purpose of study- 

 ing this branch of agriculture in situ. In 

 that district the culture of grass has attained 

 such great perfection, that the produce of 

 their meadow-land far exceeds that obtained 

 in any other part of Germany. This is ef- 

 fected simply by preparing the ground in 

 such a manner as to enable it to be irrigated 

 both in spring and in autumn. The surface 

 of the soil is fitted to suit the locality, and 

 the quantity of water which can be com- 

 manded. Thus if the meadows be situated 

 upon a declivity, banks of from one to two 

 feet in height are raised at short distances 

 from each other. The water is admitted by 

 small channels upon the most elevated bank, 

 and allowed to discharge itself over the sides 

 in such a manner as to run upon the bank 

 situated below. The grass grown upon 

 meadows irrigated in this way is three or 

 four times higher than that obtained from 

 fields which are covered with water that is 

 deprived of all egress and renewal. 



It follows from what has preceded that the 

 advantage of the alternation of crops is ow- 

 ing to two causes. 



A fertile soil ought to afford to a plant all 

 the inorganic bodies indispensable for its ex- 

 istence in sufficient quantity and in such 

 condition as allows their absorption. 



All plants require alkalies, which are 

 contained in some, in the GraminecB for ex- 

 ample, in the form of silicates ; in otners, 

 in that of tartrates, citrates, acetates, or ox- 

 alates. 



When these alkalies are in combination 

 with silicic acid, the ashes obtained by the 

 incineration of the plant contain no carbonic 

 acid ; but when they are united with organic 

 acids, the addition of a mineral acid to their 

 ashes causes an effervescence. 



A third species of plants requires phos- 

 phate of lime, another phosphate of mag- 

 nesia, and several do not thrive without car- 

 bonate of lime. 



