280 HANDY-BOOK OF HUSBANDBY. 



" beet-root or turnips will not be impeded, if the soil, at the same 

 " time, contain a proper quantity of alkalies. 



" When a soil contains silicates not prone to disintegrate, it 

 " may be able, in its natural state, to liberate by the influence of 

 " the atmosphere, in three or four years, only as much silica as 

 " suffices for one crop of wheat. In this case, such a crop can 

 '' only be grown on it in a three or four years' rotation, assuming 

 " that the phosphates necessary for the formation of the seeds 

 " exist in the soil in sufficient quantity. But we can shorten this 

 " period by working well the soil, and by increasing its surface, 

 '' so as to make it more accessible to the action of the air and 

 " moisture, in order to disintegrate the soil, and to procure a greater 

 " provision of soluble silicates. The decomposition of the sili- 

 " cates may also be accelerated by the use of burnt lime ; but it is 

 " certain that, although all these means may enable us to insure 

 *' rich crops for a certain period, they induce, at the same time, 

 " an earlier exhaustion of the soil, and impair its natural state of 

 " fertility. 



* * * . * * * * 



" It follows, then, from the preceding observations, that the 

 " advantage of the alternate system of husbandry consists in the 

 " fact that the cultivated plants abstract from the soil unequal 

 *' quantities of certain nutritious matters. 



" A fertile soil must contain in sufficient quantity, and in a 

 *' form adapted for assimilation, all the inorganic materials indis- 

 " pensable for the growth of plants. 



" A field artificially prepared for culture contains a certain 

 '' amount of these ingredients, and also of ammoniacal salts and 

 " decaying vegetable matter. The system of rotation adopted on 

 " such a field is, that a potash plant (turnips or potatoes) is suc- 

 " ceeded by a silica plant, and the latter is followed by a lime 

 " plant. All these plants require phosphates and alkalies — the 

 "potash plant requiring the largest quantity of the latter and the 

 "smallest quantity of the former. The silica plants require, in 

 " addition to the soluble silica left by the potash plants, a consid- 

 " erable amount of phosphates ; and the succeeding lime plants 



