159 ROTATION OF CROPS. 



He.Vtnthus 

 Peas.* Wheat. Rye. tuherosus. Turnip3.t 



117 112-43 77-05 122 37'84 



According to the preceding views, plants must obtain from 

 the soil certain constituents, in order to enable them to reach 

 perfect maturity — that is, to enable them to bear blossoms and 

 fruit. The growth of a plant is very limited in pure water, in 

 pure silica, or in a soil from which these ingredients are absent, 

 If there be not present in the soil alkalies, lime, and magnesia, 

 the stem, leaves, and blossoms of the plants can only be formed 

 in proportion to the quantity of these substances existing as a 

 provision in the seed. When phosphates are wanting, the seeds 

 cannot be formed. 



The more quickly a plant grows, the more rapidly do its 

 leaves increase in number and in size, and therefore the supply 

 of alkaline bases must be greater in a given time. 



As all plants remove from the soil certain constituents, it is 

 quite obvious that none of them can render it either richer or 

 more fertile for a plant of another kind. If we convert into 

 arable land a soil which has grown for centuries wood, or a 

 vegetation which has not changed, and if we spread over this 

 soil the ashes of the wood and of the bushes, we have added to 

 that contained in the soil a now provision of alkaline bases, and 

 of phosphates, which may suffice for a hundred or more crops of 

 certain plants. If the soil contains silicates susceptible of disin- 

 tegration, there will also be present in it soluble silicate of 

 potash or soda, which is necessary for the rendering mature the 

 stern of the siliceous plants ; and, with the phosphates already 

 present, we have in such a soil all the conditions necessary to 

 sustain uninterrupted crops of corn for a series of years. 



If this soil be either deficient or wanting in the silicates, but 

 yet contain an abundant quantity of salts of lime and of phos- 

 phates, we will be enabled to obtain from it, for a number of years, 

 successive crops of tobacco, peas, beans, &c, and wine. 



But, if none of the ingredients furnished to these olants be again 

 returned to the soil, a time must come when it can no longer 



♦Heavily manured. t Heavily manured. 



