58 



AGRICULTURAL CHEMISTRY. 



Silicic acid is the first solid substance 

 taken up by plants; it appears to be the ma- 

 terial from which the formation of the wood 

 takes its origin, acting like a grain of sand 

 around which the first crystals form in a so- 

 lution of a salt which is in the act of crys- 

 tallising. Silicic acid appears to perform 

 the functions of woody fibre in the Equise- 

 tacece and bamboos,* just as the crystalline 

 salt, oxalate of lime,, does in many of the 

 lichens. 



When we grow in the same soil for seve- 

 ral years in succession different plants, the 

 first of which leaves behind that which the 

 second, and the second that which the third 

 may require, the soil will be a fruitful one 

 for all the three kinds of produce. If the 

 first plant, for example, be wheat, which 

 consumes the greatest part of the silicate of 

 potash in a soil, whilst the plants which 

 succeed it are of such a kind as require 

 only small quantities of potash, as is the 

 case with Leguminosce, turnips, potatoes, 

 &c., the wheat may be again sowed with 

 advantage after the fourth year ; for during 

 the interval of three years the soil will, by 

 the action of the atmosphere, be rendered 

 capable of again yielding silicate of potash 

 in sufficient quantity for the young plants. 



The same precaution must be observed 

 with regard to the other inorganic constitu- 

 ents, when it is desired to grow different 

 plants in succession on the same soil : for a 

 successive growth of plants which extract 

 the same component parts, must gradually 

 render it incapable of producing them. 

 Each of these plants during its growth re- 

 turns to the soil a certain quantity of sub- 

 stances containing carbon, which are gra- 

 dually converted into humus, and are for the 

 most part equivalent to as much carbon as 

 the plants had formerly extracted from the 

 soil in a state of carbonic acid. But al- 

 though this is sufficient to bring many plants 

 to maturity, it is not enough to furnish their 

 different organs with the greatest possible 

 supply of nourishment. Now the object of 

 agriculture is to produce either articles of 

 commerce, or food for man and animals ; 

 but a maximum of produce in plants is al- 

 ways in proportion to the quantity of nutri- 

 ment supplied to them in the first stage of 

 their developement. 



The nutriment of young plants consists 

 of carbonic acid, contained in the soil in the 

 form of humus, and of nitrogen in the form 

 of ammonia, both of which must be sup- 

 plied to the plants, if the desired purpose is 

 to be accomplished. The formation of am- 

 monia cannot be effected on cultivated land, 

 but humus may be artificially produced ; and 

 this must be considered as an important ob- 

 ject in the alternation of crops, and as the 

 second reason of its peculiar advantages. 



* Silica is found in the joints of bamboos, in the 

 form of small round globules, which have received 

 the name of Tabasheer, and are distinguished by 

 their remarkable optical properties. 



The sowing of a field with fallow plants, 

 such as clover, rye, buck-wheat, &.C., and 

 the incorporation of plants, when nearly at 

 blossom, with the soil, affect this supply of 

 humus in so far, that young plants subse- 

 quently growing in it find, at a certain pe- 

 riod of their growth, a maximum of nu- 

 triment, that is, matter in the process of de- 

 cay. 



The same end is obtained, but with much 

 greater certainty, when the field is planted 

 with sainfoin or lucerne.* These plants are 

 remarkable on account of the great ramifi- 

 cation of their roots, and strong develope- 

 ment of their leaves, and for requiring only 

 a small quantity of inorganic matter. Until 

 they reach a certain period of their growth, 

 they retain all the carbonic acid and ammo- 

 nia which may have been conveyed to them 

 by rain and the air, for that which is not 

 absorbed by the soil is appropriated by the 

 leaves ; they also possess an extensive four 

 or six-fold surface, capable of assimilating 

 these bodies, and of preventing the volatili- 

 zation of the ammonia from the soil, by 

 completely covering it in. 



An immediate consequence of the pro- 

 duction of the green principle of the leaves, 

 and of their remaining component parts, as 

 well as those of the stem, is the equally 

 abundant excretion of organic matters into 

 the soil from the roots. 



The favourable influence which this ex- 

 ercises on the land, by furnishing it with 

 matter capable of being converted into hu- 

 mus, lasts for several years, but barren spots 

 gradually appear after the lapse of some 

 time. Now it is evident that, after from six 

 to seven years, the ground must become so 

 impregnated with excrements that every 

 fibre of the root will be surrounded with 

 them. As they remain for some time in a 

 soluble condition, the plants must absorb 

 part of them and suffer injurious effects in 

 consequence, because they are not capable 

 of assimilation. When such a field is ob- 

 served for several years, it is seen that the 

 barren spots are again covered with vegeta- 

 tion, (the same plants being always sup- 

 posed to be grown,) whilst new spots be- 

 come bare and apparently unfruitful, and so 

 on alternately. The causes which produce 

 this alternate barrenness and fertility in the 

 different parts of the land are evident. The 

 excrements upon the barren spots receiving 

 no new addition, and being subjected to the 

 influence of air and moisture, they pass into 

 putrefaction, and their injurious influence 



* The alternation of crops with sainfoin and lu- 

 cern is now universally adopted in Bingen and its 

 vicinity, as well as in the Palatinate ; the fields in 

 these districts receive manure only once every 

 nine years. In the first years after the land has 

 been manured turnips are sown upon it, in the 

 next following years barley, with sainfoin or lu- 

 cerne ; in the seventh year potatoes, in the eighth 

 wheat, in the ninth barley ; on the tenth year it ia 

 manured, and then the same rotation again takes 

 place. 



