ROTATION OP CROPS. 



115 



that in cases of mixed salts in water, the plant absorbed all in equal 

 proportions. Daubeny states, that if a particular salt is not present, 

 the plant frequently takes up an isomorphous one. 



231. The differences in the absorption of solutions depend, per- 

 haps, on the relative densities alone, and not on any peculiar selecting 

 power in roots, for it is well known that poisonous matters are absorbed 

 as well as those which are wholesome. The following experiments show 

 that poisonous matters in solutions, varying from half a grain to five 

 grains to the ounce of water, are taken up by roots, and that some 

 substances which are poisonous to animals do not appear to act ener- 

 getically upon plants : 



Chloride of zinc, 

 Sulphate of zinc. 

 Sulphate of copper, 

 Nitrate of copper, 

 Acetate of copper, 



Bichloride of mercury, 



Arsenious acid, 

 Arseniate of potash, 

 Acetate of lead, 



Bichromate of potash, 



Nitrate fe sulphate of iron, 

 Chloride of barium, 

 Nitrate of baryta, 



Nitrate of strontia, 



Muriate, sulphate, and) 

 nitrate of lime, ) 



Sulphate and muriate) 

 of magnesia, / 



Phosphate of soda, 



Chloride of sodium, 



Growing Plants. . 

 i beans, 



. cabbages, and wheat, 

 , beans, 

 . beans, 

 cabbages, 

 (beans, 

 < wheat, 

 (cabbages, 

 cabbages and wheat, 

 barley and cabbages, 

 beans, 



cabbages, beans, barley 



beans, 

 beans, 

 cabbages and wheat, 



beans, 

 beans, 



beans and cabbages, 



beans and cabbages, 

 beans and cabbages, 



J- quickly destroyed. 



"I weak solutions did not 

 / destroy. 



destroyed in a few days. 

 f destroyed unless much 

 "| diluted. 



destroyed in a few days. 



V quickly destroyed. 



J plants uninjured, except 

 ( solution strong. 

 < improved when very di- 



luted, 

 injured, and if strong 



destroyed. 



v 



} no injury when diluted. 



232. Rotation of Crops. As the inorganic materials which enter 

 into the composition of plants vary much in their nature and relative 

 proportions, it is evident that a soil may contain those necessary for 

 the growth of certain species, while it may be deficient in those 

 required by others. It is on this principle that the rotation of crops 

 proceeds ; those plants succeeding each other in rotation which require 

 different inorganic compounds for their growth. In ordinary cases, 

 except in the case of very fertile virgin soil, a crop, by being constantly 

 grown in successive years on the same field, will deteriorate in a 

 marked degree. Dr. Daubeny has put this to the test of experiment, 

 by causing plants to grow on the same and different plots in successive 

 years, and noting the results : 



