FERTILITY OF SOILS. 121 



This disease generally attacks turnips, gourds, and peas, 

 when the soil is drenched with sudden and violent rain, after 

 continued dry weather, at the time when the plants are near, but 

 have not attained maturity ; it is also necessary for its occur- 

 rence, that dry weather should again happen after the rain. 



By the rapid evaporation of the water absorbed by the roots, 

 a laiger quantity of salts enters the plants than they are able to 

 use. The salts effloresce on the surface of the leaves, and 

 when they are juicy, act as if the plants had been treated with 

 solutions of salts, in greater quantity than their organism could 

 bear. Of two plants of the same kind the one nearest maturity 

 is most liable to this disease ; if the other plant has either been 

 planted at a later period, or if its development has been restrain- 

 ed, the causes, which exercised injurious effects upon the first 

 plant, accelerate the development of the latter. The germ 

 springing out of the earth, the leaf on coming out of the bud, the 

 young stem, and the green sprouts, contain a much larger quan- 

 tity of salts with alkaline bases and give ashes on incineration 

 much richer in alkaline ingredients, than parts of the matured 

 plant. The leaves, being the part in which the absorption and 

 decomposition of carbonic acid is effected, are much richer in 

 mineral ingredients than other parts of the plant. 



The simple fact that a plant is restrained in growth by the 

 want of rain to convey to it alkalies, proves completely that these 

 alkalies play a most important part in vegetation. 



Although it was found by Saussure that wheat before blossom- 

 ing yielded -j^j, in blossom -j-j^r, an d after the ripening of the 

 seeds only half this quantity of ashes ; it cannot hence be con- 

 cluded that the ingredients of the soil present in the young and 

 growing plants, were again returned to the soil. Equal quanti- 

 ties of young plants yield twice the amount of ashes that matured 

 plants do ; but this evidently arises from the circumstance, that 

 new quantities of organic constituents are added to the carbon, 

 hydrogen, and nitrogen, previously existing in the young plant. 

 The amount of ashes remains the same in both plants, although 

 their relative proportions have become different. 



We may feel assured that the alkalies contained in the vine, 

 in the potatoe, and beet, and found in tho juices, united with tar- 

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