INTRODUCTION. 19 



elements. Too abundant feeding of our cultivated plants has created 

 a danger which the farmer of to-day must face. 



Formerly the method of cultivation gave a mediocre and irregular 

 yield, and the farmer did not disturb himself. There was in the opinion 

 of our fathers, which was fatal, good years and bad years. Diseases 

 existed even then, but they did not in their opinion contribute much to 

 the annual variation in the yields. In our days they have a much 

 more important role, for the cultivation expenses being higher, owing 

 to increased attention and to the use of various chemical manures, the 

 yield ought to compensate for the pecuniary efforts expended. 



Exhaustion of the Soil. — In spite of the annual supply to the 

 soil of the elements required by the plant for its intensive growth, it 

 is found that a time comes when the plant ceases to profit from the 

 nutritive elements and thrives no longer. This is due to the fact 

 that the enemies of the cultivated plant are accumulated in the soil. 

 The ancient farmers attributed this condition to the exhaustion of the 

 soil, and intercalated the bare falloiv between the crops when this ex- 

 haustion manifested itself. In bare fallow the fields remained several 

 years without a crop. Without being aware of it they thus abolished 

 the provision stores of the parasites, and these disappeared or became 

 reduced to their natural proportion. By this time the field had 

 acquired new vigour, and might be again cultivated. This method 

 cannot be adopted to-day, because it is a loss of time and money. 

 The altei'nation of crops or of different plants having consequently 

 different parasites succeeded each other, and where the same plant did 

 not appear in the rotation except at long intervals it caused a 

 great improvement in this condition of the soil. Rotations would * 

 give perfect results in the absence of polyphagia parasites : Nematoides, 

 Elaterides, grey and white worms which attack all our crops indiffer- 

 ently, and the exaggerated multiplication of which operates through- 

 out the most different crops ; the spores of Ustilaginece (smut, bunt, 

 etc.), which resist the weather for several years, excepted. Against the 

 exhaustion of the soil from the exaggerated development of these para- 

 sites no efficient remedy exists, except disinfection of the soil by car- 

 bon disulphide. This must be done either in a complete manner, 

 and in massive doses every ten years, or in small doses each autumn. 

 It frees our cultivated fields from all the parasites which our methods 

 of cropping have allowed to accumulate in too great number. This 

 method finds more adherents every day, as it enables rotations to be 

 dispensed with and to cultivate the same plant intensively for several 

 years in succession. Artificial manures as well as the metallic salts 

 intended to stimulate the growth of plants should be used with dis- 

 cretion, so as not to predispose the plant by a modification of the sap 

 to certain diseases which formerly it escaped. Laurent found that 

 bacteria, not parasites of the potato in a normal state, might invade 

 it after manuring with lime. The Jerusalem artichoke becomes less 

 resistant to the Sclerotina Libertiana ^ after phosphatic manure. These 



1 Note by Translator. — Fungus which ravages potato, haricot beans, hemp, 

 cucumbers, swedes, zinnias, petunias, chrysanthemums. Remedy. — Apply soot or 

 lime to soil. 



