76 ON THE ROTATION OF CROPS. 



to yield for many years without the application of fresh manure; the varieties 

 of trefoil, especially clover and sainfoin, are of this sort. 



PRINCIPLE 5. That all plants are not equally favourable to the growth of 

 weeds. It is said that a plant fouls the soil, when it facilitates or permits the 

 growth of weeds, which exhaust the earth, weary the plant, appropriate to 

 themselves a part of its nourishment, and hasten its decay. All plants not 

 provided with an extensive system of large and vigorous leaves, calculated to 

 cover the ground, foul the soil. The grains from their slender stalks rising 

 into the air, and their long narrow leaves, easily admit into their intervals 

 those weeds that grow upon the surface, which being defended from heat and 

 winds grow by favour of the grain they injure. 



From the principles here laid down, says our author, we 

 may draw the following conclusions: First. That however 

 well prepared a soil may be, it cannot nourish a long succession 

 of crops without becoming exhausted. Second. Each harvest 

 impoverishes the soil to a certain extent, depending upon the 

 degrees of nourishment which it restores to the earth. Third. 

 The cultivation of spindle roots ought to succeed that of run- 

 ning and superficial roots. Fourth. It is necessary to avoid 

 returning too soon to the cultivation of the same, or analogous 

 kinds of vegetables, in the same soil.* Fifth. It is very un- 

 wise to allow two kinds of plants, which admit of the too ready 

 growth of weeds among them, to be raised in succession. 

 Sixth. Those plants that derive their principal support from 

 the soil should not be sown, excepting when the soil is suffi- 

 ciently provided with manure. Seventh. When the soil ex- 

 hibits symptoms of exhaustion from successive harvests, the 

 cultivation of those plants that restore most to the soil, must 

 be resorted to. 



These principles are confirmed by experience; they form 

 the basis of a system of agriculture, rich in its products, but 

 more rich in its economy, by the diminution of the usual quan- 

 tity of labour and manure. All cultivators ought to be govern- 

 ed by them, but their application must be modified by the 

 nature of soils and climates, and the particular wants of each 

 locality. 



To prescribe a series of successive and various harvests, 

 without paying any regard to the difference of soils, would be 

 to commit a great error, and to condemn the system of cropping 

 in the eyes of those agriculturists who are too little enlightened 



* In addition to the reasons I have given why plants of the same or analo- 

 gous kinds should not be cultivated in succession upon the same soil, there is 

 another which I will here assign. M. OLIVER, Member of the French Insti- 

 tute, has described with much care all the insects which devour the neck of 

 the roots of grain; these multiply infinitely if the same or analogous kinds of 

 plants be presented to the soil for several successive years; but perish for 

 want of food whenever plants not suited to be food for their larvae, are made 

 to succeed the grains. These insects belong to the family of Tipulae, or that 

 of flies. (Sixteenth Vol. of the Memoirs of the Royal and Central Agricul- 

 tural Society of Paris.) 



