184 SCIENTIFIC AGRICULTURE. 



the air for some time, before they can be safely mixed with 

 the surface soil. 5. The soil often contains more or less 

 organic matter which is inert, or decays so slowly as to be 

 almost unavailable to vegetation : by leaving this to decompose 

 and become fitted for the food of plants, the crop which fol- 

 lows will grow more luxuriantly and yield more abundantly. 

 6. The nitrates, which are very favorable to vegetable growth, 

 are more rapidly formed when the land lies in naked fallow 

 than when covered with crops. 7. The fragments of rocks of 

 various kinds are disintegrated and decomposed faster during 

 fallow than during cropping. 8. The saline and other sub- 

 stances, such as ammonia, magnesia, the nitrates, &c., which 

 are brought down by rains, accumulate in the soil during fal- 

 low. 9. The clay, oxide of iron, and organic matter of the 

 soil, have the power of extracting ammonia from the air; and 

 this is the more rapid, the greater the extent of surface which 

 is uncovered and exposed to the passing air. 10. The light 

 soils sometimes become too loose to afford sufficient mechani- 

 cal support to the roots of crops, and require time to settle 

 together and resume their cohesion and compactness. 



No doubt the period usually allowed to land to lie in fallow 

 may in many cases be very much abridged, and in some cases 

 altogether dispensed with. Whenever follow is beneficial, it 

 must be ascribed to some one or more, if not all the above 

 causes combined. 



ROTATION OF CROPS. 



By rotation of crops, is implied, the alternate production of 

 different plants in regular succession on the same land. Expe- 

 rience has shown that the same crop cannot be produced 

 successively on the same field for an indefinite period of time. 



The grasses and forest trees seem to present an exception 

 to this principle : but it must be observed that the grasses are 

 mowed or pastured down before arriving at maturity, for, if 

 they were allowed to perfect their growth and ripen their 



