On Arjricultural Chemistry. 



251 



stores of nitrogen are generally found, are exactly those which 

 derive the least benefit from a rotation of crops. The amount of 

 nitrogen existing in a sandy soil may hardly be appreciable by 

 analysis, but by the free circulation of air through its pores, the 

 accumulation from the resources of the atmosphere through the 

 medium of green crops, and especially of turnips, to a certain ex- 

 tent counterbalances the deficiency. The actual value of a tur- 

 nip crop must vary very much, according to the texture of the 

 soil. On heavy clays, the decomposition of soil by means of a 

 summer-fallow aided by lime, will often render available more 

 ammonia as well as mineral mixtures, than could be obtained by 

 means of a turnip-crop. Upon light soils, however, nothing can 

 advantageously substitute the collective powers of the turnip. In 

 one soil the accumulation of available stores may be effected by 

 combustion or lime, in the other they must be supplied by a dif- 

 ferent process. As almost every soil contains mineral matter in 

 an undecomposed state, it must evidently be advantageous to 

 favour its liberation by every possible means ; for the more pro- 

 duce a soil can be made to yield without manure, the less manure 

 it will require to bring its produce up to a maximum. It was at 

 one time supposed that by repeatedly hoeing and stirring the soil, 

 it could be made to yield perpetual crops without manure ; and 

 although this was carrying the principle too far, it undoubtedly 

 proves the benefit of mechanical operations. Draining, however, 

 offers advantages to the agriculturist superior to any as a means 

 of obtaining the influence of the atmosphere upon the soil. Not 

 only is the surface of the soil exposed to the action of the air, but 

 its influence extends to the depth of the drains themselves. In 

 addition to this advantage, what may be considered as an artificial 

 climate is to some extent obtained. An increased temperature, 

 and the absence of moisture, conditions so essential to the pro- 

 duction of grain of fine quality, are the result of draining the soil. 

 Thermometers placed in two soils equally exposed to the sun's 

 rays, one of which is moist and the other dry, indicate very differ- 

 ent degrees of temperature. The rays of the sun, which only 

 serve to evaporate moisture in the one, will raise the tempera- 

 ture of the oilier. It follows that plants would grow more rapidly 

 upon a well- drained soil than upon one in an opposite condition, 

 especially during the spring. It will be remembered how large 

 a quantity of ammonia I found it necessary to supply to my soil 

 each year to restore the substances removed in the previous crop. 

 Besides being expensive, this ammonia cannot be procured in the 

 market in any large quantities ; but by cultivating turnips and 

 the leguminous plants, a large amount of this substance is col- 

 lected by them from the atmosphere. A rotation of crops may in 

 one sense therefore be considered as an economical process for 



