lyii^] on. Recent Advances in AijricuUural Science. 515 



The (]uestion of the duration of the fertility of the land under 

 continual cropping has excited much attention of late, chiefly because 

 the United States has begun to take alarm about the reduced produc- 

 tion of some of its most fertile lands, as, for instance, the old prairie 

 lands of the middle "West— a reduced production which, amongst other 

 causes, has helped to set in motion a stream of migrants from the 

 United States to tlie newer lands of the Canadian North-West. In 

 the development of agriculture three distinct stages may be observed. 

 In the first place, we may have a process of pure exploitation of the 

 initial resources of the soil, when the farmer is to all intents and 

 purposes mining in its fertility. This is the process which, in the 

 main, has been going on in America, and, indeed, in all the newer 

 countries which have been opened up to agriculture during the last 

 two centuries. Not all virgin soils are rich, and the system of crop- 

 ping alternately with wheat or maize which prevails over so much of 

 North America has reduced great areas of the land in the eastern 

 States to such a poverty-stricken condition that it has been allowed 

 to go derelict. In the great plains, however, where the first settler 

 found four or five feet of black soil, containing nearly half per cent, 

 of nitrogen, the land has kept up its productivity almost unimpared for 

 nearly a century. If we suppose the black soil only extended to a 

 depth of three feet, and contained three-tenths per cent, of nitrogen, 

 both limited estimates, there would still be 80,000 lbs. of nitrogen per 

 acre - that is to say, nitrogen enough for five hundred crops larger 

 than the American farmer has been accustomed to win from that land 

 — and yet in less than a century such soils are beginning to show 

 signs of exhaustion. The farming of the kind just described is de- 

 structive ; but in the older lands of the west of Europe, which have 

 been under cultivation for something like a century, a conservative 

 system has been devised which is capable of keeping up the produc- 

 tive power of the soil, though not, perhaps, to a very high pitch. 

 Perhaps the best example of this may be seen in the Norfolk four- 

 course rotation prior to the introduction of artificial fertilizers. In 

 this system a turnip crop, which was either consumed on the ground 

 or converted into manure and so returned to the soil, was followed by 

 barley in which clover was sown, and the clover, which also got back 

 to the soil, was followed by wheat. The farming covenants prevented 

 the sale of anything more than barley and wheat grain, and the meat 

 that was produced by the consumption of the turnips and hay. Thus 

 but a small proportion of the nitrogen taken out of the soil by the 

 crop left the farm ; the rest was returned and used over again, 

 although considerable losses of gaseous nitrogen occurred during 

 the making of the dung. Both losses, however, were more than re- 

 placed by the nitrogen which the clover crop gathered from the atmo- 

 sphere during its growth. At any rate, we find that under such a 

 conservative system of farming the productivity of the land remained 

 pretty constant at about a level of twenty bushels to the acre from the 



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