MATTERS TO BE RESTORED VARY. 241 



turnip produce of three hectares the seed-constituents 

 of four wheat-crops, besides about 1000 Ibs. of potash. 

 A supply of phosphoric acid alone will not suffice, in 

 this case, to keep up the productiveness of the land. 



The grower, of commercial plants, such as tobacco, 

 hemp, flax, the vine, &c., must in like manner strictly 

 attend to the law of restitution, which, properly inter- 

 preted, does not imply that he should bestow tne same 

 anxious care upon the replacement of all constituents 

 alike which have been taken away in the crops. It 

 would, for instance, be the height of absurdity to re- 

 quire the tobacco planter who grows his crops on a lime 

 or marl soil, to replace the lime carried off in the leaves 

 of the plant. But it tells him that not all that goes by 

 the name of manure is useful for his fields, and it shows 

 him the difference between manures : it informs him of 

 the loss inflicted upon the soil by the preceding crop, 

 and the supply required to insure future harvests ; it 

 teaches him never to allow himself to be guided in his 

 proceedings by the opinions of persons who do not take 

 the slightest interest in him and his land, but always to 

 act upon his own observations. A careful study of the 

 weeds that spring up spontaneously in his fields may 

 frequently prove more useful in this respect than a heap 

 of hand-books on agriculture. 



If after the foregoing statements the condition of 

 the cultivated land in Europe, and the decline towards 

 which agriculture is tending by the prevailing system 

 of farm-yard manuring, should still be a matter of 

 doubt to many persons unacquainted with the natural 

 sciences, and who trust only to definite numbers as 

 palpable facts, that doubt may, perhaps, be removed 

 by statistical data on the corn produce of the land in 

 different parts of Germany, which have been collected 

 partly by order of the government. 



For a correct appreciation of the importance of 

 these data in the matter, it is necessary in the first 

 place to understand clearly what is meant by an 

 ' average ' crop. By this term is designated the aver- 

 age produce, expressed in numbers, of a field, or a 

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