REASONS FOR INTERCHANGE OF CROPS. 271 



well established, that the produce of many plants diminishes, 

 when cultivated for several years on the same soil. 



5. But on the other hand, it is also fully settled, that when 

 a field has become unfitted for one species of grain, it is not 

 therefore unfitted for another ; but that a succession of plants 

 will flourish well without the addition of a large quantity of 

 manure; hence has arisen the modern system of rotation. 

 It now becomes a question of the first importance whether 

 these facts can be so explained, as to aid us in pointing out 

 the best system of rotation. If we can fully ascertain the 

 causes of the failure of the successive cultivation of the same 

 crop, and of the favorable effects of rotation, we shall be pro- 

 vided with the best hints for constructing a proper system. 

 These causes are to be found in the structure of plants, in 

 their composition, and in the influence of the matters which 

 they excrete by their roots. 



I. The structure of plants, such as their roots, stalks and 

 leaves, afford one important reason for the rotation of crops. 

 Each family of plants have similar roots, leaves, etc. Their 

 action upon the soil is therefore similar. The spindle roots, 

 for example, like the carrot and beet, extend their roots deeply 

 into the soil, while the common grains lie near the surface. 

 Clover and some of the grasses penetrate to a considerable 

 depth, and branch out in all directions ; hence, when one kind 

 of crop is planted in the same soil for several successive 

 years, the effect both mechanically and chemically is the 

 same. Chaptal supposes that the roots exhaust only those 

 portions of the soil which are in contact with them, 

 and hence similar roots exhaust the soil in the same parts; 

 but this effect could not take place when the land is ploughed 

 between each crop, though it might apply to trees. This 

 theory is wholly set aside by the fact, that the roots form a 

 galvanic battery with the soil ; and, as in all galvanic circles, 

 the matter would be transferred from some distance around, 

 so that the plant could stand in no need of food, provided it 



