252 



THE ROTHAMSTED EXPERIMENTS. 



Importance 

 of mineral 

 constitu- 

 ents. 



Nitrogen 

 in rotation 

 crops. 



Assimila- 

 tion of 

 nitrogen by 

 roots. 



small, the very fact that the different crops require for their 

 growth, not only very different amounts of individual consti- 

 tuents, but require these to be available within the soil in 

 very different conditions, both of combination and of distri- 

 bution, points to the conclusion that, in any explanation of 

 the benefits of an alternation of crops, the position, and the 

 rdle, of the mineral constituents must not be overlooked ; and 

 the less can it be so, when their connection with the very 

 important element — the nitrogen of the crops — is considered. 



As to the nitrogen : — It has been seen that, although very 

 characteristically benefited by nitrogenous manures, the cereal 

 crops take up and retain much less nitrogen than any of the 

 crops alternated with them. In fact, the root -crops may 

 contain two, or more, times as much nitrogen as either of the 

 cereals, and the leguminous crop, especially the clover, much 

 more than the root-crops. The greater part of the nitrogen 

 of the cereals is, however, sold off the farm ; but perhaps not 

 more than 10 or 15 per cent of that of either the root-crop, or 

 the clover, or other forage leguminous crop, is sold off in 

 animal increase or milk. Thus, most of the nitrogen of the 

 straw of the cereals, and a very large proportion of that of the 

 much more highly nitrogen-yielding crops, returns to the land 

 as manure, for the benefit of future cereals and other crops. 

 Indeed, it is, as a rule, only a comparatively small proportion 

 of the very much increased amount of nitrogen obtained in 

 rotation compared with that in continuous cereal -cropping 

 (chiefly due to the interpolated crops), that is lost to the land 

 in the saleable products. 



As to the source of the nitrogen of the so-called " restora- 

 tive crops," it has been shown that certainly in the case of 

 the roots it is not, as has sometimes been assumed, that such 

 plants take up nitrogen from the air by virtue of their ex- 

 tended leaf-surface. Both common experience and direct 

 experiment demonstrate that they are as dependent as any 

 crop that is grown on available nitrogen within the soil, which 

 is generally supplied by the direct application of nitrogenous 

 manures — natural or artificial. Under such conditions of 

 supply, however, the root-crops, so to speak, gross feeders as 

 they are, and distributing a very large amount of fibrous feed- 

 ing root within the soil, avail themselves of a much greater 

 quantity of the nitrogen supplied than the cereals would do 

 under similar circumstances ; this result being partly due to 

 their period of accumulation and growth extending even 

 months after the period of collection by the ripening cereals 

 has terminated, and at the season when nitrification within 

 the soil is the most active, and the accumulation of nitrates 

 in it is the greatest. Lastly, full supply of both mineral con- 



