ROTATION OF CROPS. 225 



The Amounts of Nitrogen in the Rotation, and in the 

 Continuous Crops. 



Table 61 (p. 226) shows the average amounts of nitrogen 

 per acre per annum, over the eight years, in the rotation, and 

 in the continuous crops, respectively. 



The Nitrogen in the Boot-crops. — Without manure, with Wo man- 

 extremely small crops, but very abnormally high percentage ure ' 

 of nitrogen in them, the amounts per acre were, in the 

 continuously grown crops only about twice as much as 

 annually conies down as combined nitrogen in the rain and 

 minor aqueous deposits from the atmosphere ; whilst, even in 

 the rotation crops, the amounts averaged but little more than 

 in the continuous. 



With superphosphate alone, much larger crops, but much With 

 lower percentages of nitrogen, there was very much more su P e fP hos ~ 

 nitrogen taken up than without manure ; in fact, when grown 

 in rotation from three to four times as much, and when grown 

 continuously more than twice as much. There was, too, very 

 much more in the rotation than in the continuous crops. 

 The detailed results published elsewhere, relating to the con- 

 tinuous growth of root-crops afford conclusive evidence that 

 the increased amount of nitrogen taken up by the crop under 

 the influence of phosphatic manures is derived from the re- 

 sources of the soil itself, by the aid of the greatly enhanced 

 development of fibrous feeding root induced by such manures. 



With the mixed manure containing nitrogen there was, as Mixed 

 with superphosphate alone, much more nitrogen taken up manure - 

 under rotation than with continuous growth. But, under 

 rotation, there was about twice as much taken up with the 

 mixed manure containing nitrogen as with superphosphate 

 without nitrogen ; and with continuous growth there was 

 nearly three times as much taken up as with superphosphate 

 without nitrogen. It is clear, therefore, that the crops, 

 whether grown in rotation or continuously, took up much 

 of the nitrogen supplied by the manure. Indeed, it cannot Sources of 

 be doubted that, beyond the small amount of combined nitro- f^ roots. 

 gen annually coming down from the atmosphere in rain and 

 the minor aqueous deposits, the source of the large amount 

 of nitrogen of root-crops is the store of it within the soil, 

 whether this be due to accumulations, or to direct supply by 

 manure. On the other hand, the large amounts of produce Sources of 

 obtained by the aid of nitrogenous manures on land to which < r a J^ n ^ 01 

 no carbonaceous manure has been applied for about fifty 

 years is evidence that the atmosphere is at any rate the chief, 

 if not the exclusive, source of the carbon of the crops. 



Lastly, as to the results in the table relating to the Swed- 

 VOL. VII. p 



