52 



THE ROTHAMSTED EXPERIMENTS. 



Table 15 remainder from the atmosphere. The results given in Table 

 expained. -^ afford pretty conclusive evidence against such a view. 

 There is there given the average produce of mangel-wurzel 

 — root, leaf, and total crop — over five years : — 



1. By superphosphate of lime and potassium sulphate. 



2. By the same mineral manures with, in addition, am- 



monium-salts, supplying 7.8 lb. nitrogen per acre per 

 annum. 



3. The same mineral manures and ammonium-salts, sup- 



plying 86 lb. nitrogen per acre per annum. 



Wed of 

 large and 

 small sup- 

 plies of 

 nitrogen. 



Soil the 

 source of 

 nitrogen 

 for roots 

 and cereals. 



TABLE 15. — Mangel-Wurzel. Average produce, 5 years, 

 1876-80. Quantities per acre per annum. 



, J Superphosphate of lime and po-1 



\ tassium sulphate 

 9 / As 1, and 36| lb. ammonium-salts 1 

 \ ( = 7.8 lb. nitrogen) 



and 400 lb. ammonium-salts'! 

 :86 lb. nitrogen) 



("As 1, 



Roots. 



tons. cwt. 



4 10 



6 



14 



Leaves. 



tons, cwt, 



1 



1 6 



2 16 



Total. 



tons. cwt. 



5 10 



7 6 



16 16 



Thus the annual application of 7.8 lb. of nitrogen increased 

 the crop by only 30 cwt. of roots per acre per annum ; and 

 it may be mentioned that the increased yield of nitrogen in 

 the crop was even less than that supplied in the manure. 

 The application of 86 lb. of nitrogen, however, further in- 

 creased the crop of roots by 160 cwt. more, or by 190 cwt. 

 in all. It is obvious that the application of the small 

 amount of nitrogen (7.8 lb.) did not enable the plant to 

 take up any from the atmosphere, and that it required a 

 further supply by manure to obtain a further increase of 

 crop. 



It cannot be doubted that beyond the small amount of 

 combined nitrogen which annually comes 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 less 

 recent accumulations or to direct supply by manure. Further 

 confirmation of the conclusion that the source of the nitrogen 

 of root-crops, as of cereals and others, is the supplies within 

 the soil, is to be found in the fact that after many years of 

 the growth of such crops by mineral manures without nitro- 

 gen, the surface-soil showed a lower percentage of nitrogen 

 than has been found in any of the other experimental fields. 

 It is indeed certain that if root -crops are to yield large 



