126 THE FORAGE AND FIBER CROPS IN AMERICA 



soluble nitrates of the soil and from the free nitrogen of the 

 air has never been determined. Nevertheless the fact that 

 clovers and other leguminous plants do in practise improve the 

 crop-producing power of the soils, especially for cereals and 

 true grasses, indicates, although it does not prove, that the 

 amount of free nitrogen taken from the air may not be incon- 

 siderable. 1 "It is one of the secrets of profitable farming to 

 draw from the air as much nitrogen as possible by the alter- 

 nation of crops." 2 



133. Influence of Root-tubercles on the Growth of Plants. 



In the case of a number of leguminous plants it has been shown 

 that those which contain tubercles on their roots grow more 

 vigorously than those which do not. The poorer the soil in 

 available nitrogen the greater has been found the difference 

 between plants possessing tubercles and those which do not. 

 In some instances, and, perhaps, with all our more commonly 



1 There is a considerable number of facts scattered throughout the literature 

 of the subject which may be taken, if not as an absolute, at least as a very 

 satisfactory proof that the amounts of atmospheric nitrogen fixed may be 

 quite large. Hall ("On the accumulation of fertility by land allowed to run 

 wild," The Journal of Agricultural Science, Vol. I, part 2, May, 1905) shows 

 that a portion of the Broadbalk field at Rothamsted left to itself for 20 years 

 apparently gained nitrogen at the rate of more than 100 pounds per annum. 

 Schultz-Lupitz (Zwischenfruchtbau auf leichten Boden, Berlin, 1901, p. 8) 

 states that the fixation of atmospheric nitrogen by legumes is not accompanied 

 by a reduction in the nitrogen content of the soil. One soil area at Lupitz 

 bore 28 successive crops of lupines while receiving annually applications of 

 kainit alone. Notwithstanding the very large amounts of nitrogen removed 

 in the 28 crops, the soils were steadily gaining in nitrogen, as was demonstrated 

 by the soil analyses made at different times by Maercker, Orth, Frank, and 

 Holdefleiss. The investigations of Vogel in 1894 showed fixation of nitrogen 

 at the rate of 140 kilos a hectare, and he estimated the total nitrogen harvest 

 in the fall of that year (011 250 morgen) at 17,200 pounds, equivalent in round 

 numbers to 1,100 zentners of sodium nitrate. The box experiments of the 

 New Jersey Station (Bui. No. 180) may be taken as a further proof that 

 leguminous crops may not only yield large quantities of nitrogen in their 

 substance, but leave the soil richer in combined nitrogen. 



2 George Ville: Artificial Manures, p. 37. 



