SOIL NITROGEN 17 



Table 12. — Nonleguminous Crops. Pounds per Acre 



of Dry Matter and Nitrogen, 1924-1935. Plots 7 



and 9, Without Nitrogen Fertilizer since 1882 



Year Crop Dry Matter Nitrogen 



1924 Grass 990 8.7 



1925 Sweet corn 2,120 28.5 



1926 Japanese millet 4,760 33.4 



1927 Sweet corn 2,630 44.4 



1928 Grass 3,370 30.0 



1929 Grass 1,230 12.0 



1930 Japanese millet 6,800 70.9 



1931 Oats 2,110 27.0 



1932 Hungarian millet 3,580 29.2 



1933 Barley Plowed under 



1934 Hungarian millet 3,900 32.9 



1935 Oats 2,290 25.6 



Total 33,780 342.6 



The two series of nonlegume crops differ in the crops grown and their suc- 

 cession. The early series is estimated to have removed in its 9 crops, 39.5 

 pounds per acre of nitrogen per year. The 11 crops in the late series are 

 calculated to have removed 31.1 pounds per acre yearly, but with wide varia- 

 tions from the average. Grasses cut once in a season remove much less nitro- 

 gen than is contained in the turf and roots. Corn leaves only a small part of 

 its nitrogen in its stubble and roots. Spring crops are less benefited by soil 

 organisms than summer crops, which have a warmer soil at their time of growth. 



Omitting the first crop of corn in the early series because it had the benefit 

 of the grass sod, the other six corn crops averaged 40.2 pounds per acre of 

 nitrogen per year. The late series, including the six summer crops of sweet 

 corn, Japanese and Hungarian millets, had a yearly average of 39.9 pounds of 

 nitrogen. The soil of these two plots again supplied nitrogen to crops under 

 summer conditions at the rate of 40 pounds per acre, without measurable 

 deterioration, although 40 years had elapsed during which any additions of 

 nitrogen must have been through soil microorganisms. 



This experiment is not absolute proof of the fixation of nitrogen by soil 

 organisms in the absence of legumes. Circumstantial evidence, however, favors 

 such fixation. 



Of the five crops grown on the nonlegume subplots with residual nitrogen, 

 grasses alone showed striking response to the fertilizer residue when compared 

 with the subplots without nitrogen fertilizer. Six crops on the same subplots 

 with nitrogen fertilizer exceeded those on the subplots without it, yet they 

 had recovered less than half of the added nitrogen in comparison with that 

 taken wholly from the soil of the other plots. One-half the applied nitrogen 

 seems unnecessary because some soil agent appears capable of keeping up a 

 moderate supply, year after year. 



Bradley and Fuller have studied the nitrogen-fixing powers of soils taken 

 from these different subplots and of bacteria isolated from the soils, results of 



