420 



University of California Publications in Agricultural Sciences [Vol. 3 



Nitrogen Fixation^ 



Diablo clay adobe. — This type shows the highest quantity of nitro- 

 gen fixed, 9.6 nig., with the subsoil quantities, much lower than the 

 surface. The variation within the type is seen to be the largest of that 

 in any of the types. 



Altamont clay loam. — The surface samples have 1.0, 4.7, and 9.1 

 mg. nitrogen (table 34 and fig. 20b). The soils shows a wide diver- 

 gence between the surface samples and between the surface and sub- 

 soils. This is to be expected in the heavier soils. 



7.5 



c 



B 

 2.5 



10 



11 



12 



13 17 



Mg N. Fixed 



18 



21 



26 Soils 



Fig. 21b. Graph showing nitrogen fixation in the three horizons of the 

 eight samples of San Joaquin sandy loam. 



San Joaquin sandy loam. — The quantity fixed in the A horizon 

 (table 35 and fig. 21b) is small and quite variable. It is between 

 nothing and 5.5 mg., with the average of 1.9 mg. Instead of nitrogen 

 fixation denitrification took place in a number of cases, especially in 

 horizon C. Considering the wide variation in textures of the horizons, 

 it is rather odd that there should not be a greater variation between 

 the soils from the various depths. 



Han ford fine sandy loam. — The amount of nitrogen fixed by the 

 surface soil (table 36, and fig. 22b) averages much higher, 5.7 mg., 

 than that in the San Joaquin sandy loam, though the range of varia- 

 tion is about the same. It is noticeable that the amounts of nitrogen 

 fixed by the B and C horizons of the soils nos. 14 and 19 are much 



28 All of the figures on nitrogen fixation refer to the milligrams of nitrogen 

 fixed per gram of mannite in 50 grams of soil (table 33 and figs. 9-13). 



