48 



BULLETIN 



nitrates recovered from the different layers of the six soils 

 under consideration after they had stood under the mulched 

 and unmulched conditions where all other conditions were the 

 game so far as we know, except in so far as the condition of the 

 two surfaces affected the soil moisture and soil air relations, 

 and through those the differences in water-soluble salts recover- 

 able by our method of washing. The table following shows the 

 differences developed at different depths below the depth of the 

 3-inch mulch and below 3 inches in the soil not mulched : 



Differences in the amounts of nitrates, expressed as NO 3 , in six soil 



types at different depths below surface under 3-inch mulches 



and where surface was firm. 



It will be seen that we have, in every one of the six soil 

 types, profound differences in their nitrate content at the sev- 

 eral depths below the surface, which not only emphasizes the 

 point under consideration but also the influence of tillage on the 

 water-soluble content of the soil already referred to. The larger 

 amounts of nitrates shown here under the mulched surfaces are 

 not due simply to the fact that less has been carried above the 

 3-inch layer, where the soils were mulched, for the total 

 amounts recoverable from the full 18 inches were larger. The 

 figures in the table show in an emphatic manner how influential 

 the 3-inch mulch has been in holding the nitrates in the zone 

 of greatest root development, where they are* needed and can 

 best be obtained by the plants. 



The amounts of silica, chlorine and of bicarbonates were also 

 determined in these soils at the same time and the mean values 

 for the six soil types are given in the next table, together with 

 those for the other ingredients : 



