Soils and fertilizers . as winterrains will have leached from the soil nitrates that accumu-lated during the preceding year, the plants often sufferseriously from lack of nitrogen. It is not often that the soil for several inches belowthe surface becomes hot enough, even in midsummer, tointerfere with nitrate formation. Crops that make theirgrowth in late spring or summer are not Hkely to sufferfor nitrates unless the total supply of nitrogen is deficient. 171. Effect of sod on nitrate formation. — In soil onwhich there is a good stand of grass very little nitrate isever found. Sod apparently
Soils and fertilizers . as winterrains will have leached from the soil nitrates that accumu-lated during the preceding year, the plants often sufferseriously from lack of nitrogen. It is not often that the soil for several inches belowthe surface becomes hot enough, even in midsummer, tointerfere with nitrate formation. Crops that make theirgrowth in late spring or summer are not Hkely to sufferfor nitrates unless the total supply of nitrogen is deficient. 171. Effect of sod on nitrate formation. — In soil onwhich there is a good stand of grass very little nitrate isever found. Sod apparently has a depressing influence onnitrate formation. On the same type of soil as that used inthe experiment last described, the average quantities ofnitrates for each month of the growing season in the surfaceeight inches of sod land, as compared with corn land underthe same manuring, were as follows : Table 31. — Nitrates in Soil Under Sod and Under Corn MoN^ AprilMay-JuneJulyAu^st NiTRATsa IN Dry Soil, Parts perMillion. There was more nitrogen contained in the corn crop thanthere was in the timothy crop, so that the larger quantity THE GERM LIFE OF THE SOIL 135 of nitrates in the corn land cannot be attributed to failureof the plants to remove it. Grass appears to have a decidedly-depressing effect on the process of nitrate formation, andthis may be one reason why grass is generally a detrimentto the growth of young orchards. 172. Depths at which nitrate formation takes place. —It is probable that the processes by which nitrates are formedare, in humid regions, confined largely to the furrow sliceof soil. Nitrates found below that point have probablybeen, in large measure, washed down from above. The sub-soil in such a region is not a verj^ favorable medium for theseprocesses. In arid and semi-arid regions, however, the caseis different. Here the distinction between surface soil andsubsoil is not so marked, and owing to the rich and porousnature of thes
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