112 SCIENCE AND SOIL 



If the culture experiments and the ultimate chemical analysis of the soil agree 

 in the deficiency of any plant-food element, then the information is conclusive 

 and final; but if these two sources of information disagree, then the culture 

 experiments should be considered as tentative and likely to give way with 

 increasing knowledge and improved methods to the information based on 

 chemical analysis, which is absolute. " l 



The plant food in the subsurface and subsoil is unquestionably 

 )f some value, but even the total supplies of nitrogen and phos- 

 phorus that are held within the feeding range of ordinary plant 

 roots are not unlimited when measured by crop requirements in 

 permanent agriculture. However, the thing of first importance is 

 to maintain a rich surface soil, for no subsoil is of much practical 

 value if it lies beneath a worn-out surface. On the other hand, if the 

 subsoil will act as a reservoir for moisture, then a rich top soil will 

 produce large crops. Manures and fertilizers are applied to, and 

 incorporated with, the plowed stratum only. On the Rothamsted 

 fields' where chalk exceeding 100 tons per acre was applied to the 

 land a hundred years ago, practically no calcium carbonate is 

 found below the plowed soil even after a century of cultivation, 

 although 50 tons of the chalk applied still remain in the surface soil ; 

 and the land fertilized with nitrogen, phosphorus, and potassium, 

 which has yielded more than 30 bushels of wheat per acre as an 

 average of fifty years, contains, as an average, no more plant 

 food in the strata below the surface 9 inches than is found in 

 the same strata where the land has been unfertilized and has 

 produced an average yield of only 13 bushels of wheat for the 

 same fifty years. 



\i The supply of nitrogen in soils is contained only in the organic 

 matter; and thus the amount of nitrogen in the subsoil of normal 

 soil types is relatively small, as will be seen from a study of Tables 

 15, 16, and 17, six million pounds of subsoil containing, as a rule, 

 less nitrogen than two million pounds of surface, except where the 

 surface is much worn. The small amount of humus in the subsoil 

 is also quite inactive, and the liberation of nitrogen from its decom- 

 position is very slight. Furthermore, in all humid regions there is 

 large loss of nitrogen in drainage waters; so that in practice the 

 addition of nitrogen to the surface soil must be somewhat greater 



1 "Cyclopedia of American Agriculture," Vol. I, page 475. 



