72 



CROPS AS FEEDERS 



In general, the surface soil contains more nitrogen than the subsoil, 

 owing to the presence of more organic matter. Some deep, black 

 soils may have as high a percentage of nitrogen in the subsoil 

 (to a limited depth) as is contained in the surface stratum. 



The percentage of phosphorus in the surface layer is commonly 

 greater than or equal to that contained in the subsoil. There is 

 often a close relationship between the phosphorus content and the 

 amount of organic matter in mineral soils. This accounts for 

 the higher phosphorus content of the upper strata. During the 

 virgin state the roots of grasses and other plants brought up 

 considerable phosphorus from the subsoil from a depth of about 



Si 



dations^ 



soils. 



The new analysis, as compared with the 

 average of 100 fertile soils of Illinois, shows 

 as follows and shows better than our own 



original analysis: 



Average Average 

 % Wis. % III. 



Nitrogen 1.23 .30 



Potash 75 .25 



Phosphates 14 .24 



The re] 

 of th 



Fig. 28. — How some peat land was advertised. This particular Wisconsin, peat was 

 compared with the best Dlinois prairie soil, which averages about 25 per cent nitrogen, 

 1.8 per cent potassium and 08 per cent phosphorus. 



two or three feet, and this became stored in the organic matter 

 accumulated in the top soil. On exhaustive cropping, the higher 

 phosphorus content of the surface soil is gradually reduced until 

 it equals at least the percentage contained in the subsoil. 



The potassium content is usually greater in the subsoil, espe- 

 cially when they are fine textxn-ed. More potassium is found in 

 subsoils in humid climates because of the presence of more fine 

 particles which are not only richer in potassium than the coarser 

 surface particles, but which absorb much of the potassium leached 

 down from the surface stratum. 



In arid and semi-arid §oils the phosphorus and potassium con- 

 tent of the surface soil is very much the same as that of the subsoil. 



