AVAILABLE PLANT FOOD 113 



than that removed in crops, if the productive power of the soil is 

 maintained. 



The phosphorus of the soil exists in both organic and mineral 

 forms. While the supply of mineral phosphorus is likely to be smaller 

 in the surface and subsurface than in the subsoil, the organic 

 phosphorus, like nitrogen, varies with the organic matter, which, 

 as a rule, decreases rapidly below the plowed soil. As a consequence, 

 there is usually less phosphorus in the subsurface than in the sur- 

 face, unless the surface is very poor in organic matter; and also 

 there is less phosphorus in the subsurface than in the subsoil, 

 unless the subsurface is much richer in organic matter. A larger 

 supply of phosphorus in the surface than in the subsurface is suffi- 

 cient to prove that plants secure some phosphorus from below the 

 surface soil, because the excess of phosphorus in the surface is 

 contained in the decomposition products of plant residues from the 

 centuries gone by. Where the subsoil is richer in phosphorus than 

 the subsurface, it indicates either that phosphorus has been lost 

 from the subsurface by leaching or that the plant roots have with- 

 drawn phosphorus from the subsurface to a greater extent than 

 from the subsoil. 



With the brown silt loams and black clay loams of the corn belt, 

 the surface stratum rich in organic matter is always richer in phos- 

 phorus than the subsurface; and, with one exception, the subsur- 

 face, which is also quite well supplied with organic matter, is richer 

 in phosphorus than the subsoil, but the other upland soils contain 

 less organic matter in the surface and much less in the subsurface, 

 and the subsoil, with a single exception, is richer in phosphorus than 

 the subsurface, equal weights of soil always being considered. 



The potassium is contained almost solely in the mineral part of 

 the soil, and the supply regularly increases with depth, the sub- 

 surface being richer than the surface and the subsoil still richer. 



