26 Mineral Nutrition of Plants 



integrates, but also loses material as its upper portion weathers more 

 completely from gravel to soil and becomes incorporated as a part of 

 the layer above. 



Thus we see that in the development of soils from rock, it is natural 

 and inevitable that the characteristics of the soil should vary from the 



A- HORIZON 



TOPSOIL 



B- HORIZON 



SUBSOIL 



C- HORIZON 



PARENT MATERIAL 



BEDROCK 



Figure i. A vertical cross-sectional view of a soil showing the horizons 

 (layers) that make up a soil. This is called a soil profile. 



surface downward. Although this variation is gradual, it is sufficiently 

 marked in most cases to allow ready observation of three layers of the 

 kind described. For convenience of reference as illustrated in Figure i, 

 the pedologist calls the upper layer the A horizon, the middle layer the 

 B horizon, and the lower layer just above the bedrock the C horizon. 

 Strictly speaking, the true soil, called the solum, consists of horizons A 

 and R; horizon C is raw material out of which the soil proper is made. 



