80 The Substratum 



as a particularly desirable type of storehouse since materials are re- 

 leased from it only gradually. As will be discussed more fully in 

 Chapter 8, the critical plant nutrients of the soil consist of such in- 

 organic materials as nitrate, phosphate, potassium, and calcium, as 

 well as of certain organic substances. These are derived in part from 

 the humus itself and in part from the breakdown of the mineral com- 

 ponents of the soil. Many of the nutrient materials are held by the 

 colloidal complex in loose chemical combination or physical adsorp- 

 tion on the surfaces of particles. The reactions involved in the de- 

 composition of soil components and in the association of nutrient mate- 

 rials with colloids are extremely complicated, and the reader should 

 turn to a treatment such as Lyon, Buckman, and Brady ( 1952 ) for a 

 further discussion. The general point emphasized here is that the 

 colloidal complex fulfills the important function of providing for the 

 slow release of nutrient materials in such a way that they can be 

 absorbed by plant roots as needed. 



The difference between the gradual delivery of nitrogen from the 

 supply in the organic matter and the rapid exhaustion of soluble nitro- 

 gen that is freely mobile in the soil has no doubt been observed by the 

 reader for his own lawn or garden. When nitrogen fertilizers are 

 added to the soil, they tend to be rapidly dissolved. In this condition 

 the nutrient salts may be quickly leached away by rain and ground 

 water, or they may produce a "flash" growth of the plants present. 

 More desirable for the growth of cultivated plants, as well as for vege- 

 tation in general, is the slow availability of nitrogen from the organic 

 colloidal complex and from the decomposing organic matter, although 

 nitrogen fertilizers are often beneficial if used properly. 



The Soil Profile. A broader aspect of the organization of the soil 

 and another on which animals and plants exert a profound influence 

 is the soil profile. If you look closely at a fresh vertical section 

 through the soil, as exposed in an excavation or a road cut, you will 

 see a succession of layers, or horizons as they are called, that together 

 form the soil profile (Fig. 3.7). This arrangement of the soil mate- 

 rial in layers is the result of the action of the living components and 

 the climatic influences of the region on the original parent material. 

 In some regions this master organization of the land substratum has 

 been in the process of formation for thousands of years. The nature 

 of the soil profile is of crucial concern in respect both to the natural 

 vegetation and to commercial crops. We should accordingly think 

 twice before allowing agricultural procedures that may permanently 

 destroy the established layering of the soil. 



The depth and composition of each horizon of the soil profile differ 



