374 NORMAN 



clay colloids are fully saturated with bases, whereas in an acid soil there is 

 partial hydrogen replacement. It is these cations which are available for 

 exchange with plant roots, and it is the clay complex that constitutes the 

 reserve of these cations. 



Several different types of clay colloid have been recognized, and even within 

 a single type there is some range in properties. However, the different types 

 behave as though they were different kinds of insoluble acids. They have 

 different total exchange capacities, and the ease of replacement of various 

 cations retained by them may be different. In equilibrium with an extracting 

 solution, or with plant roots, the partition of exchangeable bases would not 

 be the same. Some of the major differences between soils in physical properties 

 and nutritional properties are explicable only on the basis of the different 

 types of clay which they contain. 



The total exchange capacity of a soil depends primarily on the amount and 

 nature of the clay component. The organic fraction of soils also possesses 

 the property of retaining cations by attachment to carboxyl and hydroxyl 

 groups, and in soils of high organic content the fraction of the total exchange 

 capacity due to the organic matter may exceed that due to the clay. Examples 

 of the exchange capacity of some representative soils are given in table 2 . 



Table 2. Cation-exchange capacities of some representative surface soils 



Cass Sandy Loam, Nebraska 



Wabash Silty Clay Loam, Nebraska . . 

 Marshall Silty Clay Loam, Nebraska . . 

 Sharpsburg Silty Clay Loam, Nebraska 

 Crete Silty Clay Loam, Nebraska . . . . 



Pima Clay Loam, Arizona 



Mohave Sandy Clay Loam, Arizona . . 



Tucson Sandy Loam, Arizona 



Laveen Sandy Loam, Arizona 



Davidson Loam, North Carolina 



Hiwassee Clay Loam, North Carolina . 

 Utuado Sandy Loam, Puerto Rico . . . . 

 Mucara Sandy Loam, Puerto Rico . . . . 



aLipps, R. C, AND L. Chesnin. 1950. Soil Sci. Soc. Amer. Proc. 15:329-333. 

 " Flocker, W. J., AND W. H. Fuller. 1956. Soil Sci. Soc. Amer. Proc. 20:387-391. 

 «=Nyun, M. a., and S. B. McCaleb. 1955. Soil Sci. 80:27-41. 

 d Abruna, F., and R. M. Smith. 1953. Soil Sci. 75:411-420. 



The array of bases present in substantial amounts includes calcium, mag- 

 nesium, potassium, and sodium, usually in descending order quantitatively. 



