PLANT NUTRITION 263 



soil is believed to be an exchange, principally between the hydrogen 

 ions from the plant roots and the cations from the soil complex. The 

 absorption of cations is thought to take place in two ways. In the 

 first way the exchangeable ion is absorbed from the soil solution, but 

 in the second the absorption is thought to take place through contact 

 exchange between plant roots and soil particles. This direct-contact 

 exchange is believed to take place without having the ions become a 

 part of the soil solution. 



FERTILIZERS 



A system of intensive agriculture which removes most of the plants 

 from the farm on which they are produced sooner or later results in 

 decreased crop yields unless certain practices in soil management are 

 followed. In order to continue to produce good yields of crops, addi- 

 tional nutrients must be made available, either directly or indirectly. 



Elements required for the maintenance of soil fertility are stored in 

 soil minerals from which they may be released by growing plants. A 

 system of rotation can be followed in which one crop is raised for the 

 purpose of mining nutrient elements from soils. This plant crop 

 when returned to the soil furnishes organic matter as well as available 

 nutrients to help maintain soil fertility. 



One of the best ways to make use of farm crops for the maintenance 

 of soil fertility is to feed the plants to animals and to return the ma- 

 nure to the soil. The use of farm manure as a fertilizer and factors 

 affecting its value will be discussed later in this chapter. 



However beneficial the practice of returning plants to the soil may 

 be, often it is not feasible to rely on this one method for the main- 

 tenance of soil fertility. Other sources of nutrient elements must be 

 added. Materials that supply the lacking mineral elements are called 

 fertilizers. 



The usual meaning of the term fertilizer in commerce includes only 

 the principal nutrient elements, nitrogen, phosphorus, and potassium, 

 omitting calcium, magnesium, sidfur, and micronutrient elements. 

 However, commercial fertilizers contain many elements in addition 

 to the principal three given above. Thus sulfur, chlorine, sodium, 

 calcium, magnesium, and small quantities of many other elements may 

 be present. Nevertheless, fertilizers are sold on the basis of their con- 

 tent of nitrogen, phosphorus, and potassium. This content is stated as 

 percentages of N, PoO-,, and KoO, given in that order. Therefore, a 



