136 MINERAL SALTS ABSORPTION IN PLANTS 



and are also easily removed from soil by leaching. Exchangeable 

 and non-exchangeable ions in soil are not so immediately available 

 but they represent a large reservoir of mineral salts from w^hich ions 

 are slowly released into the soil solution. As a result, soil remains 

 fertile for a long time in spite of the continual depletion of the soil 

 solution through absorption by plants and leaching by rainfall. 



B. The Soil Solution 



The soil solution comprises water and dissolved substances held 

 in the soil against gravitational forces, but displaceable by liquids 

 or by gas under pressure. Burd and Martin (1924) forced water 

 under pressure through a tall column of soil, and collected the 

 effluent in small aliquots. The first few samples had a similar 

 composition and these were taken to be uncontaminated soil 

 solution. The concentrations of various ions in solutions obtained 

 in this way from a cultivated soil are given in Table 13 A. It can be 

 seen that the concentration of certain ions, particularly potassium, 

 nitrate and phosphate was low, and this is characteristic of many 

 soil solutions. The data of Table 13A show also that the con- 

 centration of salts in the soil solution decreases during the course of 

 a growing season, and increases when the ground remains fallow. 



The composition of soil solutions varies considerably between 

 different soils, depending mainly on the nature of the parent material 

 from which the soil was formed and the vegetation it supports (Table 

 13B). Saline and non-saline clay soils yield solutions which are 

 especially rich in calcium, sodium and chloride, whereas in loams 

 the soil solution tends to contain less of these ions relative to the 

 amount of potassium present. Even in phosphorus-rich soils, the 

 soil solution seldom contains more than about 1 part per million 

 of this element. In a particular soil, the concentration of soil 

 solution tends to be inversely proportional to the moisture content 

 of the soil. This relationship holds particularly well for ions such 

 as nitrate and chloride which are present in soil mainly in the 

 dissolved state. The concentration of phosphate is more independent 

 of soil moisture content, presumably because the soil solution is 

 nearly saturated with rather insoluble phosphates, and when it is 

 diluted more salt goes into solution from the solid phase. As soil 

 moisture content is reduced, the concentrations of potassium and 



