15 



VII. SOIL-PLANT RELATIONS 



A. Basic Aspects 



The soil and plant components of the soil-plant system are individually com- 

 plex, as is evident from the preceding sections. The combination of these two com- 

 ponents increases the difficulties of understanding and generalization. Both systems, 

 independently and together, are dynamic. Plant growth requires the continuous net 

 removal of ions from the soil into the plant. On the other hand, changes in mois- 

 ture and the removal of ions by the plants continually change the quantity of the ions 

 available to the plant. 



B. Competing and Carrier Cations 



The kinds and amounts of the complementary ions affect the availability of a 

 given ion (48, 87). Two types of processes can be distinguished: the exchange reac- 

 tions governing the distribution of ions between clay and solution (37, 48) and the 

 competitive effects during the course of ion absorption by plants (17, 20, 21, 27, 28). 

 Since several cations compete for the same carrier site, increasing the concentration 

 of one should decrease the uptake of others in the same group. Examination of this 

 hypothesis in greenhouse and field experiments has shown this to be true within 

 certain ranges. 



Increasing the calcium concentration in nutrient solution from zero to two 

 milliequivalents per liter reduces the uptake of strontium (43). Further increases 

 in calcium reduce strontium uptake only slightly. A fourfold reduction of strontium 

 uptake in field experiments appears to be the maximum that can be achieved by the 

 addition of calcium to acid, low-calcium soils. Even smaller reduction occurs in 

 soils richer in calcium. 



The addition of stable strontium has little effect on radioactive strontium up- 

 take because of the similarity of strontium to calcium and the thousandfold greater 

 abundance of calcium in soils (68, 127). In one experiment, no effect of stable stron- 

 tium was observed (43) and a slight increase in strontium-90 uptaike was found in 

 another experiment (104). It was postulated that small increments of strontium dis- 

 placed some of the strontium-90 from the exchange complex into solution. It is esti- 

 mated that five tons of strontium amendments per acre would be needed to reduce the 

 strontium-90 uptake appreciably (104). 



A depressing effect of potassium on plant uptake of calcium, magnesium, and 

 strontium has been observed (47, 54, 65). Potassium treatments decreased stron- 

 tium uptake 20 per cent in wheat plants (47) and 40 per cent in radish plants (54). 



In a field comparison of plant concentrations of different elements with the cor- 

 responding soil concentrations (57), it was found that varying levels of calcium and 



