PHYSIOLOGY AND MINERAL NUTRITION 101 



tinct loss in germination. Pons et al. (65) stored peanuts at minus 18° C, 

 1° C. and 27° C. for 4 years. Those stored at 27° C. were not viable, 

 while those stored at lower temperatures germinated perfectly. In view of 

 these results it appears that the viability of peanut seed may be good for 

 from 3 to 5 years, depending upon the variety and the temperature of 

 storage. 



The viability of seed is usually determined by a germination test. Re- 

 cently Brewer (16) reported good agreement between the tetrazolium 

 chloride chemical test and germinability by an ordinary method. If a 

 chemical method for determining the viability of peanut seed could be 

 perfected, it might be of considerable advantage to persons involved 

 in germination studies. 



ABSORPTION OF MINERAL ELEMENTS 



Mineral elements considered essential for plant growth, with minor 

 exceptions, are absorbed by the roots from the soil. It is generally assumed 

 that plant roots absorb ions either from the soil solution or by a root- 

 colloid exchange. 



Cations such as calcium, magnesium, potassium, sodium and hydro- 

 gen are sorbed by the soil colloids. Through the phenomenon of base ex- 

 change the cations are liberated to the soil solution and thus become 

 available for intake by the roots. For example, carbon dioxide resulting 

 from root respiration or from the decomposition of organic matter can 

 react with the soil water to form carbonic acid. The hydrogen ions from 

 the carbonic acid may displace cations attached to the soil colloids. The 

 cations released to the soil solution as the result of ionic exchange can 

 then be absorbed by the plant. The base exchange reaction is reversible 

 and the amount of cations present in the soil solution at any time will 

 depend on several factors. Jenny and Overstreet (46) contend that by a 

 root-colloid exchange mechanism there can be a direct exchange of 

 ions between the root and the soil colloid. According to this theory, ions 

 sorbed on colloids may be as readily available to plants as ions free in 

 solution. 



The plant absorbs anions from the soil solution. Anions, with the 

 possible exception of the phosphate ion, are not retained in any appreci- 

 able quantities in well-drained soils and unless used by crops are usually 

 leached out of the soil rather rapidly. The water-soluble phosphate com- 

 pounds are thought to be precipitated in the soil as insoluble or relatively 

 insoluble compounds which largely prevent their leaching. The supply 

 of soil phosphorus available to the plant depends on a series of complex 



