THE BALANCING FUNCTION OF SALTS 123 



text, behaves like a negative salt when placed in contact with a 

 negative colloid. All salts can thus be divided into two groups— 

 those that are " negative" and those that are " positive." Acids 

 behave like positive salts owing to the very active hydrogen ion, 

 and alkalies behave like negative salts. The size of the radical or 

 ion also plays a part, because the charge concentrated on the sur- 

 face will be more intense the smaller the ion. 



Thus we see that CaCl 2 and NaCl are toxic because both pro- 

 duce abnormal changes in the protoplasmic membrane, owing to 

 the electrical charges which they possess. Antagonism results 

 because these effects are opposite, and when the salts are mixed 

 in the proper proportions the electrical charges are neutralized. 

 All toxicity disappears in a balanced solution because the salts 

 and their ions are balanced electrically. 



It is noteworthy that of the six elements which plants take from 

 the soil in any quantity, viz., N, Ca, K, Mg, S, and P, three occur 

 in the anion with five negative charges while the three in the cation 



/ - -- + + ++ +\ 



have five positive charges ^ N o 3 , S0 4 , HP0 4 , Ca, Mg, and KJ- 



Iron is generally added as iron phosphate f jr e> P0 4 )> anc * tne 

 two latest additions to the list of essential elements (Mn and B) 

 are absorbed as ]y[ n and g 4 Q 7 - While these facts do not prove 



anything, they can hardly be mere coincidences. 



The Soil Salts; Base Exchange. — In the cultivation of plants, 

 fertilizers containing various plant nutrients, as discussed in the 

 previous chapter, are added to the soil. But the soil is an extremely 

 complex mixture of microorganisms, decaying organic matter, 

 and fine particles of disintegrating rocks, ranging in size from stones 

 or pebbles down to particles of eolloidal size and even to mole- 

 cules and ions in solution. The result is that physicochemical 

 reactions that occur between the various particles of the soil 

 (not counting the biological activities) are so varied and interre- 

 lated that it is difficult to know exactly what salts are reaching 

 the plant even when one knows what has been added to the soil. 

 Colloids of the soil absorb on their surfaces some of the salts 

 added, and in some cases one ion of a salt may be absorbed more 

 than the other. Likewise the plants may absorb one ion more 

 than another. This differential adsorption and absorption of 



