THE BALANCING FUNCTION OF SALTS 125 



water is 1/10,000,000 Normal in respect to the H and the OH ions. 

 Now instead of saying that a neutral solution contains 10 -7 g. 

 mol. of hydrogen ions, which would be a long and inconvenient 

 way of stating the matter, we say that water "has a pH of 7. 



This type of equation applies equally well to all solutions of 

 acids and bases, and it is possible in all cases to express the active 

 acidity by an appropriate pH number. It will be seen that if 

 there were more hydrogen ions present in the solution, the number 

 of hydroxyl (OH) ions would decrease, since K is a constant. 

 When a number with a negative exponent increases, the numerical 

 value of the exponent decreases (e. g., 10~ 7 is less than 10~ 5 ). 

 When we express the active acidity by the "pH value," we are 

 using this numerical value of the hydrogen ion exponent; there- 

 fore, the smaller the pH value, the larger the number of hydrogen 

 ions and the greater the active acidity. 



Also it will be seen that when we know how many H ions there 

 are in a solution, we also know how many OH ions are present, 

 since their product is a constant. For this reason, the expression 

 of the pH value is extremely convenient. The reader should also 

 be able to see that a solution of pH 4 has 10 times as many H ions 

 as one of pH 5 (1/10 2 is how many times 1/10 3 ?). 



With these chemical preliminaries out of the way, we are now 

 ready to proceed with our physiology. The development of most 

 species of plants is possible only within rather narrow limits of 

 H + ion concentration. Soils generally have a pH value between 

 3 and 11, but the former would be very acid and the latter very 

 alkaline. Some plants prefer soils of a certain pH value and others 

 of another. In fact, in buying a farm or judging the value of a 

 soil, the nature of the plants growing thereon furnishes a very 

 valuable guide and a fairly reliable index to the present cultural 

 value of the soil as far as the acidity is concerned. 



By far the vast majority of plants thrive best near the neutral 

 point. Alfalfa, clover, lupines, rye, spinach, and wheat grow best 

 at pH 6.0-7.5; buckwheat, corn, oats, strawberries, onions, po- 

 tatoes, soy beans, timothy, and turnips can thrive on somewhat 

 more acid soils at pH 5.0-6.5; while barley, sugar beets, and to- 

 bacco thrive best on soils somewhat alkaline (pH 7.0-8.0). These 

 are all seen to be associated with soils not extremely far from neu- 

 tral, but coffee is said to require soils as acid as pH 4.0-5.0, and 

 pineapples grow best at pH 4.0-6.0. 



