32 PRINCIPLES OF AGRICULTURAL CHEMISTRY 



For instance, suppose all conditions were favorable to the produc- 

 tion of 100 bushels of corn, with the exception of phosphoric 

 acid ; then the size of the crop would depend upon the quantity of 

 phosphoric acid it could secure; if only enough for 10 bushels, 

 then ten bushels it would be. Suppose the soil were very rich, as 

 it often is in arid regions, and there were little water, then 

 quantity of water would limit the size of the crop. An excess 

 of water would also limit decrease the crop. If all conditions 

 of soil and water were favorable, the limiting conditions might be 

 the quantity of light the plant received, the temperature, or the 

 individuality of the plant. 



The conditions which limit plant growth may be kind of seed, 

 light, water, space, temperature, total ash, phosphoric acid or any 

 other plant food, insects, injurious diseases, or the condition, 

 nature or situation of the soil. The size of the crop depends 

 upon the least favorable of the conditions which surround it. It 

 is exceedingly important in practical agriculture to ascertain the 

 limiting conditions, and render them more favorable. 



Mitscherlich 1 gives mathematical expression to the law of 

 minimum. Under ideal conditions, a certain maximum yield 

 would be obtained. The yield is depressed if some essential 

 factor is deficient. If now the deficiency is overcome to a 

 certain extent, the yield becomes greater, and is the larger, the 

 greater the previous depression. According to Mitscherlich, the 

 increase in crop produced by a unit increment of the lacking 

 factor is proportional to the decrement from the maximum. 

 The mathematical expression is : 



dy 

 -fe = (A y}k or log e (A y} = c kx. 



When x is the amount of the factor present, y is the yield, and 

 A is the maximum yield possible with an excess of the factor. 



Permanent and Variable Limiting Conditions. The character 



of the soil and the plant food which it supplies are more or less 



permanent during the growth of the crop, but the soil moisture 



and the weather conditions are more variable. The limiting con- 



1 Landw. Versuchs-stat., 1911, p. 231. 



