306 ELEMENTARY STUDIES IN BOTANY 



16. Limiting factors. It is evident that in cultivating 

 plants we are not dealing with one condition or one substance, 

 but with a complex of conditions and substances. The 

 various conditions and substances may be spoken of as 

 factors that have to do with successful plant growth. We 

 have discussed six important factors and have indicated 

 several more. It is a very common mistake to suppose that 

 if some one factor is supplied, plants will do well. For 

 example, some will say that a proper water supply will insure 

 good plants; others will claim that a supply of nitrates is 

 all that is necessary for plants to do well ; still others are just 

 as strenuous in reference to phosphates as the only cure for 

 feebleness in plants. In this way a great number of so-called 

 " fertilizers " have been devised, each claiming to supply all 

 plants whatever they need. It must be evident, when it is ap- 

 preciated that numerous factors in combination are affecting 

 the plant, that any factor or even several factors may be favor- 

 able, and yet the plant cannot do well if any one necessary 

 factor is unfavorable. In other words, a plant cannot do any 

 better than the most unfavorable factor permits, and such a 

 factor is called a limiting factor. For example, the necessary 

 nitrates or phosphates may be supplied, but if the water 

 supply is too scanty or too abundant, the water is a limiting 

 factor, and the nitrates or phosphates cease to be serviceable. 

 In another case the water supply and the phosphates may 

 be just right, but if the supply of nitrates is not right, then 

 the nitrates form the limiting factor and impede the plant 

 in spite of its favorable water and phosphate supply. Any 

 one of a half dozen or more factors, therefore, may be a 

 limiting factor, and it is the limiting factor that determines 

 the success of a plant. It is like a group of men walking; 

 the group as a whole advances no more rapidly than the 

 slowest walker. In other words, the slowest walker is the 

 limiting factor. 



When plants are not doing well, therefore, the course to 



