PLANT FOOD AND PLANT GROWTH 45 



Common functions. Some common functions may be performed 

 by several elements. Thus, if there is need for a base to correct an 

 excess of acid that has developed in the plant, sodium may serve 

 as well as potassium, although with enough potassium provided, 

 no sodium is needed. As already stated, the largest use of calcium 

 appears to be in this line, in which, perhaps, manganese, magne- 

 sium, or iron might serve equally well if they were present in the 

 plant in sufficient amount. Likewise, in solvent compounds, chlorin 

 may serve as well as nitrogen or phosphorus, but cannot take their 

 place in living tissue. 



We shall also consider in the following pages the value to plants 

 of certain materials when applied to certain soils, which serve not 

 as plant food, but rather as soil stimulants, having power to liberate 

 from the soil some essential plant-food element more rapidly than it 

 would otherwise become available an action that may result in 

 temporary profit and ultimate land ruin. Caustic lime, salt, 

 gypsum (land-plaster), and, under certain conditions, commercial 

 fertilizers, and even farm manure, clover, and green manures, may 

 act in part, at least, as soil stimulants; and, to guard against such 

 injurious action, practice must be controlled by science (knowl- 

 edge). 



