WHAT FERTILIZERS DO FOR PLANTS 147 



nitrogen is supposed to stimlate leaf and stalk development; phos- 

 phorus is presumed to increase seed production and to hasten 

 maturity, and potassium is presumed to have a vital connection with 

 early maturation of a crop, and with sweetness and quality in fruit. 

 Many other such statements have been made in the literature with 

 respect to the specific effects of these three chemical elements in 

 plant growth. Moreover, some authors have gone so far as to say 

 that certain colors, textures, or other characteristics of plants in 

 growth indicate a lack of sufficiency of a given one of the three 

 elements in question. 



There is little or no basis for all such statements and assump- 

 tions, because it is a fact that color of foliage may in most cases be a 

 guide to the determination of whether or not an ample or a deficient 

 supply of available nitrogen is present in the soil or other growing 

 medium. Thus, a deep green foilage is usually an indication of a 

 plentiful supply of available nitrogen. Yellow, or light green foliage 

 is frequently an evidence of a defficiency of available nitrogen. But 

 even those indications are not always correct and particularly is this 

 true of the yellow color of leaves. Likewise, an excess of nitrogen 

 does seem to encourage rank stalk and leaf development, resulting 

 with the cereals even in "lodging," but this too might be taken as 

 evidence of a badly unbalanced condition of the nutrient medium of 

 growth (the soil solution) rather than a specific effect of nitrogen. 

 It is, nevertheless, true that the encouragement of heavy stalk and 

 leaf production by nitrogen is very marked with some plants and 

 comes nearest being a manifestation of a specific growth-producing 

 effect of an element which we know about. As for the idea of the 

 specific effects mentioned above as being characteristic of phos- 

 phorus and potassium and others found in the literature which I 

 have not mentioned, there is aboslutely no unexceptionable evidence 

 to support it. We have no data upon which to base the common 

 belief that phosphorus hastens maturity of plants; or that a lack 

 thereof manifests itself in some abnormal appearance of the plant 

 which has been definitely recognized and correlated with it. The 

 same statements will apply to the potassium question, and we thus 

 find no justification for the idea that there has been established a 

 certain well-defined correlation between the appearances, colors, or 

 other qualities of plants and a deficiency of any one of the essential 

 elements to plant growth, unless, possibly, it be nitrogen. This does 

 not mean, however, that each of the mineral elements does not per- 

 form a specific function in the metabolism of the plant, for there is 

 some good evidence that it may. It does not even mean that every 

 one of the elements may not specifically affect the plant by its 

 sufficiency or by its inadequacy in such a manner as to be indicated 

 characteristically in the appearance or quality of the plant which 

 human senses can discern. It does mean that if such specific effect 

 or effects for any of the mineral elements except nitrogen exist, we 

 know nothing about them to date. 



The denials of the common beliefs and of the current teachings 

 in our text-books, which are made above, are not intended, however, 

 as disclaimers of the existence of other visible effects on plants which 



