SOILS AND FERTILIZING MATERIALS 31 



either contain an excess of one or other of these, 

 or are better able to obtain some one or other of 

 them from the soil, than are other crops, it is 

 frequently economical to apply a special manure 

 to meet the needs of such crops. For what is 

 known as to the requirements of individual crops 

 a work on agricultural chemistry must be con- 

 sulted. It thus happens that special manures 

 are divided into phosphatic, nitrogenous, and 

 potash manures. It is necessary, in order for a 

 manure to be efficient, that it shall not only con- 

 tain the requisite constituent or constituents of 

 plant food, but that the nutriment shall be in an 

 assimilable form, and it has been ascertained that 

 in whatever condition the plant food may be 

 actually absorbed, the absorption occurs the more 

 rapidly the more soluble the food constituents of 

 the fertilizer. 



There is no way to tell, without experiment, 

 what food constituents a soil lacks. The crops 

 themselves give valuable suggestions. As a rule 

 lack of nitrogen is indicated when plants are pale 

 green in color, or when there is small growth of 

 leaf or stalk, other conditions being favorable. A 

 bright, deep green color, with a vigorous growth of 

 leaf or stalk, is, in case of most crops, a sign that 

 nitrogen is not lacking, but does not necessarily 

 indicate that more nitrogen could not be used to 

 advantage. An excessive growth of leaf or stalk, 

 accompanied by an imperfect bud, flower, and fruit 

 development, indicates too much nitrogen for the 



