52 THE SOILS AND CROPS OF THE FARM. 



A complete manure is one which contains all the 

 elements of plant food. Such a manure will increase 

 the growth of almost any crop if applied to almost 

 any kind of soil. It may increase the growth of the 

 stalk and leaves at the expense of the seed. Stable 

 or barn-yard manure is the typical, complete or gen- 

 eral manure. 



A special manure is one which supplies some but 

 not all the elements needed by plants, or which is 

 specially prepared for certain classes of crops or of 

 soils. This class is known as artificial manures, 

 chemical manures or commercial fertilizers. They may 

 be by-products of manufacturing establishments ; they 

 may be specially compounded, or they may be ob- 

 tained from the great stores of possible plant food na- 

 ture has stored up, as in beds of marl, of gypsum, or 

 of phosphatic rocks. 



Stable manure ordinarily acts in all three of the 

 ways named. Artificial manures usually are help- 

 ful by directly adding plant food, or by aiding chemi- 

 cal action in the soil, thus hastening the preparation 

 of a sufficient supply of available plant food. 



It has already been stated that the only mineral 

 elements of plant food probably in insufficient sup- 

 ply in the soil are phosphorus and potassium; and 

 that nitrogen, in a form in which plants can make 

 use of it, is the only gaseous element of which plants 

 find it difficult to secure a sufficient supply. To sup- 

 ply some one, or two, or all three of these scarce or 

 "precious" — because scarce — elements, is the purpose 

 of most artificial manures. If prepared to supply one 

 or both the mineral elements, they may be called by 

 their names; if to supply all the mineral elements 



