182 SOILS AND MANUEES 



manure, and artificial manures are supplied to the differ- 

 ent crops to increase production. 



Adaptation of Manures to Circumstances. Artificial 

 manures may contain all the fertilising ingredients 

 nitrogen, phosphoric acid and potash or only one or 

 two. They are commonly referred to as nitrogenous, 

 phosphatic or potash manures according to the nature 

 of the predominant constituent. There are several kinds 

 of each, differing more or less in their properties, and 

 they must be used with discretion in order to obtain 

 satisfactory results. Comparisons are sometimes drawn 

 between the effects of artificial manures and farmyard 

 manure, but no just comparison can be made except 

 when one is used as a substitute for the other. If it be 

 proposed to employ artificial manure as a substitute 

 for farmyard manure, i.e., for purposes of restitution, 

 all three constituents must be supplied. They may be 

 derived from one or several sources, and may be applied 

 on one or several occasions, but the total quantity of 

 each should be equivalent to the quantity contained in 

 the dressing of farmyard manure to which the artificial 

 manure corresponds. Even if chemically equivalent, 

 the artificial manure would not, of course, produce the 

 physical and biological effects that farmyard manure 

 does. When artificial manures are employed as addi- 

 tive substances, i.e., to increase production beyond the 

 natural capacity of the soil, they must be properly 

 adapted to the requirements of the soil and of the crop 

 to which they are applied. 



Requirements of the Soil. In considering the require- 

 ments of the soil, both the physical properties and the 

 chemical composition should be taken into account. 

 Manures in which the plant foods only become available 

 as a result of oxidation, act better on light, open soils 

 than on those of closer texture ; those which are very 



