THE PHOSPHORUS GROUP OF FERTILIZERS 35 



and two kinds of nitrogen. For the early growth of practically 

 all crops a rich surface is necessary. When, however, the 

 plants have grown it is desirable that the fertilizing ingredients 

 should be deeper in the soil, to prevent an excessive develop- 

 ment of surface root, with the subsequent susceptibility 

 to drought. 



Guano. This old-established and favourite type of 

 manure is produced on rocky situations with little rainfall, 

 from the accumulations left by sea-birds during the nesting 

 season. Where the rainfall is very scanty the amount of 

 nitrogen in the guano may be as high as n per cent. Where 

 the rainfall is considerable the nitrogen may be removed by 

 washing till it falls to i per cent. In guano produced under 

 dry conditions the phosphoric acid is partially soluble in 

 water ; but in that produced in wet situations the constituents 

 are all insoluble. A small quantity of potash is often 

 present, say i per cent. The varieties of guano may be 

 divided into those whose value is chiefly due to the nitrogen 

 and those whose value is chiefly due to the phosphorus. 

 The phosphatic kinds will barely differ in their properties 

 from bone flour. Those of the nitrogenous kind will be of 

 a more complex character, containing both nitrogen and 

 phosphorus in various degrees of solubility. Some of the 

 less valuable guanos are treated with sulphuric acid to render 

 them more soluble. 



A great variety of artificial mixtures are put upon the 

 market to supply both nitrogen and phosphorus. As a 

 rule the basis of these is super-phosphate, to which has been 

 added some bone, an} 7 of the nitrogenous organic manures 

 described above, and not infrequently a miscellaneous 

 collection of materials of lower value. Some materials, in 

 themselves almost worthless, can be so treated as to bring 

 them into use for this group. For example, leather in itself 

 is of little manurial value, but it can be treated with sulphuric 

 acid and thereby dissolved. The acid is not lost in the 

 process, but is still capable of dissolving mineral phosphates. 

 Such a mixture will contain the leather in a digested form, 

 as well as soluble and insoluble phosphate. 



