PHOSPHORUS FERTILIZERS 151 



It would not be wise to attempt to be too successful in dust- 

 ing, since the superphosphate needs to become wetted after 

 application to the soil. With a moderate amount of care, 

 a superphosphate " dusted " in this way will stand a con- 

 siderable amount of pressure without adhering to the drill 

 or other implement used for distribution on the soil. 



During the operation of removing from the den, a fair 

 amount of drying takes place. It is therefore desirable 

 that plenty of air should obtain access to the substance 

 while it is still hot. Where the work is carried out by hand 

 labour, and the meal is quickly and immediately riddled, 

 much drying automatically takes place. It is very important, 

 at this stage, that the superphosphate should be pressed 

 together as little as possible, otherwise it forms a sticky 

 mass, which cannot be reconverted to a fine dry condition. 

 In some works the workmen are provided with respirators 

 to prevent them breathing any poisonous gas, but where the 

 dens are properly constructed with good fans, and the front 

 of the den is placed in a situation where there is a good 

 natural draught, there is no necessity for this precaution. 

 The works should be so designed that the front of the den, 

 screening floor and cup elevator are all situated where there 

 is usually a draught, as by these means the danger of 

 asphyxiating fumes can be removed without any measurable 

 effort or expense. 



As a rule, it is possible to begin to extract the super- 

 phosphate from the den in 1-2 hours from the time 

 when it was filled, but it is not desirable to work at such a 

 pace, as the reaction is by no means completely over, and 

 removing superphosphate from the den before the reaction 

 is complete is bad practice if the material is chilled. The 

 reaction is therefore still further delayed, and the probability 

 of turning the superphosphate into a sticky mass is much 

 increased. Manufacturers, therefore, should have several 

 dens, so that they may not be compelled to empty them too 

 frequently. Forced by the large amount of labour needed 

 for extracting the superphosphate, and the expensive cha- 

 racter of the work, manufacturers have attempted to design 



