40 MISSOURI BOTANICAL GARDEN BULLETIN 



this will give a test of the fineness of the particles of which the 

 paste is made up. 



The most important point in buying arsenate of lead in any 

 form is to know its strength in terms of arsenic oxide. As 

 the poisonous properties of the substance are due to the 

 arsenic present and not to the lead, its efficiency from this 

 point of view depends upon the arsenic content. Most re- 

 liable firms will guarantee the arsenic strength of the particu- 

 lar brand sold by them. Fifteen per cent of arsenic is an 

 average strength of the paste, and this content is assumed in 

 directions given for mixing the spray. 



Another element entering into the efficiency of any spray 

 used is the fineness of the jet as it is applied and the amount 

 put on each tree. Where a very fine spray jet is used, it is 

 sometimes desirable to double the strength of the arsenate of 

 lead and use but half the normal amount on each tree. Too 

 much stress cannot be laid upon the importance of using a 

 nozzle which will give the finest possible spray. The idea is 

 to envelop the tree in a fog or mist. If the droplets are too 

 large the liquid drips off the leaves, carrying the arsenate of 

 lead with it, and the efficiency of the operation is greatly re- 

 duced, besides causing an undue loss of material. It is im- 

 possible to give more than approximate figures as to the 

 proper amount and strength of spray, since so much depends 

 upon the character of the spray, the method of application, as 

 well as the size of the tree and the state of its foliage. The 

 following table, however, will furnish a fair idea of what is 

 necessary to bringing about satisfactory results. 







Weight of arsenate 



Spread of tree 



Amount of spray 



of lead paste for 





for 100 trees 



100 trees 



7 feet 



200 gallons 



8 pounds 



10 feet 



270 gallons 



11 pounds 



15 feet 



460 gallons 



I.S. 1 ; pounds 



20 feet 



720 gallons 



29 pounds 



25 feet 



1120 gallons 



45 pounds 



30 feet 



1500 gallons 



60 pounds 



25 feet 



2200 gallons 



88 pounds 



40 feet 



2500 gallons 



100 pounds 



For a smaller number of trees the approximate amount of 

 spray and the number of pounds of arsenate of lead needed 

 can be easily figured by taking the fraction obtained by divid- 

 ing 100 by the number of trees to be sprayed. 



Time of Spraying. — Owing to the great variation in the 

 time at which trees leaf in different seasons, it is impossible to 



