DIPPING AND FUMIGATION OF NURSERY STOCK 11 



The trees are placed in the box or house. The water and sulphuric acid 

 are mixed in an earthen jar and the jar placed in the box or house. The 

 potassium cyanide is then dropped in and the box or house closed just as 

 quickly as possible. The hydrocyanic-acid gas which is generated is deadly 

 poisonous and the person doing the fumigating must be very careful not 

 to breathe any of it. It requires about 45 minutes to fumigate nursery stock, 

 altho some authorities say that better results can be obtained by letting 

 the stock remain an hour. At the end of this time the fumigating box or 

 house is opened and the gas allowed to escape and in from 15 to 20 minutes 

 the trees can be safely removed. 



It is never advisible to fumigate trees while they are damp or wet. It is 

 claimed that under such conditions the gas is more likely to injure the stock. 

 However, the writer's experiments to date fail to corroborate this, though 

 they do show that less scale is killed under those conditions. 



Some states require by law that all nursery stock grown within its 

 borders or shipped in from outside nurseries be fumigated and,, as a result, 

 all of the larger nurseries in the United States have constructed special 

 fumigating houses or boxes. 



Chemical composition of hydrocyanic-acid gas. In fumigating work 

 hydrocyanic-acid gas is generated, as has already been explained, by placing 

 together potassium cyanide (KCN) sulphuric acid (H 2 SO 4 ) and water 

 (H 2 0). 



The sulphuric acid, which is sold commercially, has a strength known 

 as 66 Baume which corresponds to the 96 per cent pure sulphuric acid. 

 Commercial sulphuric acid, however, contains some impurities and is seldom 

 more than 93 or 94 per cent pure. 



The potassium cyanide which is purchased on the market runs about 

 98 per cent pure. 



When the sulphuric acid and the potassium cyanide are brought to- 

 gether, the chemical reaction that takes place is as follows: 

 2 KCN + H.SCX = K 2 SO 4 + 2 HCN 



In the above reaction, 1 ounce (avoirdupois) of potassium cyanide (100 

 per cent pure) requires 0.75 ounce (avoirdupois) sulphuric acid or .81 ounce 

 of commercial sulphuric acid containing 93 per cent sulphuric acid which 

 would be equal to 0.42 fluid ounces. 



Under conditions met with in fumigating work, the above reaction can- 

 not be obtained and result in the best yield of hydrocyanic-acid gas. More 

 sulphuric acid must be used and this causes acid potassium sulphate to be 

 formed as is shown in the following equation: 



KCN + H 2 S0 4 = KHSO. + HCN 



In this reaction 0.84 fluid ounce of 93 per cent sulphuric acid is required 

 for each ounce (avoidupois) of potassium cyanide. This amount in round 

 numbers equals 1 part cyanide to 1 part acid which gives the best results 

 in field work. In order to get the best yield of hydrocyanic-acid gas only 

 two parts of water should be used, but in field practice when only two parts 

 of water are used, the residue in the generating jar often solidifies and in 

 order to prevent this, three parts of water are used. Thus the 1-1-3 formula 

 is used in fumigating nursery stock. 



