564 INSECTS AFFECTING VEGETATION 



of three years will very greatly reduce the amount of disease, though 

 it may not be entirely exterminated after five, or even ten, years. 



The principal point to be kept in mind in choosing rotation 

 crops is to increase the amount of organic acids in the soil by plowing 

 under green crops, such as rye, clover, vetch, etc., which by their 

 decay tend to produce an acid reaction in the soil. It is possible that 

 the soil can be further favorably modified by the use of fertilizers, 

 such as acid phosphate, muriate of potash, land plaster, etc., which 

 leave an acid residue. It seems at first thought illogical to advise 

 plowing under green crops on soils largely made up of organic mat- 

 ter and to suggest fertilizers where there appears to be no lack of 

 plant food. No such positive recommendation is intended, but in the 

 absence of knowledge of the subject such experiments to test this 

 should be undertaken. The beneficial effect of such treatment is well 

 recognized in the Eastern states. 



The chemical treatment of the soil to reduce scab promises but 

 little. Applications of sulphur at the rate of 300 pounds per acre 

 have been used with some degree of success in Delaware and New 

 Jersey and might give good results in California. 



The most effective method of disinfecting seed potatoes is by 

 the use of formaldehyde gas liberated by mixing the commercial 

 solution with potassium permanganate. This method is adapted to 

 the disinfection of large quantities of potatoes in a short time. To 

 use it, an air-tight shed should be constructed of sufficient size to 

 hold whatever quantity it may be desired to treat at one time. This 

 may be made of rough lumber, lined with building paper and pro- 

 vided with a tight door. The potatoes may be treated in sacks, but 

 .these sacks must be piled so as to allow a free circulation of air all 

 around them. It is suggested that they be piled in tiers, with two 

 2 by 4 inch scantlings between each layer of sacks. Space should be 

 left in the center of the building for placing the charge of formalde- 

 hyde, which should be set off in shallow pans, such as galvanized 

 washtubs. For each 1,000 cubic feet 23 ounces of potassium per- 

 manganate and 3 pints of formaldehyde should be allowed. After 

 the potatoes are properly stacked and everything is made ready, the 

 permanganate should be spread in a thin layer on the bottom of the 

 pan, the required quantity of formaldehyde poured in and stirred 

 quickly, and the building vacated. The building should then be kept 

 closed tight for twenty-four hours, when it may be opened and the 

 potatoes taken out. 



Formaldehyde is a nonpoisonous but highly irritant fluid which 

 can be purchased in small lots at about 50 cents a pound, in carboy 

 lots at 20 cents, or in barrel lots at about 12 cents. Potassium per- 

 manganate is a reddish brown, crystalline substance, purchasable at 

 from 13 to 25 cents a pound. 



The precaution should be taken not to pile any potatoes directly 

 over the pans or within 3 feet laterally, as the gas there might be 

 strong enough to injure the potatoes and destroy their germinating 

 power. The formaldehyde works best in a humid atmosphere. It is 

 therefore advised that the floor of the shed be wet down before the 



