io8 DISEASES OF CROP-PLANTS 



disinfection of the soil. For this there are two kinds of treat- 

 ment available : by heat and by chemicals. Primitive methods 

 sometimes used are the roasting of the surface soil by covering 

 the bed with trash and brushwood, which is fired ; or, alternatively, 

 the heating of the soil in some kind of pan. The more modern 

 way is to use live steam for the purpose. A large shallow box 

 of sheet iron (about lo inches deep) with one or more perforated 

 pipes in its base, is inverted over the bed and the edges pressed 

 into the soil. The pipes connect with the boiler of a portable 

 engine, and steam at 80-100 lb. pressure is forced into the 

 soil. About 30 minutes is the average exposure given. 



Alternatively a stationary boiler may be used and the soil 

 brought to it in a covered box which has perforated pipes ar- 

 ranged for the passage of the steam. 



The formalin drench method is carried out with a one in 50 

 solution of commercial formalin, applied to the prepared bed 

 slowly enough to sink in, and at the rate of two quarts to the 

 square foot. The soil should be covered to retain the fumes 

 for a day or two, and then left open several days before planting. 



The steaming method is the better of the two, but both are 

 too expensive to use with any but a valuable and delicate crop. 



A cheaper method, using sulphuric acid, is described in papers 

 by C. Hartley and R. G. Pierce. The experiments of these 

 authors relate to the seedlings of coniferous trees, and its suit- 

 ability for any other type of plant should be determined by 

 preliminary trials. For the requirements of nurserymen the 



treatment consists in the application, in standard soils, of three- 

 sixteenths of a fluid ounce of commercial sulphuric acid to each 

 square foot of seed-bed, applied in solution in water immediately 

 after the seed is sown and covered. This has proved more 

 reliable than the more expensive methods mentioned above. 



The amount of water used to carry the disinfectant does not 

 appear to be a matter of importance, provided that the necessary 

 amount of the acid is applied to each unit of area. The quantity 

 used by the authors varies from i pint per square foot when the 

 soil is wet to 2 pints when the soil is dr3\ There is a possibility, 

 especially in light soils, of a concentration of the acid by evapora- 

 tion to a strength injurious to the root tips, which in practice has 

 been found to be completely avoidable by watering the beds 

 frequently during the period of germination. When the root- 

 tips have penetrated to a depth of half an inch this is no longer 

 necessary. 



There are differences in the amount of acid required for 

 successful results in different soils. In sandy soil which was 

 probably somewhat alkaline, a heavier application, one-fourth 

 to three-eighths of an ounce, was indicated. In a fine sandy soil 

 which was probably already acid, chemical injury to seedlings was 

 more difficult to avoid and reduction of the acid to one-eighth of 

 an ounce was advisable. On heavier soils the use of five-eighths 



