DAMPING-OFF OF CONIFEROUS SEEDLINGS. 27 



necessary to treat beds some time before sowing, this period varying 

 from two days to two weeks in different soils. The generally pre- 

 scribed practice of covering the beds to prevent reinfection or pre- 

 mature evaporation of the formaldehyde during this period is 

 troublesome and expensive. Further tests may prove it unnecessary. 



In considering disinfectants, the ultimate as well as the immedi- 

 ate results should be taken into account. No data are at hand to 

 show the effects of repeated treatments of soil with toxic salts or 

 formaldehyde. The effects of single treatments seem to be purely 

 temporary. Treatments of one-eighth and three-sixteenths ounce 

 of copper sulphate per square foot (equivalent to 341 and 511 pounds 

 per acre, respectively), which are expected to be quite sufficient for 

 use against damping-off at most nurseries, involve additions of only 

 one-sixth and one-fourth, respectively, of the amount which has 

 been used in single treatments at Garden City without any notice- 

 able permanent effect. Sulphuric acid might ultimately bring about 

 an acid condition of the soil if used repeatedly, but this could be 

 easily remedied at any time by the addition of lime. The only 

 possible bad effect of continued use of the acid would therefore be 

 an accumulation of sulphates. The sulphur added in a treatment of 

 three-sixteenths fluid ounce of acid per square foot would be, roughly, 

 300 pounds per acre. If repeated every year, this would, of course, 

 mean a considerable change in sulphur content. However, it would 

 be a very rare thing for a treatment to be applied two consecutive 

 years on the same soil, and where rotation is practiced five or six 

 years commonly elapse between the growing of two crops of seed- 

 lings of susceptible species on the same soil. This minimizes the 

 likelihood of any bad cumulative effect by the treatments. 



The comparison of the four disinfectants considered in the fore- 

 going paragraphs does not make possible a final statement as to 

 their relative value. At most of the localities listed in Table III there 

 have not been enough tests to give conclusive results. It is believed 

 from the somewhat incomplete evidence secured that at most nur- 

 series soil treatment with sulphuric acid will be found a satisfactory 

 and probably the most satisfactory means of decreasing damping- 

 off and that where it is not satisfactory success can be secured with 

 some one of the other disinfectants — copper sulphate, zinc chlorid, 

 or formaldehyde. 



It is, of course, recognized that the treatments so far devised are 

 not as simple and effective as are desired. Further tests of these 

 disinfectants and of numerous others are under way. The problem 

 of damping-off control is also being attacked from other directions 

 than that of simple soil disinfection. It is hoped that a single disin- 

 fection method may be found which can be used on any soil and which 

 will not require any unusual precautions against chemical injury; or, 

 failing this, that some less direct procedure against the parasites 



