4 MASS. EXPERIMENT STATION BULLETIN 351 



others. The measure of growth was green weight of seedlings. The most common 

 evidence of stimulation observed was when Rhizoctonia interfered with growth 

 of seedlings in untreated soil, without killing them. Clayton (20) also noticed 

 that plants grew more rapidly in soil which had been freed of Rhizoctonia by 

 formaldehyde, and the writer is of the opinion that much of the so-called stim- 

 ulatory effect of formaldehyde and other soil disinfectants is thus explainable. 

 This would be less true, however, when only Pythium was involved, for it is more 

 likely to kill quickly than merely to retard the growth of seedlings. 



Seedlings were usually weighed at that age and stage of development at which 

 they are ordinarily transplanted for the first time. What the effects might have 

 been had plants been allowed to grow until flowering, was not determined. 



"Damping-off" means post-emergence damping-off unless otherwise indicated, 

 in which case the reference is to the killing of seeds or of seedlings before they 

 had emerged from the soil, and this is more commonly discussed in connection 

 with the effect of treatment on germination. Such effects, provided of course that 

 germination is uninjured by the treatment, are less important, however, than are 

 numbers which germinate and live; in other words, final stands. Final stands 

 are a measure of protection against both pre-emergence and post-emergence 

 damping-off, for approximately the same number of seeds of a given species was 

 used with each treatment in any one experiment. 



MEDIA FOR SEEDAGE AND THEIR RELATION TO DAMPING-OFF 



As has been well demonstrated by Dunlap (31), there is little or no damping-off 

 of seedlings in washed sand. To provide the necessary plant food, he added to 

 the sand, before seeding, about 2.8 gm. potassium nitrate in 142 cc. water per 

 square foot (1 ounce in 3 pints water for 10 square feet). He found that some 

 species grew better in a mixture of sand and peat moss than in sand. 



Some use was made of Dunlap's method, comparing soil, washed sand, sand 

 and sphagnum, and sand and peat moss (about half and half by volume in both 

 cases). Five different sands were used. The media were 3 to 4 inches deep in 

 flats with narrow seams in the bottom. Sands always received potassium nitrate, 

 before seeding, at the rate of 2.8 or 3.0 gm. in 140 or 150 cc. water per square 

 foot. 



It was soon evident that sand-sphagnum and sand-peat moss need the nutrient 

 about as much as does sand, for the addition of potassium nitrate increased the 

 weights of plants as follows; 



Percentage Increase due to Fertilizer 



In Sand-peat In Sand-sphagnum 



Calendula 14 11 



Cress 55 50 



Beet 59 38 



In all later experiments, therefore, potassium nitrate was applied to these media 

 as well as to sand. Soil, of course, received no nutrient. 



Results are summarized in Tables 1 and 2, both of which include the means 

 of several experiments. There was little or no damping-off in sand. There was a 

 little more damping-off in sand-sphagnum and in sand-peat moss than in sand, 

 but much less than in soil. 



Damping-off was as completely prevented and final stands were as much 

 improved by the use of washed sand as a medium for seedage as by the disinfec- 

 tion of soil with formaldehyde. 



