SOIL TREATMENTS AND SEED GERMINATION 35 



typical instance were: check, 6.4; 6 gm., 6.2; 9 gm., 5.7; 12 gm., 5.5; and 16 gm., 

 5.0. 



The severity of damping-off, most of it caused by Pythium, some by Rhizoc- 

 tonia, was lessened by 9 gm., but infection was not satisfactorily controlled by 

 less than 12 or, more often, 15 gm. 



These and even smaller quantities were injurious to most species. Growth 

 of beet and cucumber was retarded by 6 gm., that of English daisy by 9 gm. 

 Twelve grams interfered with the germination of Delphinium, Salpiglossis, Viola 

 cornuta, and Canterbury bells, but not of Dianthus, the genus least injured 

 by this acid. Growth of Dianthus barbatus, D latifolius, D. plumarius and D. 

 alpinus was a little injured at first by 14 gm., but most or all of this injury was 

 later outgrown. 



Sulfuric acid 10 gm. improved germination of seeds of Rhododendron calendula- 

 ceum, R. carolinianum, R. calawbiense and R. Schlippenbachii in a sandy, peaty 

 soil (initial pH value 5.8), without injury to growth. The use of sulfuric acid, 

 in this way, with ericaceous plants, is now being investigated further 



Of the other plants used, only the species of Dianthus were sufficiently tolerant 

 of sulfuric acid to suggest that it may possibly be used with them. 



Sulfur 



As is well known, the addition of sulfur to soil may, by increasing soil acidity, 

 have an inhibiting effect on certain soil fungi. There is no reason to suppose that 

 damping-off would be controlled in this way, however; for, as has already been 

 noted, the disease ma\' be severe even in an acid soil, and Buddin (16) concluded 

 from his work that sulfur does not sterilize soil. Townsend (89), too, found sulfur 

 ineffective in protecting (lettuce) plants against a disease caused by Rhizoctonia. 

 It is nevertheless occasionally mentioned in horticultural literature as a soil 

 treatment for the prevention of damping-off and some experimental use was 

 accordingly made of it. 



Flowers of sulfur, up to 20 gm. per square foot or more than 1900 pounds per 

 acre, was well mixed with soil immediately before seeding. Soil pH values, 5 

 weeks later, were lowered from 6.3 to 5.8 by 8 gm. and to 5.0 by 20 gm., but 

 damping-off, caused by Pythium, was no less severe with 6 to 20 gm. than it 

 was in untreated soil. 



Six grams (or more) of sulfur interfered with the growth of beet and cucumber 

 seedlings, but foxglove was uninjured by 8 gm., and sweet pea, for the first ten 

 weeks, by 10 gm. 



SUMMARY 



Damping-off is not altogether controllable in practice by regulating and ad- 

 justing the environment (soil temperature, moisture, and reaction), for condi- 

 tions preferred by the fungi are too nearly the same as those preferred by the 

 plants. For example, the watering of small seeds from below is advisable for other 

 reasons, but damping-off is not always prevented by it. 



In washed sand or in sand-sphagnum or in sand-peat moss, there was very 

 little damping-off as compared with that in soil. However, growth of all species 

 was poorer in sand, even with nutrient, than in soil and usually better in sand- 

 sphagnum or in sand-peat moss (with nutrient) than in sand. 



