94 BOTANICAL GAZETTE [january 



seem to have a marked effect upon water loss. With the latter phenomenon 

 is associated an abundance of pulvini. 



The report is to be commended as an attempt to apply quantitative 

 methods in an almost untouched field. — 'Geo. D. Fuller. 



Heated soils. — Johnson 10 has done a very critical and exhaustive piece of 

 work on the effect of heating soils at various temperatures on the germination 

 of seeds and later growth of plants in such soils. The heating at 114-116 C. 

 was done in an autoclave ; at higher temperatures the heating was done with 

 air-dry soils in dry ovens. The duration of heating was about 2 hours. 



Soils heated at 100-115 C. gave temporary retardation of germination and 



seedling growth, followed later by a great increase in rate of growth. The 

 extent of these varied greatly with the soil, seed, and plants used, and with 

 other environmental conditions. The injury increased as the temperature 

 rose up to 250 C. As the temperature rose above 250 C. the injury decreased 

 until it was nil with heating at 350 C. or above. The time of recovery from 



the toxic effects was proportional to the intensity of the toxicity. Soils 

 showed considerable variation in the degree of effect of heating. This variation 

 cannot be explained on the basis of any one characteristic of the soil, but seems 

 to result from a combination of a number of its characters. 



Seeds varied in their sensitiveness. Lettuce and clover are very sensitive, 

 and wheat, buckwheat, and flax are resistant. Gramineae and Cucurbitaceae 

 are usually resistant, while Leguminosae and Solanaceae are more sensitive. 

 There is great variation in the response of the growing plants. Heated soils 

 that proved very injurious to some plants, as tomatoes, may be beneficial to 

 others, as wheat. In general, but not always, there is a parallel between the 

 sensitiveness of germination and of the later growth of the seedling. Pyronema, 

 some other fungi, and some bacteria grow best in soils heated to 250 C, and 

 fall off in growth rate with soils heated to higher or lower temperatures. 



The ammonia content of soils is highest in those heated at 250 C, and 

 diminishes as the temperature of heating rises or falls. The same is true of 

 the concentration of the soil solution, so that there is a rough parallel between 

 these characters of the soil and the degree of toxicity or later increased growth. 

 Adsorptive capacity of the soil modifies the action of the toxic substance. In 

 soil extracts the toxicity is more nearly correlated with the concentration of 

 the ammonia. Additions of ammonia to soil produce effects similar to heating. 

 The author believes the toxic action of heated soils is largely due to ammonia 

 existing as ammonium carbonate. He thinks other factors are involved in 

 so-called "chemical" injuries. 



The toxic material in heated soils is volatile. It is also changed into 

 non-toxic form when the soil is kept under conditions favoring growth of 

 organisms. The latter is due to soil flora, and, contrary to Pickering, does 



10 



Soil Science 7:1-87. 1919. 



germination 



