258 SCIENCE AND FRUIT GROWING 



a temperature of 57, and the cysts at 70, so that if their exist- 

 ence were the only, or the main consideration, soils heated to 

 70 should prove more fertile than any others, and heating beyond 

 that point should cause a reduction in fertility, till, with a tem- 

 perature of 125, a minimum would be reached, all the bacteria 

 having then been killed, and growth depending on chance re- 



300 



50" .00- 150 



Temperature of Heating. 



FIG. 28. GROWTH OF PLANTS IN SOIL HEATED 

 TO DIFFERENT TEMPERATURES. 



inoculation ; but from the Wo burn results, which are shown 

 diagrammatically in Fig. 28, it appears that the maximum effect 

 occurs in the case of non-grasses with soil heated to 100, not to 

 70, and in the case of grasses with soil heated to 125 or 150 

 (see p. 254), according as the roots are excluded or included. 



Such a fact argues strongly in favour of chemical change being 

 the principal factor in the case. 1 



1 Cf. " The Chemistry of Steam-heated Soil," U.S. Dept. of Agric. Bureau 

 of Soils, Bull. 89, 1912, by Schreiner and Lathrop. This paper also contains 

 many references to earlier work. 



