250 SCIENCE AND FRUIT GROWING 



instance, is as high as 150 C., the soluble inorganic matter is 

 increased threefold, and the organic matter twelvefold, it being 

 chiefly in the nitrogenous organic matter that the largest increase 

 occurs. These values vary very greatly, as might be expected, 

 with the nature of the soil, and the amount of water present 

 in it during the heating ; but, whilst the maximum effect is not 

 produced till about 200, it is still well in evidence when the 

 temperature is as low as 60, and, when the results are set out for 

 a series of samples heated to different temperatures, as shown 

 in Fig. 24, it is clear that, whatever* changes the heating has pro- 

 duced, these are continuous down to a point below even 25 C. ; 

 that is, they are continuous with changes occurring in soils which 



Soluble Organic 

 15 Matter in Soils aftei 

 heating. 



10 



50 100 150 



Temperature of Keating. 



FIG. 24. RELATION BETWEEN TEMPERATURE AND 

 ORGANIC MATTER. 



have not been heated artificially above atmospheric temperatures 

 (XIII, Appendix, p. 267). 



When such heated soils are kept for any length of time, 

 further changes of a somewhat complex nature occur in them, 

 for, as might naturally be expected, there are many substances 

 of different degrees of stability formed by the heating, and the 

 relative proportions of these depend on the temperature reached ; 

 but the general results are clear, namely, that if air is excluded, 

 or if the soils are kept dry, the heated soils undergo but little 

 change, whereas, if there is free access of air and moisture, the 

 proportion of soluble matter decreases gradually, the excess 

 present in the heated soil becoming, after about 100 days, 

 reduced to half its original amount. 



The behaviour of such soils towards plant life was first 

 examined by studying the germination of seeds in them, for 

 this presented the twofold advantage of its being a method 



