HORTICULTURAL RESEARCH 



503 



125° or above, in which all of them would have been killed, the 

 soil should be much less fertile than even unheated soils or any 

 increase in fertility which it exhibited would be of a very 

 irregular character, depending on chance reinoculation with 

 bacteria. 



The results of growing plants in soils heated to different 

 temperatures do not tally with these requirements. Those 

 already alluded to are set out in fig. 6, the curve ab representing 

 those with tobacco, tomatoes and spinach, the curve ac repre- 

 senting those with three grasses. This latter has been some- 

 what smoothed, as the values were not very regular. Neither 

 of these curves shows a maximum at 50 : ab does show a 



o 

 an 



o 



300 



100 



maximum but this occurs at ioo° and ac shows no maximum 

 at all. Moreover, neither curve shows any marked irregularity 

 from 125 to 150 or any tendency to give lower values than that 

 for the unheated soil : the results, in fact, seem to show that the 

 circumstances conditioning them are continuous from the lowest 

 to the highest temperature. 



On the other hand these results are quite in harmony with 

 the chemical explanation given above of the effect of heating 

 soil — the formation of a toxic substance which becomes 

 oxidised to form a plant-food, different plants being sensitive 

 in different degrees to the toxic action. It is question- 

 able, however, whether the actual quantity of plant-food thus 

 liberated by heating to the lower temperatures, up to, say, ioo°, 

 is sufficient to explain the extra vigour of plants grown in such 

 soil ; in such cases, no doubt, the bacterial explanation of 

 increased fertility becomes important. Both explanations are 

 probably correct but neither alone affords a full explanation 

 of the facts. 



