644 CONTRIBUTIONS TO OUR KNOWLEDGE OF SOIL-FERTILITY, xiii., 



But after making allowance for apparent discrepancies, it appears 

 that, with the smallest amount of dextrose, viz., 0005%, the pass- 

 ing of air over the soil makes it at first more toxic, then more 

 nutritive. This is presumably caused by the decomposition or 

 decay of toxin rapidly formed from the small quantity of sugar. 

 With 0*01% of dextrose, aeration makes the soil toxic in about 

 five days, with an air-temperature running about 25°. Simple 

 storage of the soil containing the smaller amount of dextrose, 

 results in the formation of toxin in five days, while with the 

 larger quantity, a toxic effect does not become apparent in that 

 time. On the whole, aeration appears to hasten the formation 

 and destruction of toxins formed from the sugar. 



These experiments with dextrose and soil show that the 

 presence of small quantities of sugar undoubtedly increases the 

 toxicity of the soil, when a sufficient time is given for the sugar 

 to become decomposed. The determination of the time will 

 naturally depend upon the conditions which govern microbic 

 fermentation, and include the quantity of sugar, the aeration, 

 the temperature, etc. 



The preceding experiments were made with the test-organism 

 Bac. prodigiosus, which can give direct evidence of toxicity in 

 soil-extracts. The soil-bacteria have not been found to do this; 

 they show the toxicity indirectly by a diminished growth under 

 the conditions of the experiments, as has been shown in previous 

 papers. A diminished development of bacteria is also seen in 

 the following experiment in which a rather large quantity of 

 dextrose, viz., 0-1%, was added to the soil. This contained 8*3% 

 of moisture, and was stored at laboratory-temperature. The 

 bacteria used for infecting the extracts and water-controls were 

 obtained by shaking some of the same soil with sterile water, 

 and using the suspension. As in other experiments, the growth 

 was determined by counting the bacteria after an incubation at 

 22° for 20 hours. (See Table on next page). 



A diminution of the bacterial growth is evident, but, unfor- 

 tunately, the gradual loss of nutritive effect, in the al)sence of a 

 direct evidence of toxic action, may be capable of two explana- 

 tions. The gradually diminished growth may be caused by the 



