482 Journal of Agricultural Research 



Vol. V. No. II 



found in the living condition, while in soils of very open structure where 

 little or no surplus water is available they would seldom, if ever, be- 

 come active. This point requires further investigation. 



The question at once arises, How are protozoan cysts transported to 

 the different soils? This process is likely to be brought about by wind 

 action, by flowing water, and by mechanical means in the case of culti- 

 vated soils. Likewise, if the protozoa do not exist in the active state 

 in the soil, can they and do they multiply ? Under certain abnormal con- 

 ditions of moisture they will become active and remain active as long 

 as there are sufficient moisture and food and the absence of toxic or 

 decomposition products. During this period multiplication takes place. 

 When the conditions become unfavorable, no doubt some die, while the 

 greater number encyst until conditions again become favorable for them 

 to become active. 



EFFECT OF MOISTURE ON THE ACTIVITY OF PROTOZOA IN THE SOIL 

 UNDER CONSTANT AND VARIABLE TEMPERATURES 



Large samples of three soils which had previously been used by the 

 wnter (i) in his study of protozoa were collected. The first was a 20 

 per cent manure shale, greenhouse soil, the second, a clay loam orchard 

 soil which had received no applications of manure for the last 20 years; 

 and the third, a sandy loam field-plot soil that for a period of 20 years 

 had been receiving annual applications of manure at the rate of 20 tons 

 per acre. (Hereafter throughout this study the first soil will be desig- 

 nated as the "greenhouse soil," the second as the "orchard soil," and 

 the third as the "field soil.") The soils were air-dried at laboratory 

 temperature and then sieved through a 20-mesh sieve. The optimum 

 moisture content of these soils was determined. Twenty 50-gm. portions 

 of each soil were weighed into 4-ounce bottles. With each soil one series 

 of five samples was left air-dried. To one series sufficient sterile tap 

 water was added to make the moisture content half of the optimum. 

 To another series enough water was added to increase the water content 

 to the optimum. To a fourth series sterile tap water was added so that 

 the resulting mixture would be equivalent to one and a half of the opti- 

 mum. At one and one-half of the optimum the soils could take up all 

 the moisture without any free water being present. The soils were well 

 mixed with a stirring rod, so that the moisture content was homogeneous 

 throughout. In order to prevent condensation on the sides of the 

 bottles, they were left unplugged. The flasks containing four samples 

 of each soil, representing four moisture contents, were incubated at 5° 

 to 7° C, one series at 15° to 17°, one at 22° to 24°, one at 32° to 33°, 

 and one at the outdoor temperature. The samples were weighed daily, 

 and the slight amount of moisture lost by evaporation was replaced. 

 Each sample of soil was then examined for active protozoa not fewer 



