422 EXPERIMENT STATION RECORD. [Vol.43 



exponent indicated liy tlie liydrogen-electrode potentials were very considerable, 

 the soils becoming less intensely acid. Accompanying the development of reduc- 

 ing conditions was a production in most cases of a foul odor. Different soils v; 

 differed in the rate at which they became highly reducing. The addition of \ 

 dextrose to one soil favored the development of reducing conditions, the effect 

 of 0.1 per cent of dextrose being very pronounced. It is thought that sourness 

 of soils includes something beyond acidity, and that the residual unfavorable 

 quality may be a high intensity of reduction. A discussion is given of the 

 significance of reducing conditions and of reduction potentials in soil study. 



Forty references to literature bearing on the subject are included. 



Number of colonies for a satisfactory soil plate, H. A. Noyes and G. L. 

 Grounds (Proc. Ind. Acad. ScL, 1918, pp. 93-101, flys. 7). — Data fronj work 

 conducted at Purdue University are reported, the results of which, together 

 with results from other tests, are taken to indicate that 30 is near the optimum 

 number oL' colonies for a petri plate 100 mm. in diameter. The averages of a 

 sufficient number of plates carrying between 10 and 100 colonies are considered 

 to be satisfactory for computing bacterial numbers. 



The length of time to incubate petri plates, H. A. Noyes, E. Voigt, and 

 J. D. LucKETT (Proc. Ind. Acad. Sci., 191S, pp. 102-109).— The results of experi- 

 mental work conducted at Purdue University are reported, which are taken to 

 indicate that " counts made after 10 daj's' incubation at 20° C. of petri plates, 

 made from bacterial dilutions of soil, give reliable results as to the bacterial 

 content of the soil, providing the number of colonies present per plate is small 

 enough for all organisms to develop into colonies. The rapidity with which 

 bacteria develop into colonies has been shown to vary with the soil and to be 

 influenced by soil temperature, moisture, and aeration. Much of the lack of 

 confidence in results obtained by the plate method is due to having too many 

 colonies present per plate and not allowing suflicient time of incubation of the 

 petri plates." 



The etching of marble by roots in the presence and absence of bacteria, 

 E. B. Fred and A. R. C. Haas (Joiir. Gen. Physiol., 1 (1919), No. 6, pp. 631-638, 

 figs. 5; ahs. in Abs. Bact., 3 {1919), No. 6, p. 338). — Studies were made at the 

 Wisconsin Experiment Station of the effect of soil bacteria upon the etching 

 power of the roots of Canada field peas upon polished marble. These studies 

 showed that the presence of the bacteria from field soils in pure cultux-e in- 

 creased the etching power of the roots. 



The growth of higher plants in soils free of microorganisms, E. B. Fred 

 (Jour. Gen. Physiol., 1 (1919), No. 6, pp. 623-629, figs. 3; abs. in Abs. Bad., 

 3 (1919), No. 6, p. 338). — In this paper, a contribution from the Wisconsin 

 Experiment Station, the author calls attention to the advantages of growing 

 plants under sterile conditions when studying the action of soil bacteria, and 

 describes an improved apparatus for this purpose. 



The seeds are sterilized by mercuric chlorid in partial vacuum. The plants 

 are grown in a special pot fitted with a tall cylinder of pyrex glass, the upper 

 end of which is covered with a pyrex beaker. A neck at the side of the pot, 

 ordinarily plugged with cotton, provides a means of planting the seed and 

 watering. 



Bacteria in frozen soil, H. A. Noyes (Proc. Ind. Acad. Sci., 1918, pp. 110- 

 116, figs. 3). — The work of others bearing on the subject is reviewed and experi- 

 ments conducted at Purdue University are reported, the results of which are 

 taken to indicate that the number of bacteria present in soil is not increased 

 when the soil is frozen. 



Soil improvement for maize. — I, Manures and fertilizers, H. Wenholz 

 (Agr. Gaz. N. S. Wales, 31 (1920), Nos. 1, pp. 29-35; 2, pp. 111-116; 3, pp. 



