i;>-'0] SOILS FERTILIZERS. 421 



contour farming. Tho practice of leaving permanent strips of sod inliillside 

 valleys is also strongly recommended. When hillside valleys are filled by 

 plowing in, the use of obstructions made of short stakes and straw set in 

 a crescent curved downstream is advisable. 



Part II deals with the checking of overfalls. It is stated that practically 

 all ditches in Iowa are caused by overfalls working back. "A successful 

 method of preventing the overfall from working back is to ease the water 

 from the higher to tlie lower level by putting in an obstruction of straw and 

 brush staked down." The obstruction should come up almost as high as 

 tlie top of the overfall. These obstructions are built by first setting one or 

 more posts firmly in the ground in the bottom of the gulley and within 2 ft. 

 of the place where the water falls over. The number of posts to use varies 

 with the size of the overfall, the best results being secured where they were 

 put from 2i to 3 ft. apart around the bottom of the fall. The straw is packed 

 around the posts and tight against the raw surface of the earth, being held 

 in place by brush interwined between the posts and well trampled down. The 

 brush is Ji(>ld down by crosspieces nailed to the posts. 



The lime requirement of soils according to the Veitch method, com- 

 pared with the hydrogen-ion concentration of the soil extract, A. W. 

 Blaik and A. L. Pkince (Soil ScL, 9 (1920), No. J,, pp. 253-259, fujs. 2).— Experi- 

 ments conducted at the New Jersey Experiment Stations are reported, in which 

 lime-requirement data secured by the Veitch method were compared with those 

 indicated by the hydrogen-ion concentration of water extracts from loam soil. 



For the samples tested there appeared to be a fairly close correlation between 

 the hydrogen-ion concentration of the soil extract and the lime requirement as 

 determined by the Veitch method. Certain inconsistencies appeared, which may 

 be due to the inaccuracies of the Veitch method or to the lack of uniformity 

 in the samples, or to the influence of buffer substances. 



" With normal soils a determination of the hydrogen-ion concentration of the 

 soil solution may give one some idea of the amount of lime water required by 

 the Veitch method, and thus considerably shorten this method. Of the soils under 

 consideration, those which have a hydrogen-ion concentration of about pH=6.7 

 or over are alkaline by the Veitch method. With further studies along this line 

 It may be rxjssible, with normal soils, to assign a fairly definite lime require- 

 ment to a given hydrogen-ion concentration, so that in many cases at least a 

 determination of the hydrogen-ion concentration would make a lime-requirement 

 determination unnecessary." 



Reduction potentials of bacterial cultures and of water-logged soils, 

 L. J. GiLLKSPiE (Soil Sci., 9 (1920), No. 4, pp. 199-216, figs. 4).— In a contribu- 

 tion from t^e U. S. Department of Agriculture oxidation and reduction potentials 

 are discussed as the intensity factor of oxidation and reduction. 



Measurements of the reduction potentials of bacterial cultures or suspensions 

 showed, when suitable precautions were taken, constant potentials for the 

 facultative anaerobe Bacillus coli and for mixed cultures of soil microorganisms 

 grown in a deep layer, the values for the reduction potentials being near the 

 hydrogen-electrode potentials. Mea.surements with aerobes (B. subtilis, B. 

 mycoitles, and a third aerobe) showed progressively increasing reduction po- 

 tentials with laj)se of time, but in no case did the reduction potential approach 

 the hydrogen-electrode potential as closely as 0.3 volt. It is thought that this 

 difference between anaerobes and aerobes may hold in general, but the evidence 

 is not conclusive. 



When soils were treated with an excess of water they became highly reducing, 

 as evidenced by their reduction potentials. At the same time their hydrogen- 

 electrode potentials changed. In the cases noted the changes of hydrogen-ion 



