1916] SOILS FERTILIZEES. 513 



A preli mi nary statement on the present status of the humus nitrogen 

 problem in arid soils, C. B. Lipman {Soil Set., 1 {1916), No. S, pp. 285-290). — 

 The author reviews experimental work by himself and others conducted for the 

 most part at the University of California. 



The results are taken to indicate that " the method of determining humus 

 nitrogen in the ammonia extract of soils is a seriously faulty one, no matter 

 how much care is employed in boiling the extract with magnesia. The method 

 is so faulty as to deserve immediate rejection by all those who are at all con- 

 cerned with the correct determination of nitrogen in humus. If the results 

 given are considered in connection with the largest part of the humus nitrogen 

 data furnished by Loughridge [E. S. R., 34, p. 324], there can be no question 

 that the prevalent belief in the high nitrogen content of the humus of arid soils 

 is in error. The facts in hand do not justify any belief in the higher nitrogen 

 content of the humus in either the arid or the humid group of soils over each 

 other." 



A list of five references to literature bearing on the subject is appended. 



A contribution to the subject of the factors concerned in soil productivity, 

 C. Hoffmann {Kans. Univ. Sci. BuL, 9 {1914), No. 7, pp. 19-99, pis. 5). — 

 Laboratory experiments conducted at the University of Wisconsin with normal 

 peat, sandy, and marsh soils to determine (1) the influence of the growth of 

 pure cultures of soil organisms in sterile soil on the growth of wheat, corn, and 

 clover seedlings in the extract of such soil, and (2) the influence of the gi'owth 

 of corn, oats, and clover in the same soil on pure cultures of bacteria grown in 

 the extracts of such soil are reported. 



It was found that " the growth of individual species of bacteria in a soil pro- 

 duces changes in the soil solution which manifest themselves by an increased 

 or decreased development of plant seedlings when grown in extracts made from 

 such soils. Invariably this influence seems to affect the root development rather 

 than the leaf development. In repetitions of the same experiment consistent 

 results are secured as long as all three factors, soil, bacterial species, and crop, 

 are the same. A change of any one factor modifies the results secured. The 

 growth of individual species of crops in a soil produces changes in the soil 

 solution which manifest themselves by an increased or decreased bacterial mul- 

 tiplication in extracts made from such soils. The extracts made from a marsh 

 soil cropped by corn, oats, or clover in all cases stimulated bacterial multiplica- 

 tion. This stimulation was consistently greatest in the case of the corn-cropped 

 soil. In contrast to the marsh, the extracts from the cropped loam and sand 

 soils invariably retarded the multiplication of the bacteria grown in the same. 

 There is a definite relation between the growth of crops in soil and the growth 

 of bacteria therein. . . . 



" Owing to the variability of the three factors involved, soil, crop, and bac- 

 terial flora, it is impossible to establish any hard and fast laws for all cases." 



Incubation studies with soil fun^, S. A. Waksman and R. C. Cook {Soil 

 Sci., 1 {1916), No. S, pp. 275-284, flQ- !)• — Ammonification experiments con- 

 ducted at the New Jersey Experiment Stations with Mucor plumbeus, Penir 

 cillium sp., and Monilia sitophila in pure culture in a gravelly loam soil, using 

 dried blood and cotton-seed meal as ammoniates, are reported. 



The results obtained are taken to indicate that "optimum moisture conditions 

 for ammonia accumulation by fungi lie near the physical optimum. The proper 

 incubation period depends entirely upon the organism. A 12-day incubation 

 period is preferable to a shorter one for practical work. A correlation exists 

 between the biological stage of the organism and the periods of ammonia 

 accumulation; the largest amount seems to accompany the periods of spore 



