sept. 4. 1916 Influence of Barnyard Manure on Soil Bacteria 909 



applied during the previous season exerted no effect which carried over 

 the winter. In midsummer the nitrifying powers of the soil receiving 5 

 tons of manure are apparently less than the soil receiving no manure. 

 The plots receiving 15 tons per acre are much more active in nitrifying 

 dried blood than are the others. The lower applications of irrigation 

 water apparently exert a favorable influence on all the plots, but the 

 greater applications exert a depressing influence. It is, however, no 

 more marked in the heavily manured soils than in the others; therefore, 

 if there be any denitrification taking place, it must be attributed to the 

 production of anerobic conditions by the water, and not due to the 

 manure applied. In November the beneficial influence of the 5 tons of 

 manure applied becomes more regular than at any other time of the year. 

 Here also the influence of the water becomes more perceptible. Taking 

 the results as a whole they do not show the influence of either manure or 

 water as well as it was shown by the potted soils; nor do they bring 

 out the difference as clearly as it is brought out by the ammonification 

 series. The relationship actually existing in the various treated soils is 

 brought out graphically in figure 5. 



On the base line is indicated the irrigation water applied in inches per 

 acre, while on the perpendicular line is given the milligrams of nitric 

 nitrogen produced in 100 gm. of soil to which 2 gm. of dried blood had 

 been added. Taking the average nitric nitrogen produced in the unma- 

 nured soil as 100 per cent, the soil receiving 5 tons of manure becomes 

 105 per cent, while that of the soil receiving 15 tons becomes 486 per 

 cent ; or the average increase per unit of manure applied is much greater 

 when 15 tons of manure are applied than when only 5 tons are applied. 

 In this respect it differs markedly from the ammonification series. 



Taking the average of the unirrigated plots as 100 per cent, the irri- 

 gated plots then arrange themselves in the order — 5 inches, 94 per cent; 

 10 inches, 75 per cent; 20 inches, 85 per cent; 30 inches, 76 per cent; and 

 40 inches, 65 per cent. In every case the average for the season on all 

 plots shows the water to have a depressing influence upon nitrification. 



FIELD EXPERIMENTS ON CROPPED PLOTS 



The same number of plots, arranged and treated exactly the same as 

 those in the preceding part except that they were cropped, were sampled. 

 These had grown corn continuously since the spring of 191 1. They were 

 sampled at the same time of the year, and bacterial counts made as was 

 done on the fallow soil. The average results are given in Table VII. 



These results are very similar to those obtained on the fallow soil. 

 The number of organisms obtained is slightly lower and we do not find 

 as great an increase during the summer months as we do on the fallow. 

 The count as obtained in the spring is low for the unmanured soil, 

 higher for that receiving 5 tons per acre of manure and still higher for 

 the soil receiving 15 tons of manure. While the difference is marked, 



