914 Journal of Agricultural Research vol. vi, No. 23 



by crop and cultural methods. The variation between the differently 

 treated soils during the same part of the year is qualitatively similar to 

 that noted in the fallow soil. 



The influence of the manure is very pronounced throughout the entire 

 season. The ammonifying powers of the unmanured soils are all low, while 

 those of soils receiving 5 tons of manure per acre are higher. Those of soils 

 receiving 15 tons of manure per acre are very high. This difference is 

 probably slightly greater during the spring months than during the fall. 



The irrigation water applied is found to exert an influence upon this 

 group of bacterial activities. Measured in terms of ammonifi cation, the 

 unmanured soils and those receiving 5 tons of manure per acre are 

 benefited greatly by small quantities (10 and 20 inches) of irrigation 

 water, while the soils receiving 15 tons of manure per acre have the 

 highest ammonifying powers when they receive 20 or 30 inches of water. 

 During the spring it is greatest in those soils from plots receiving 30 

 inches of irrigation water. Forty inches of water produce a marked 

 depression in the ammonia formed, being pronounced in the soils receiv- 

 ing 15 tons of manure not only in the cropped soil but also in the fallow 

 and potted soils. It is clear, therefore, that large quantities of water 

 applied to a soil rich in organic matter depress the beneficial bacterial 

 activities of that soil. The fallow unmanured soils and soils receiving 

 5 tons of manure per acre showed a slight decrease in the ammonifying 

 powers of the soil, owing to the larger applications of irrigation water; 

 but this does not appear in the cropped soil and is probably caused by 

 the removal of large quantities of water by the growing crop, so that 

 enough water does not accumulate in the presence of these small quan- 

 tities of organic material to injure the ammonifying powers of the soil. 

 These facts are brought out clearly in figure 7. 



If we take the average of the quantity of ammonia produced in the 

 unmanured soil as 100 per cent, the others then become with 5 tons 129 

 per cent and with 15 tons 183 per cent. Here the average increase per 

 ton of manure applied is about the same whether 5 or 15 tons of manure 

 be applied per acre. If the average of the plots receiving no irrigation 

 water be taken as 100 per cent, the others then become with 5 inches of 

 water 105 per cent; with 10 inches, 1 14 per cent; 20 inches, 118 per cent; 

 30 inches, 112 per cent; and 40 inches, 109 per cent. It thus reaches 

 its maximum when 20 inches of water are applied, while the fallow 

 reached its maximum when only 10 inches were applied. This is a 

 difference which is undoubtedly due to the great quantities of water 

 removed by the growing plant. The average increase per acre-inch of 

 water, however, is greatest in the cropped soil where only 10 inches of 

 irrigation water were applied. 



The nitrifying powers of the same soils were tested by the method 

 previously given, the results of such tests being given in Table IX as 

 milligrams of nitric nitrogen produced during 21 days in 100 gm. of 



