340 



THE IRRIGATION AGE. 



NITRIFICATION OF THE SOIL 



The process of oxidizing ammonia to nitric acid is 

 termed nitrification. Scientists are bending much of 

 their energy toward soil improvement along this line. 

 One step necessary in the direction of larger crops is a 

 study and understanding of the soil. 



If the summer working of fallow land helps to retain 

 water in the surface soil, this water may have a power- 

 ful influence upon the production of nitrates for the 

 next crop. 



A growing crop may reduce the nitrate supply for 

 the next crop in two ways it may use up all the nitrates 

 actually present, and it may so dry the land that nitri- 

 fication in the interval between crops is reduced to a 

 minimum. 



The moisture requirements for nitrification suggest 

 that in dry seasons a single flooding of fallow land might 

 be followed by good results. 



Nitric acid or nitrate contains nitrogen in the highest 

 state of oxidation. Ammonia is a compound of nitrogen 

 which has no oxygen. To convert ammonia into nitric 



Fig. 7. Lower Latham Reservoir, Greeley, Colo. Destruction by Wave 

 Action. April, 1'Jlsi. 



acid, its nitrogen must be oxidized. This happens in 

 fertile soil, and the oxidation is brought about by certain 

 bacteria. 



Besides nitrogen and oxygen plants require eight 

 other elements in their food materials. Each of these 

 must be in the highest state of oxidation commonly oc- 

 curring in Nature. Phosphites, sulphites, nitrites, and 

 ferrous salts either kill the plant outright, or they give 

 an unsatisfactory return. This is also true of ammonia 

 for ordinary crop plants. Ammonia is not poisonous in 

 practical amounts, but it gives a much smaller yield than 

 the highly oxidized nitrogen found in nitrates. 



If moisture is necessary for nitrification, it follows 

 that conservation of soil moisture has a double purpose. 

 The moisture is held in reserve for the use of a future 

 crop, but its presence meanwhile tends to the pro- 

 ductio'n of nitrates. To find how far exactly, and within 

 what limits the presence of soil moisture has an effect 

 ipon the production of nitrates was the object of recent 

 experiments. 



The method of investigation was as follows: Air- 

 dry soil equal to 300 grams dry soil was placed in a 

 bottle of 3 inches diameter, 6 inches high, neck \y> inches, 

 and of 500 c. c. capacity. Sufficient tapping was given 



to make the soil compact. A mark was placed on the 

 bottle at the soil level, the soil was replaced by water to 

 the same mark, and from the weight of water required 

 the apparent S. G. of the soil was determined. From 

 this and the real S. G. as found by the displacement 

 method, the porosity of the soil, and from this the total 

 water-holding capacity was found by calculation. There 

 were two sets of experiments in consecutive periods, using 

 about 100 bottles each. In the first experiments water 

 was added to bring the original soil moisture up to 10, 

 20. 30, 40, 50, 60, and 70 per cent of its total water- 

 holding capacity in different bottles. In the second, the 

 steps were 10, 20, 30, 50, 70, and 90. All the bottles were 

 marked to the same volume, and the same weight of dry 

 soil was used in every case. 



Before charging the bottles, the weighed soil was 

 thoroughly mixed with its proper quantity of water in a 

 VVedgewood mortar. One-half of the bottles got water 

 only, the other half got 5 c. c. standard ammonium sul- 

 phate solution in their water. After filling, the bottles 

 were placed in a dark cupboard in the laboratory, and 

 the temperature of the cupboard was read daily at 9 a. m. 

 and 5 p. m. The bottles in the cupboard were corked. 

 Twice weekly each bottle was taken out. uncorked, as- 

 pirated for 5 seconds with the suction 

 pump, corked and replaced in the 

 locker. 



The method of using closed bot- 

 tles and aspirating seemed better than 

 the method of using open bottles and 

 adding water lost. In order to judge 

 of the two methods, six bottles were 

 left open, and the results compared 

 with closed aspirated bottles after a 

 period of twenty-eight days. 



With closed bottles the losses 

 were insignificant, while with open 

 bottles usually more than one-half of 

 the original moisture was lost. The 

 lost water in open bottles cannot be 

 replaced in a satisfactory manner, be- 

 cause at the point where the water is 

 added the soil will be much wetter 

 than at other points in the soil mass. 

 To demonstrate this, a glass tube of 

 1-inch bore was packed with soil to 

 '- the same degree as in the experiment 

 bottles, and to a depth of 10 inches. 

 Water was then added from the top 

 equal to 10.70 parts per 100 dry soil 

 (40 per cent of its water-holding ca- 

 pacity). At the end of seventeen days 

 the tube was cut into four sections of 

 2 l /2 inches and the moisture in each 

 determined. Of the total water in 

 the soil there was found in the top 

 section 35; in the second, 31; the 

 third, 24; and in the fourth, 10 per 

 cent. With a smaller quantity of 



water, or with a shorter time allowed, the differences 

 would probably have been greater than here noted. 



In each experiment the ammonia bottles and the con- 

 trol blanks were done in duplicate, and the average results 

 are taken where the figures are close, when wider, the 

 average is marked doubtful. This happens in two instances 

 with the very wet soils of the second experiments. 



In the chemical analysis the larger percentage of lime, 

 and particularly of carbonic acid, in the clay is notable as 

 representing available base. The sandy soil with .032 carbonic 

 acid falls below the minimum requirements, and is appar- 

 ently deficient in available lime. 



Between 40 and 70 per cent of the water-holding capacity, 

 rate of nitrification, did not vary in any important degree. 

 At the various steps below 40, there was a notable falling away ; 

 10 per cent practically stopped nitrification, and at 20 the 

 rate was only one-seventh of the best case. Another dupli- 

 cate set of bottles set up at the same time, but analyzed one 

 week earlier, showed results on a lower plane for each degree 

 of moistness, but placed them in the same order of merit. 



Nitrification is inactive in these soils while they still 

 contain about three times more moisture than in their average 

 air-drv condition. 







