Transformation of nitrogen in rice soil

Nitrate, nitrite, and ammonia changes were studied in flooded and unflooded soils under greenhouse conditions. These soils were treated as follows: Green manure, sodium nitrate, ammonium sulfate, and no treatment. One-half of each series was cropped to rice and the other half left uncropped. The results from a study of these changes indicate that the nitrite content of the soil was never significant at any time. The greatest concentration was 0.25 p.p.m. No toxic effect to the plant could be expected at this concentration. 2. In soils to which ammonium sulfate was applied the following changes in ammonia were observed: For the unflooded series ammonia decreased from about 27.5 p.p.m. to a trace over a period of two months. During the same time the nitrates increased from 4 p.p.m. to about 27.5 p.p.m. and one month later they had decreased to 13 p.p.m. In the flooded series the ammonia decreased from about 20.5 p.p.m. to 11 p.p.m. in two months. The nitrates in this series were not readable after the first analysis. Nitrates were greatly reduced in the unflooded soils as a result of cropping. After a period of two months the cropped soils showed 14 p.p.m. of nitrates in comparison to 62.4 p.p.m. in soils not cropped. 3. The changes of nitrates in soils to which sodium nitrate was applied are as follows: In submerged soils the nitrates decreased from 19 p.p.m.to a trace two months after cropping. The ammonia in the same soil, during the same time usually was not readable, though in a few cases a concentration of 6 p.p.m. are recorded in the flooded soils. For similar unflooded soils the nitrates were reduced from about 41 p.p.m. to 18 p.p.m. over a period of two months. During this time traces of ammonia are recorded. It appears that in this case nitrates were assimilated by the rice plant. 4. The submerged soils to which green manure had been applied showed a steady increase (over a period of two months) from 4.1 p.p.m. to 34 p.p.m. of ammonia. During this time the nitrates were present in too small a quantity to be readable. In the dry soils the ammonia increased from a trace to 9 p.p.m. in two weeks and then decreased again to a trace after a period of six weeks. During this time the nitrates increased from 3 p.p.m. to 56.5 p.p.m. 5. Soil reaction was changed decidedly toward alkalinity in flooded soils treated with green manure, with sodium nitrate, and those receiving no treatment. No such change occurred where ammonium sulfate was applied.