SUMMARY OF RESULTS FOR NITRIFICATION Considering now the entire results for nitrification, it appears that the use of dried blood or of ammonium sulfate is eminently satisfactory as a measure of the nitrifying power of the soils. The results obtained with the two substances were in close agreement, some differences, however, being worthy of note. Thus at the first sampling the gains due to lime were proportional to the amounts applied whether dried blood or ammonium sulfate was employed as a measure of the nitrifying power. At the second sampling the differences brought out were much greater in both cases, but while the nitrate produced in the check soils with dried blood was practically the same as at the first sampling, the amount produced with the ammonium sulfate was less than that obtained at the first date. The proportional gains were considerably wider here where ammonium sulfate was used than where dried blood was employed. At the third sampling the results agreed more closely, the nitrates produced with either dried blood or ammonium sulfate being less than at the previous sampling and the proportional gains for the increasing applications were very similar. At the last date the results in either case agreed with those at the previous sampling, and the differences brought out by the lime were practically the same. THE CURVES FOR NITRIFICATION With a plotting of the curves for nitrification some of the differences and agreements just discussed are brought out more clearly. In the first place, with the exception of those plotted from the results of the third sampling, the curves for the nitrification of dried blood and ammonium sulfate correspond almost exactly. If placed one upon the other they almost coincide. We may conclude therefore that the effect of applications of lime on the nitrifying power of soils may be tested either with ammonium sulfate or dried blood with practically the same results. In other words, the nitrification of a protein like dried blood, seems to proceed in the same way as that of an ammonium compound. This seems strange when we recall that several more stages are necessary for the transformation of protein into nitrate than for the transformation of ammonia into nitrate, and we would naturally expect differences to manifest themselves. When we remember, however, that the effect of liming on the ammonifying power of the soils corresponded very closely to that on the nitrifying power, the mystery is solved, for increased ammonia production is always an indication of increased nitrate formation. Considering now the possibility of any relation between numbers and nitrifying power, we find that a gain in numbers in this experiment was coincident with a gain in nitrifying power. But while the numbers increased from the first to the second sampling and then decreased to the third and fourth, the nitrification of ammonium sulfate decreased from the first sampling to the second, further to the third, and then increased slightly at the fourth. The nitrification of dried blood, however, increased from the first to the second sampling and then decreased at the third and fourth. At these latter samplings the amounts of nitrates produced were almost identical. Thus we may conclude that the nitrification of dried blood is more closely related to the total numbers of bacteria appearing on the "modified synthetic” agar than is the nitrification of ammonium sulfate. CONCLUSIONS OF NITRIFICATION TESTS The conclusions to be reached from this experiment are: 1. Applications of lime cause increasing nitrate production from ammonium sulfate and dried blood, depending on the amount of lime applied, the gains being almost proportional to the amount of lime. 2. The curves for nitrification of ammonium sulfate and dried blood, with one exception, are practically coincident. 3. The use of the beaker method with either of the two substances is well suited for measuring the nitrifying power of a soil. 4. The nitrification of dried blood is closely related to the total numbers of organisms appearing on "modified synthetic" agar, but there is little or no relation between numbers and nitrate production from ammonia sulfate. VIII. THE NITRATES PRESENT IN THE SOILS We would naturally expect differences in nitrifying power to lead to differences in the nitrates present in uncropped soils at any one time. With this idea in mind the nitrates present in the soils at each sampling were determined. Table XVII shows the results of the determinations. At TABLE XVII. NITRATES IN SOILS IN PARTS PER MILLION the first sampling practically the same amounts of nitrates were present in all the soils, showing that the differences evidenced by the nitrifying powers of the soil at that date had not yet had opportunity to affect materially the nitrate store in the soils. At the second sampling, however, there had been a gradual increase from 7.92 parts per million nitrogen for the check to 11.91 parts per million nitrogen for the three ton application. There was a correspondence here between the amount of nitrates present and the larger differences in nitrifying powers already discussed. At the third date, while the amounts of nitrate present were greater than at the previous samplings, the differences were smaller, due perhaps to the effect of some unexpected factor. At the last sampling the differences were even wider than at the second, showing an accumulation of nitrates from 6.79 parts nitrogen per million in the check soils to 16.93 parts per million of nitrogen in the soils receiving the three ton application of lime. In a general way it may be said that increased nitrifying power causes a small increase in the nitrates present in the soils at any one time, but occasionally through the interference of some unknown factor, physical or perhaps bacteriological, there is an accumulation of nitrates which largely covers up the differences. This interference, however, seems only temporary and is followed by a lowering of the nitrate content of the check soils to the normal or a little below, and the differences between them and the treated soils appear much more distinctly than before. The conclusions to be reached from these results are: 1. Applications of lime lead to slightly increased accumulations of nitrates. 2. Increased nitrifying power is followed by slight increases in nitrate content of soils. 3. Natural increases in nitrates in soils obscure the differences due to treatment. IX. NITROGEN-FIXATION EXPERI- The nitrogen-fixation experiments were carried out according to both the solution and the beaker methods in order TABLE XVIII. NITROGEN FIXATION IN MANNITE |