TABLE I

Determinations in the Laboratory

Determinations incubated in the Field

influenced. Lack of aeration in the field areas is another important factor which must be considered. A third feature of importance is the excess of moisture in the field samples as is seen by the rainfall during the time that these soils were incubated.

With all the soils, in both the field and the laboratory experiments, there was a greater amount of cottonseed meal ammonified than dried blood. With one exception, twice as much cottonseed meal as dried blood was decomposed. There seemed to be very little difference in the ammonifying power of the different soils. That stirring the soil affects biological activities to a marked extent is apparent when one compares the relative amounts of ammonia distilled from the small areas which had been stirred with those which remained undisturbed.

TABLE II

THE VARIATION IN NITROGEN CONTENT OF SMALL AREAS
6 x 6 INCHES AND 18 INCHES APART

Inasmuch as there was an excessive amount of rainfall during the time that the nitrification and nitrogen fixation experiments were carried on, the field experiments with regard to moisture particularly were under conditions very different from those affecting the corresponding tests in the laboratory. The lack of nitrates in the field areas, to which ammonium sulphate had been added, as in several instances none was found, clearly demonstrated that the heavy rains either washed it away as such or as nitrates after it had been nitrified. Likewise, the nitrates resulting from the nitrification of the dried blood must have either been leached away very soon after they had been formed, or fixed at lower depths. No doubt there occurred a solution and loss of the dextrose in the nitrogen fixation experiments which were carried out in the field, as there was a heavy fall of rain during the ten days that this experiment was under observation. Due to these very abnormal conditions of moisture, the results obtained in the nitrification. and nitrogen fixation tests in the field are not entered in this preliminary paper, as a comparison between field and laboratory test of these processes under such conditions would hardly be justified.

The data in Table II show an interesting fact in regard to the variation in the nitrogen content over a small area of a field, which was made plain during this experiment.

It is seen in Table II, that the nitrogen content of the extreme areas, Nos. 25 and 33, which were 151⁄2 feet apart, in the case of the plot soil varied as much as .0180 gm. while with the orchard soil this difference was .0236 gm. Thus it is very apparent that great care must be exercised in sampling a field if a uniform sample is desired.

From the limited amount of data presented, it appears that, to some extent at least, biological experiments can be carried out in the field. Inasmuch as the moisture is one of the controlling factors, to be most successful with such experiments, a season during the year must be selected when there is apt to be least danger of heavy rainfall.

SUMMARY

1. Biological experiments (e. g. in ammonification) can be successfully carried out in the field.

2. As a rule a greater amount of organic matter seems to be ammonified in the laboratory tests than in the field.

3. Nitrogen fixation and nitrification studies in the field are greatly interfered with by rains.

4. The nitrogen content of the soil varies considerably, even over a comparatively small area.

A STUDY OF THE ACTION OF CARBON BLACK AND SIMILAR ABSORBING MATERIALS IN SOILS'

By

J. J. SKINNER and J. H. BEATTIE, Biochemists, Soil Fertility Investigations, U. S. Department of Agriculture

Carbon black is a very good agent for purifying distilled water for plant physiological purposes, its action seeming to be one of absorbing substances of a solid or gaseous nature. It is used quite generally by plant physiologists in preparing good water for culture work. Other insoluble, finally divided materials, such as ferric hydrate, aluminum hydrate, magnesium carbonate, barium carbonate and quartz flour (1, 3) are good purifiers of distilled water and extracts of soils.

Water extracts of certain unproductive soils are improved by shaking them with carbon or ferric hydrate and filtering the solution clear (2, 3). Experiments were made to ascertain whether certain poor soils would be improved by the addition of carbon and other finely divided materials direct to the soil. These experiments were made first in pots in the greenhouse, using soils which had grown the same crop repeatedly for a number of times and had become very poor. The productivity of these soils was not restored by fertilizers. Carbon black was added to and mixed with the soil in an attempt to absorb anything of a harmful nature from the soil, but the soil was not thereby improved for plant growth. In a field experiment on the Arlington Experimental Farm carbon black was added on plots growing wheat, rye, timothy, clover, corn, cowpeas, and potatoes. This experiment was conducted for six years, the same crop having been grown on the same plot each year. The carbon had no beneficial effects on any of the plots. These experiments were made by mixing the carbon with the soil. Although the carbon might have had an absorptive action in taking up substances of a harmful nature, it was nevertheless in close contact with the soil and plant roots. On this account it might be expected that no beneficial action would be shown. Similar experiments were made, in which ferric hydrate and magnesium carbonate were used, but no uniformly beneficial results were secured.

Experiments were made in similar soils by putting the carbon in porous pots, tubes, and jars and burying them in the soil. The tubes used were very porous, permitting the moisture of the soil to pass freely through the

1 Received for publication June 30, 1916.

carbon and out again. Through the use of this method the absorbing material was not brought in contact with the soil itself or with the plant roots, but was able to absorb materials from the soil solution. If the soil contained soluble, harmful organic substances they would to a certain extent be absorbed and removed from the solution. Experiments bearing on this problem were made and are presented in the following pages. The carbon black used throughout this investigation is made by burning natural gas and collecting the carbon on cooled cylinders. It is known as the "G Elf" brand and was secured from G. L. Cabot and Co., Boston, Mass.

EFFECT OF CARBON BLACK INCASED IN POROUS MATERIAL ON GROWTH IN POTS

Several experiments were made in pots, carbon black being placed in a smaller pot of porous earthenware material and then buried in the soil in the larger pot in which the plants grew. The carbon black used in these experiments was thoroughly washed and packed in the small pot in a moist condition. The pots were the ordinary unglazed flower pots used in general greenhouse work. On account of the porous character of the pots water added to the soil during the experiment could circulate easily through the soil into the incased carbon and back again through the walls of the inner pot into the soil in which the plants were growing. The roots of the plants could in no way come in contact with the carbon black, as the top of the inner pot was covered in such a way that the soil could not mix with the carbon.

A soil was used for this first experiment which had grown cowpeas in the greenhouse, crop after crop for two years. The soil had become very poor and produced very poor cowpeas. The soil was potted in 8-inch earthenware pots and a 4-inch pot was filled with carbon and buried in the center of the 8-inch pot. In the other pot, which was to serve as a check, the small inner pot was filled with some of the same soil as the larger pot. Nine cowpea plants were planted around the circumference of the pots 1 inch from the walls. The seeds were planted March 10, 1910, and grew for 6 weeks. The growth in the two pots at an early period of the experiment is shown in Plate I, (fig. 1). Pot No. 1 is the check and No. 2 contains the carbon black. It is shown here that the growth at this early stage is better in the pot containing carbon. The green weight of the nine plants at the end of six weeks for the carbon pot was 17.8 gm. against 13.9 gm. for the check plot, an increase of 29 per cent.

A similar experiment was made growing wheat in a sandy loam soil. This soil when used in the greenhouse in pots and boxes grew good wheat. Pots of the same size as those described under the preceding experiment were used. Ten wheat plants grew in each pot and were planted around the circumference as in the case of the cowpeas. The wheat was planted