In the preliminary studies carried out in the laboratory and greenhouse, very similar effects were exerted on the bacterial activities in the soil and on the yield of oats from the use of the variously prepared artificial manures and from the application of farm manure. Well-rotted farm manure had a much greater effect on carbon dioxide production in sand cultures than did the artificial manures, but some of the artificial manures produced as large an influence as did the fresh farm manure. The sweet clover manure had a distinct beneficial effect, and the straw manure also increased the carbon dioxide production. The greater influence of the well-rotted farm manure was undoubtedly due to the greater decomposition occurring in that material than in the artificial manures.
The nitrification studies in sand cultures in the greenhouse indicated that nitrate accumulation was depressed by the additions of farm manure and artificial manures, there being just as great a depression from the farm manure in 71/2 and 15 ton applications as from the artificial manures. The nitrifying power of the sand cultures was increased by the treatments, the artificial manures, in some cases, having a greater effect than the farm manure. The straw-clover manure and the sweet clover manure showed a greater effect than the farm manure. Apparently the greater nitrification brought about by the treatments was followed by a greater assimilation and, hence, there was a lower content of nitrates at 11 dates of sampling. In this test the artificial manures and the farm manure seemed to have similar effects on nitrification, but the artificial manures, in some cases, had a greater stimulative influence on the oats grown than did the farm manure.
In the nitrate assimilation experiment in the laboratory, the application of farm manure appreciably stimulated the development of the nitrate assimilating organisms in the Carrington loam. The oats straw and the straw manure brought about a greater stimulation, but the artificial manures in general showed a smaller stimulation on this group of organisms. It is apparent that the artificial manures had no greater influence in increasing nitrate assimilation than did farm manure. These results were confirmed by later tests in the greenhouse and in the field on the Carrington loam, but it was noted that those artificial manures, which were not so well decomposed when applied to the soil, stimulated these nitrate assimilating organisms more and the effects persisted in the soil for a longer period.
In the greenhouse experiments on Carrington loam, the artificial manures and farm manure had very similar effects on nitrate production, and while the influence on nitrification was not quite so great at first, the later effects were greater than those produced by farm manure. The influence of the various manures was similar on the crop of oats grown in this test.
Field experiments on the Webster loam in 1926 and 1927 and on the Carrington loam in 1928 and 1929 with variously prepared artificial manures in comparison with farm manure indicate that nitrate assimilation, nitrification, nitrate accumulation and the numbers of bacteria, molds and actinomycetes were very similarly affected by the additions of the various manures. The yields of corn were also influenced in a very similar way by the artificial manures and farm manure, altho in some instances there were differences in yields which were out of line with the general trend of the results.
From the results as a whole it is apparent that the proper composting of straw and cellulose residues may permit the production of artificial manures which will give similar effects on bacteriological conditions in the soil and on crop yields to those occasioned by farm manure. There is a similar stimulation in the nitrification process and in the numbers of organisms, and while nitrate assimilation is also stimulated by the artificial manures, the influence on this process is no greater than that produced by farm manure. While, therefore, there may be a reduced content of nitrates in the soils following the addition of these artificial manures, there is no greater reduction than that brought about by farm manure. As farm manure has a well-known beneficial effect on crop growth, it is apparent that there need be no deleterious effect from well-decomposed artificial manures either on crop yields or on soil conditions in general. In fact it would seem that quite as large beneficial effects should be exerted, and any increase in nitrate assimilation and decrease in nitrate content is more than offset, from a crop production standpoint, by the increased nitrification which is occasioned.
Brown, P. E. and Smith, F. B.
"The effects of artificial farm manures on soils and crops,"
Research Bulletin (Iowa Agriculture and Home Economics Experiment Station): Vol. 10
, Article 1.
Available at: https://lib.dr.iastate.edu/researchbulletin/vol10/iss127/1