While it may be questioned whether the process of humification had in these materials really reached the point of sensible completion in all cases (notably in those of sawdust and cow manure), the great variability of the products from different materials is very striking. When the nitrogen-content is deducted the percentage composition of the products agrees more nearly. Considering that the nitrogen is probably present in the amid form, it is natural that hydrogen should in a measure vary with it, as in the case of the clover, flour and meat humus. Nitrogen being the most variable ingredient of humus, it seems probable that the variation of the proportion of the humus-amids present is the most potent factor in the variability of the composition of natural soil-humus. Arranging these results in the order of their nitrogen-content as in the table below, we see that the latter approximately corresponds to the original protein-content of the humified substances. Humus from meat scraps 10.96 % Nitrogen. “ “ green clover 8.24 “ “ cow manure 6.16 “ “ wheat flour 5.05 “ “ oat straw 2.50 “ “ sawdust .32 While the above data prove the correlation between the first products of humification and the original substance, it must be remembered that subsequently, under proper conditions, the nitrogen-percentage in humus may, in the course of time, increase very greatly, even to a proportion considerably above that contained in flesh itself. When we consider that ordinarily, the latter, and the albuminoid substances generally, decompose in contact with air with an abundant evolution of ammonia compounds, sometimes leaving only a little fat (adipocere) behind, it is surprising that the decomposition _within_ the soil should have exactly the opposite result, viz., an _accumulation_ of the nitrogen. The causes of this marked difference are not yet well understood, but it is probably due to the differences in the kinds of bacteria that are active in the two cases. Snyder has also shown that the richer the organic matter humified is in nitrogen, the more energetically it acts in rendering available the mineral matters of the soil for plant nutrition. Correspondingly, Ladd[55] has shown that with the increase of humus in the soil, there is also a corresponding increase in the amounts of mineral plant-food extracted from the soil by a four per cent solution of ammonia, such as is employed in the Grandeau method of humus-determination. [55] Bull., S. Dakota Station, Nos. 24-32, 35, 47. CHAPTER IX. SOIL AND SUBSOIL (_Continued_). ORGANISMS INFLUENCING SOIL CONDITIONS; BACTERIA, ETC. MICRO-ORGANISMS OF THE SOIL.