LUCTURES AXD ESSAYS READ AT INSTITUTES. 383 



of active nitrogen Avould be as available for wheat as for clover during the 

 period of active growth. Since the researches of Boussingault, Lawes, Gilbert 

 and Pugh compel us to reject the doctrine of air assimilation of nitrogen, the 

 discussion is narrowed down to this proposition, that the clover must find in 

 the soil some form of combined nitrogen from which it may derive an adequate 

 supply, and that wheat is incapable of obtaining a satisfactory supply from 

 such source ; in general terms, that clover and wheat are widely distinguished 

 from each other in their power of supplying themselves with nitrogen from 

 some inactive form of that element existing in the soil. 



In all cultivated soils we find a certain amount of organic matter called col- 

 lectively liumus, the remains of vegetable matter which has undergone more 

 or less complete decomposition in the soil. This humus contains a small 

 amount of combined nitrogen in the form of ammonia, or capable of being 

 given off in the form of ammonia when the humus is heated with some 

 alkaline substance; but the humus also contains a much larger amount of 

 nitrogen incapable of being driven off in the form of ammonia by heating 

 with alkalies, and has been called "the inert nitrogen of the soil." This 

 inert nitrogen is usually regarded as incapable of supplying plants with 

 nitrogen, and in this respect is classed with the free nitrogren of air. The 

 humus of the soil containing on an average 2 per cent of inert nitrogen, 

 there are present in each acre of soil from o,000 to C,000 pounds of 

 combined nitrogen. The amount of fixed nitrogen in every acre of cultivated 

 soil is enormous as compared with the wants of any crop, and would be abun- 

 dant for every crop if even a small per cent of it could be made active during 

 the period growth. 



There is one remarkable fact connected with humus, that however small the 

 amount of nitrogen in the vegetable material out of which the humus is formed, 

 the humus itself still contains about 2 per cent of nitrogen. This comes from 

 the fact that organic matter, in passing into the humose condition, fixes a 

 certain quantity of free nitrogen, as was first pointed out by Deherain. 



HUMUS A SOURCE OF NITROGEN FOR PLANTS. 



I now come to the question, Can this humus, as it exists in the soil, furnish 

 nitrogen for any of our cultivated crops ? Do some orders of plants supply 

 themselves largely from the nitrogen of humus, while other orders fail to 

 secure a sufficient supply from that source? Does the classification of plants 

 into nitrogen producers and nitrogen consumers have any substantial basis? 



In this investigation three facts turned my attention to the leguminous 

 plants : 1st. They are but little benefited by nitrogenous manures ; 2. They 

 produce a large amount of nitrogenous material per acre ; 3. They thrive best 

 in soils containing a good supply of humus. 



In entering upon an investigation of the capacity of humus to afford a sup- 

 ply of nitrogen to growing plants, I do not forget the valuable contributions 

 on this subject by Prof. Johnson, given in peat and its uses, and of Prof. 

 Storer, in bulletin of the Bussey Institution, 1874. These distinguished 

 chemists and experimenters used maize and buckwheat only, and did not direct 

 their attention to a comparison of different orders of plants, in respect to their 

 relative power of obtaining a supply of nitrogen from humus. 



Having had my mind stirred up on this general subject of nitrogen-supply 

 for plant growth, by a correspondence with Hon. George Geddes, of New 

 York, I laid before the State Board of Agriculture, in March, the plan for a 

 series of experiments on this subject, and with their sanction I entered upon 



