POOD OF PLANTS. 559 



such by the roots. In favour of such a view is the fact that 

 carbonate of ammonia, or ammonic carbonate (decomposable by 

 crenic and apocrenic acids), appears in many cases hurtful when 

 applied directly to the roots of plants. In addition to the ten- 

 dency of these organic acids to attract ammonia, they seem to be 

 capable even of causing its production in the soil, since in the 

 progressive oxidation of humus taking place at the expense of 

 water (H 2 O), the hydrogen of the latter possibly combines in 

 its nascent state with the nitrogen of the atmosphere to form 

 ammonia. 



It has been common in recent works to find the value of humous or 

 carbonaceous matters in the soil estimated very low ; they have been 

 regarded either as merely improving the (physically) absorbent power of 

 soil, or as sources of carbonic dioxide, already sufficiently provided by 

 the atmosphere. But the above observations, borne out by the experi- 

 ments in Turnip-growing by Lawes and Gilbert, are in favour of a higher 

 estimate of the value of decaying carbonaceous matters, and of regarding 

 them as important constituents of farmyard manures for certain purposes. 

 Lawes and Gilbert found that stimulating nitrogenous manures in excess 

 were rather detrimental to the growth of turnips, leaf-formation going 

 on at the expense of the roots ; but this was counteracted in a great 

 measure by supplying, with the nitrogenous manures, carbonaceous sub- 

 stances in considerable proportion. Corenwinder, however, states that 

 the roots exhale carbonic dioxide and do not absorb it except in very 

 minute quantities. 



Sources of Nitrogen. It was once supposed that there was a 

 power in living plants to fix free nitrogen from atmospheric air ; 

 but this is conclusively negatived by the experiments of Bous- 

 singault, Lawes and Gilbert, Pugh, and others. That ammonia 

 is not absolutely necessary for the food of plants is indicated by 

 the effect of nitrates as manures, rivalling that of salts of ammonia. 

 Moreover it has been stated that ozone (a peculiar condition of 

 oxygen) converts ammonia into nitrous acid ; and there is reason 

 to suppose that the ozone condition of oxygen is produced in 

 certain cases in the liberation of that element by plants. Schlosing 

 states that the ammonia supplied to the soil becomes (if light be 

 excluded) converted in the soil into nitrates by the agency of a 

 vegetable organism acting as a ferment. 



The amount of nitrogen supplied to the soil by rain is insufficient 

 to account for the amount found in plants; but this quantity is sup- 

 plemented by the direct absorption of ammonia by the leaves, as well 

 as from the soil. Deherain supposes that decaying vegetable matter 

 has the power of producing ammonia from the free nitrogen of the air; 

 but his experiments have not been confirmed. Schlosing confirms the 

 experiments of Sachs and Meyer as to the absorption by the leaves, 

 and also by the soil, of gaseous ammonia, and corroborates the statements 

 that the free nitrogen of the air is not available for plants except to the 



