322 



THE AGRICULTURAL NEWS. 



OCTOBEK 15, 1910. 



plant, but it was not possible, in the state of their 

 chemical knowledge, to indicate the origin of the car- 

 bonaceous matter that is added to plants during then- 

 growth. 



Experiments dealing with the nutrition of plants 

 were nevertheless continued, for in 1(5(50, Sir Kenelni 

 Digby, lecturing before the Society for Promoting 

 Philosophical Knowledge by Experiment, described 

 a trial in which the growth of young plants of barley 

 was increased by watering them with a weak solution 

 of nitre. Mayow, of the University of Oxford, went 

 further and drew attention to the fact that the added 

 nitre must feed the plants, under such circumstances, 

 because none of this substance could be extracted from 

 soils. Others who were interested in the growing of 

 plants confirmed the observations as to the increased 

 fertility of the soil to which this salt had been added; 

 but for various reasons, no more definite knowledge cm 

 the subject was obtained for .some time. 



It was not until the nineteenth centtiry, after the 

 work of Priestley, Lavoisier, de Saussure and others had 

 shown what is the true composition of the air, that it 

 was possible to gain any further knowledge as to the 

 way in which plants feed and grow. Attention was 

 given at first tn the humus in the soil, and it was c<m- 

 cluded, from the increased growth of plants in soils rich 

 in this, that their carbonaceous content was obtained 

 from it. By about 1840, however, the results of invest- 

 igations had arrived at a point at which it was possible 

 to state definitely that the carbon in plants comes 

 from the air, and the nitrogen and ash from the soil. 

 It is to Liebig that the agriculturist is indebted for the 

 general acceptance which was gained by this broad 

 theory of plant nutrition, as well as for most of the 

 influence which now makes agricultural chemistry 

 a matter of exact science. 



Enough was known at this stage to suggest that, as 

 the supply of material in the air giving the plant its car- 

 bonaceous matter is inexhaustible, the state and composi- 

 tion of the soil must be the responsible factors in influenc- 

 ing the growth of plants. In relation to this, the enor- 

 mous difference that exists between the amount of plant 

 food in the soil and that taken out by a crop was first 

 pointed out by Daubeny, Professor of Botany and 

 Rural Economy at Oxford, and the real fotmder of the 

 science of agriculture in England. It was this invest- 

 igator who first .showed experimentally that any normal 

 soil 'contains the material for from fifty to a hundred 

 field crops'. Considerations of the limit of the growth 

 of plants, even in the presence of such large amounts 

 of food, gave rise to the conception of available and 



unavailable plant food, and Daubeny attempted to find 

 a means of gauging the amount of the former by treat- 

 ing the soil with water containing carbon dioxide. He 

 did not obtain conclusive results, however, and this has 

 been the experience of others who have adopted the 

 same line of experiment, using different acids, such as 

 citric acid, instead of a solution of carbon dioxide in 

 water. The reason for this general failure is that such 

 investigations only determine one factor in soil product- 

 ivity; there are others*, as is well known, which must 

 be considered to an even greater degree, in order to 

 arrive at a true estimate as to its causes. 



Other suggestions that have been made, especially 

 in recent years, for the purpose of explaining the differ- 

 ences in productivity of soils may be passed over, as 

 they are at present matters under discussion. It is only 

 necessary to make mention of the theory put forward 

 by investigators in the United States to the effect that 

 infertility of soil arises from the presence of toxic bodies 

 that have been formed in it already by plants. Daubeny 

 had given attention to this theory in 1845, when he 

 produced objections to it that are valid at the present 

 time. There is the additional objection that, although 

 sterile soils have been shown to contain certain organic 

 bodies that may be toxic to plants, there is nothing to 

 indicate that fertile soils do not contain these in an 

 equal degree. 



It remained for Schliising and Miintz, Warington, 

 and Winogradsky, within the last thirty years, to show 

 that nitrates are produced from organic compounds and 

 ammonia in the soil by the acti<jn of tvvo bacteria, 

 called usually the nitrifying organisms, neither of 

 which can coni])lete the work without the presence of 

 the other. This suggested the importance of thorough 

 cultivation of the soil, in order that these organisms 

 may find tln-mselves among the conditions of aeration 

 and moisture that will conduce to their greatest activity, 

 and therefore to the (piickest formation of nitrates. 



The next step in progress was made when it was 

 discovered by Hellriegel and Wilfarth, in 188(5, that 

 certainbacteria actually assist in the addition of nitrogen 

 to the soil, by living in the nodules of leguminous plants, 

 to the latter of which they transfer nitrogen taken from 

 the air. The importance of this mode of adding nitro- 

 gen to the soil is too well known to recjuire further 

 comment in the present connexion. 



The results of such investigations drew attention 

 to the miero-organisiiis that live in the .soil, and their 



* See Agricultural News, Vol. IX, p. 257. 



