chairman's address. 573 



He took a further step, and attempted to determine the proportion of the plant 

 food which can be regarded as active. He argued that since plants only take in 

 materials in a dissolved form, and as the great natural solvent is water percolating 

 through the soil more or less charged with carbon dioxide, therefore in water 

 charged with carbon dioxide he would find a solvent which would extract out of a 

 soil just that material which can be regarded as active and available for the plant. 

 In this way he attacked his Botanic Garden soils and compared the materials 

 so dissolved with the amount taken away by his crops. The results, however, 

 were inconclusive and did not hold out much hope that the fertility of the soil 

 could be measured by the amount of available plant food so determined. Daubeny's 

 paper was forgotten, but exactly the same line of argument was revived again 

 about twenty years ago, and all over the world investigators began to try to 

 measure the fertility of the soil by determining as ' available ' plant food the 

 phosphoric acid and potash that could be extracted by some weak acid. A large 

 number of different acids were tried, and although a dilute solution of citric 

 acid is at present the most generally accepted solvent I am still of opinion that 

 W3 shall come back to the water charged with carbon dioxide as the only solvent 

 of its kind for which any justification can be found. Whatever solvent, how- 

 ever, is employed to extract from the soil its available plant food, the results fail 

 to determine the fertility of the soil, because we are measuring but one of the 

 factors in plant production, and that often a comparatively minor one. In fact, 

 some investigators — Whitney and his colleagues in the American Department of 

 Agriculture — have gone so far as to suppose that the actual amount of plant food 

 in the soil is a matter of indifference. They argue that as a plant feeds upon the 

 soil water, and as that soil water must be equally saturated with, say, phosphoric 

 acid, whether the soil contains 1,000 lb. or 3,000 lb. per acre of the comparatively 

 insoluble calcium and iron salts of phosphoric acid which occur in the soil, the 

 plant must be under equal conditions as regards phosphoric acid whatever the 

 soil in which it may be grown. This argument is, however, a little more suited 

 to controversy than to real life; it is too fiercely logical for the things them- 

 selves and depends upon various assumptions holding rigorously, whereas we 

 have more reason to believe that they are only imperfect approximations to the 

 truth. Still this view does merit our careful attention, because it insists that 

 the chief factor in plant production must be the supply of water to the plant, 

 and that soils differ from one another far more in their ability to maintain a good 

 supply of water than in the amount of plant food they contain. Even in a 

 climate like our own, which the textbooks describe as ' humid ' and we are apt 

 to call r wet,' the magnitude of our crops is more often limited by want of water 

 than by any other single factor. The same American investigators have more 

 recently engrafted on to their theory another supposition, that the fertility of 

 soil is often determined by excretions from the plants themselves, which thereby 

 poison the land for a renewed growth of the same crop, though the toxin may be 

 harmless to a different plant which follows it in the rotation. This theory had 

 also been examined by Daubeny, and the arguments he advanced against it in 

 1845 are valid to this day. Schreiner has indeed isolated a number of organic 

 substances from soils — di-hydroxystearic acid and picoline-carboxylic acid were 

 the first examples — which he claims to be tlie products of plant growth and 

 toxic to the further growth of the same plants. The evidence of toxicity as 

 determined by water-cultures requires, however, the greatest care in interpreta- 

 tion, and it is very doubtful how far it can be applied to soils with their great 

 power of precipitating or otherwise putting out of action soluble substances 

 with which they may be supplied. Moreover, there are as yet no data to show 

 whether these so-called toxic substances are not normal products of bacterial 

 action upon organic residues in the soil, and as such just as abundant in fertile 

 soils rich in organic matter as in the supposed sterile soils from which they 

 were extracted. 



As then we have failed to base a, theory of fertility on the plant food that we 

 can trace in the soil by analysis let us come back to Mayow and Digby and con- 

 sider again the nitre in the soil, how it is formed and how renewed. Their views 

 of the value of nitrates to the plant were justified when the systematic study of 

 plant-nutrition began, and demonstrated that plants can only obtain their supply 

 of the indispensable element nitrogen when it is presented in the form of a nitrate, 



1910. P P 



