288 WHAT RENDERS A SOIL EERTILE. 



each 11 per cent of humic acid, besides a much larger proportion of in- 

 soluble organic matter. It is obvious, therefore, that the fertility of a 

 soil is not dependent upon its containing this or that proportion of vege- 

 table matter, either in a soluble or an insoluble form. It is certaiflly 

 true that many very fertile soils do contain a considerable quantity of 

 organic matter, in a form in which it may readily yield nourishment to 

 the roots of plants. Yet such soils are not fertile merely in consequence 

 of the presence of this organic matter, as a source o{ organic food to the 

 plant. It may be present, and yet the soils, like those above-mentioned, 

 may remain barren. Where soils become fertile apparently by the 

 long accumulation of such vegetable matter in the soil, it is not merely 

 because of the increase of pujely organic substances, such as the humic 

 and ulmic acids, but, because, as I have already had occasion to mention 

 to you, the decaying vegetable matter which produces them contains 

 also, and yields to the soil, a considerable abundance of some of those 

 inorganic substances which plants necessarily require. The organic 

 matter is an indication of their presence in such soils. But they may 

 be present without the organic matter. They may either be duly pro- 

 portioned in the soil by nature — or they may be artificially mixed with 

 it, and then this use of the organic matter may be dispensed with. It is 

 of more importance to bear this in mind, because not only vegetable 

 physiologists, but some zealous chemists also, have laid great stress upon 

 the quantity of soluble and insoluble organic matter contained in a soil, 

 and have been led to consider it as a safe index of the relative fertility 

 of different soils. 



The history of science shows, by many examples, that those men 

 who adopt extreme views, — who attempt to explain all phenomena of a 

 given kind, by reference to a single specific cause — have ever been of 

 very great use in the advancement of certain knowledge. Their argu- 

 ments, whether well or ill founded, lead to discussion, to further investi- 

 gation, to the discovery of exceptional cases, and, finally, to the general 

 adoption of modified views which recognise the action of each special 

 cause in certain special cases, but all in subordination to some more ge- 

 neral principle. 



Thus, if some ascribe the fertility of the soil to the presence of the 

 alkalies in great abundance, others to that of the phosphates, others to 

 that of lime, others to that of alumina, and others, finally, to that of ve- 

 getable matter in a soluble state — all these extreme opinions are recon- 

 ciled, and their partial truths recognised, in one general principle, that 

 a soil to he fertile must contain all the substances which the plant we de- 

 sire to grow can only obtain from the soil, and in such abundance as 

 readily to supply all its wards ; while at the same time it must contain 

 nothing hurtful to vegetable life. 



III. SOILS CAPABLE OF IMPROVEMENT BT THE ADDITION OF 



MINERAL MATTER. 



On the principle above stated depends in very many cases the mode 

 of improving soils by the addition of mineral substances, as well as the 

 method of explaining the remarkable effects occasionally pcoduce* by 

 their mixture with the land. The following analyses wUi place this 

 matter in a clearer light : — 



