November 30, 1899] 



NATURE 



117 



The minimum is 3 per cent. The difference, 1-05 per cent., 

 is the measure of the •' mineral-hunger " of the plant, and 

 represents the mineral substance which does not perform any 

 special function. This excess of > mineral substance may be 

 supplied in the form of some indifferent substance, such as 

 silica. The observation is of considerable interest to the 

 farmer, for it shows that it is not economical to manure crops 

 with pure substances. 



II. Soils and Manures. 



Having ascertained in general what substances are necessary 

 as plant-lood, the agricultural chemist has next to apply this 

 general information to the manuring of soils which are more or 

 less deficient in certain ingredients. It has been found, un- 

 fortunately, that the chemical analysis of a soil is of little use as 

 ' guide unless accompanied by what may be termed a 

 ■ mechanical analysis," by which is meant chiefly a determination 

 : the amount of finely-divided constituents present in the soil, 

 ii is only the finely-divided earth which presents a sufficiently 

 large surface for the exercise of the solvent action of the water 

 and its dissolved carbonic acid. There is one case, however, 

 in which chemical analysis alone is of the greatest importance' 

 viz. when only traces of some necessary element are present in 

 a soil. Here there is no question of the need for a manure 

 containing this substance. 



If, on the other hand, large quantities of an element are 

 present, it does not follow that there is a sufficiency in the soil 

 even when the latter is in a satisfactory state of division, for 

 the substance in question may be present in an insoluble or 

 refractory form. This is commonly the case with nitrogen, 

 which exists in the soil chiefly in the form of a mixture of 

 indefinite nitrogenous substances known as humus, or mould. 

 These substances sometimes easily give up their nitrogen to 

 plants, but in other cases are very refractory. The uncertainty 

 as to their action is indeed so great that certain peaty soils are 

 known which consist almost entirely of humus, but contain, 

 nevertheless, an insufficiency of available nitrogen. 



Phosphoric acid affords another illustration. The soluble 

 phosphoric acid of the manure is absorbed by the soil as 

 dicalcic phosphate, which is comparatively easily soluble in the 

 soil-water. With time, however, it may change in the soil to 

 the insoluble tricalcium phosphate, or even to iron or aluminium 

 phosphates, which are still less soluble. 



In the case of calcium, chemical analysis has been found to 

 Ije of considerable service in determining what manuring is 

 required, since calcium is chiefly valuable in the form of 

 carbonate or humate, and these are easily estimated in the soil. 

 Since then the direct method of soil-analysis is an insuffi- 

 cient guide to manuring, it is fortunate that chemists have been 

 able to develop successfully an indirect method. This is the 

 cultivation method, by which plants are allowed to grow in the 

 soil under examination, after taking care to provide a sufficiency 

 of all plant-food stuffs except the one, e.g. phosphoric acid, whose 

 presence in available form is being tested. The plants are then 

 analysed, and the results compared with the analyses of the same 

 plants grown on soils provided with all the necessary plant-food 

 stuffs. As an important result of the method it has been found 

 that different plants take up very different quantities of the same 

 mineral substances. On this is largely based the system of 

 rotation of crops, where the second crop is so chosen that it 

 chiefly removes the ingredients of the soil which have been left 

 by the preceding crop. 



With the aid of the cultivation method it has also been 

 possible to draw up the following table, which represents the 

 relative values of the different nitrogen compounds for plant- 

 food. 



Nitrogen of Saltpetre 100 



,, ,, Ammonia ... ... ... 85-90 



,, ,, Albumen 60 



This table may be made use of in determining the nitrogen value 

 of a manure. 



The cultivation method may be used for testing the value of 

 manures of all kinds. Thus it was by a few cultivation experi- 

 ments that Wagner in Darmstadt first showed the very great 

 value for agricultural purposes of the " Thomas " Slag, pro- 

 duced as a bye-product in the manufacture of iron by the basic 

 process of Thomas Gilchrist. The million tons of phosphate 

 meal annually produced in Germany is now wholly utili.sed by 



NO. 1570, VOL. 61] 



the agriculturist, and its preparation for the fanner has become 

 an important offshoot of the iron industry. 



Similarly the demonstratibn by the cultivation method of the 

 value of potash salts in manures has given an enormous impetus 

 to the potash industry. 



Speaking generally, the method gives us complete control 

 over the fertility of a soil in so far as this depends on manuring. 

 One consequence of this has been that our views as to the value 

 of agricultural land have completely changed, for whereas 

 formerly sandy soils were generally considered poor, they are 

 now, by means of a system of intelligently directed manuring, 

 made to give yields which are scarcely inferior to those of the 

 best soils. The beet-sugar industry, which formerly could only 

 be conducted in the best soils, has now been extended with 

 marked success to sandy soils. 



III. Artificial Selection. 

 It might seem that with a perfect knowledge of the manuring 

 of plants, the need for further investigation would cease, for 

 when we have learned exactly what each plant requires to attain 

 its highest development, we have reached a certain limit. The 

 supply of excessive nourishment is a disadvantage, and only 

 tends to produce sick plants. 



There still remains, however, a method by which the fertility 

 of plants may be increased far beyond the limit which nature 

 appears to have fixed. This is the method of artificial selection-., 

 which has been applied in Germany on the most approved, 

 scientific principles. German agriculture would have long since - 

 broken down under the stress of foreign competition had it not 

 been for the perfect technology of its agriculturists. As an-, 

 example the sugar-beet may be quoted. This plant contained 

 originally but a small amount of sugar, and could only be used 

 as a source of sugar when the price of the latter was very high. 

 With the fall in price came the urgent need for increasing the 

 percentage of sugar in the beet-root. This was effected by 

 utilising the fact that sugar- richness is hereditary, so that by 

 selecting artificially the roots richest in sugar, getting seed from 

 these, planting the seed, again selecting the richest roots, and' 

 so on, a race of plants is at length obtained in which a high 

 percentage of sugar is normal.^ Accordingly the producers of 

 beet-root seed in Germany have erected great laboratories in 

 which the percentage of sugar in the roots is carefully determined. 

 By applying the principle of artificial selection with regard alsa 

 to the form and size of leaf and the purity of the sap, it has beea 

 found possible to improve the roots from year to year, so that 

 now beet-sugar can easily hold its own against cane-sugar, and 

 is indeed cheaper than flour, costing as it does in (Germany less 

 than a penny a pound. 



Similar success has attended the efforts to increase the crops 

 of different kinds of grain. The improvement in malt-barley 

 has been specially marked. 



It has been found that plants which have been highly culti- 

 vated by artificial selection easily lose their acquired characters 

 when they are exposed to unfavourable conditions of cultivation ;; 

 and this has led to many exact investigations, conducted for the 

 most part in Germany, during the last ten years, on the chemistry 

 of plants. The most interesting of these trace the chemical 

 history of nitrogen as it passes from the atmosphere to the soil, 

 then into the substance of plants, and finally back into the 

 atmosphere. 



The corresponding cycle for carbon has long been known. 

 Most plants assimilate nitrogen only in the form of com- 

 pounds. As, however, the total quantity of nitrogen compounds 

 in the atmosphere is comparatively small, there must be some 

 other source of nitrogen for plants. Now the cla.ssical researches 

 of Hellriegel have shown that there is one class of plants, the 

 Leguminosae, or nitrogen collectors, which are able to assim- 

 ilate elementary nitrogen and so to leave a soil in which they 

 have been grown richer in nitrogen compounds. It has been, 

 found that the power of acting as nitrogen collectors is always 

 associated with the presence of micro-organisms on the roots, 

 and that the assimilation of the nitrogen is in some way not 

 understood due to the micro-organisms. The recognition of 

 the power of leguminous plants to act as nitrogen collectors is 

 manifestly of great practical importance, for it shows clearly 

 that the best rotation of crops is one in which a leguminous 

 crop is followed by one of nitrogen consumers, i.e. plants 

 which cannot assimilate nitrogen directly. 



' See Kew Bulletin, 1897, pp. 317, 318. 



