102 



KNOWLEDGE. 



[May 1, 1900. 



The most abundant constituent of almost all soils is 

 Silica, of which quartz is a well-known form ; as a rule it 

 constitutes from 70 to 90 per cent of their bulk. Its 

 preponderance is due to its insoluble and indestnictible 

 character, for, unlike most of the other constituents of 

 rocks, it« composition is quite unaltered by long ex- 

 posure to the atmosphere. It exists in the soil in the 

 pure state and in innumerable combinations with other 

 elements known as silicates. The latter are nearly all 

 as insoluble in water as is Silica itself. Silica and the 

 insoluble silicates cannot be absorbed by the roots of a 

 plant, for these are only able to take in liquids and 

 substances held in solution. The greater bulk of the 

 soil — viz., that jiart of it consisting of Silica or insoluble 

 silicates — is therefore unable to contribute directly to the 

 mineral food supply of plants. Every solid particle is in- 

 directly concerned in the supply of water to the roots, in 

 the absence of which no food can enter them, and there- 

 fore even the innutritions Silica holds an important post 

 in the plant commissariat. In addition to Silica, nearly 

 all soils contain small proportions at least of the essential 

 substances named above, and these must all be accessible 

 to the roots of a growing plant. Very small quantities 

 of the latter will suffice, because they are all dissolved 

 in the soil-water, which is a means of transporting them 

 from any area containing them in abundance to another 

 where any of them may be deficient; another reasou is 

 that the amounts actually required by growing plants are 

 remarkably small. The following figures will bear out 

 this statement, at least for the plant they refer to. It 

 has been found that to produce 100 grammes (dry 

 weight) of the Oat plant, it must bo fed with the follow- 

 ing quantities of the essential mineral substances: — f 



Phosphoric Ai'id 



Potash 



Lime 



Magnesia 



Sulphuric Aeiil 



Total . . . 



Grammes. 

 0-5 

 0-8 

 0-25 

 0-20 

 0-20 



1-95 



These amounts are sufficient to enable the plants to 

 grow. As a matter of fact, 100 grammes of the Oat 

 plant contain, as a rule, about 3 grammes of mineral 

 matter ; in other words, the plant absorbs about half as 

 much again as is necessary to support its growth. This 

 extra quantity can to some extent be supplied by 

 mineral matters other than those constituting the food 

 proper. With this exception the plant refuses to recog- 

 nise any attempt to replace the needful substances of its 

 mineral food by compounds containing other mineral 

 elements. If one essential substance is absent, or is 

 present in too small quantity, then not only does the 

 plant absorb a less quantity of that which is deficient, 

 but also of all the other constituents of its food. In 

 fact, a lack of one of these ingredients has the same effect 

 as if the whole food sujjply were wanting in the same 

 degree. As one imperfect wheel throws a watch out of 

 gear so a shortness in the supply of one food constituent 

 upsets the nutrition of a plant. It seems as if the 

 appetite of the plant must be partly satisfied by definite 

 quantities of certain mineral components — those we 

 have called " essential " — under no circumstances replac- 

 able by others ; at a certain stage it becomes less 

 fastidious, and although it demands a further supply of 

 mineral food it is able to select it from a wider range of 

 substances. 



+ Emil Wolff, (juoted by Maercker ; Berichte. 1897. 



Lime is one of the most important constituents of 

 soils. " A limestone country is a rich country," as the 

 saying has it. The plant uses it not only as food, but 

 also as medicine, or, rather, poison antidote, an 

 edifying fact which will be noticed again. One of the 

 numerous parts played by Lime in the soil is interesting 

 as bearing upon the absorption of soluble silicates by 

 plants. Silicate of Potassium, one of the very few 

 silicates soluble in water, is present in most Clay soils 

 and absorbed by roots. Excess of Silica has an injurious 

 effect on plants. In the presence of certain compounds 

 of Calcium — Lime, for example — Potassium silicate is 

 decomposed, resulting in a Silicate of Calcium, insoluble, 

 and the Silica it contains is therefore unable to enter 

 the plant. Lime is, therefore, a very valuable con- 

 stituent of Clay soils, since it prevents an undue amount 

 of Silica being absorbed by the plants grown upon them. 

 In some of its forms Lime is very soluble in water, and 

 when present in the soil may be continually washed out 

 by rain ; to such an extent does this occur that the 

 soil even of a limestone country frequently contains very 

 little of it. A remarkable illustration of this is fur- 

 nished by the soil of the Bermuda Islands. The Coral 

 and Shell Limestone of these islands contains 54.5 per 

 cent, by weight of Lime ; during the disintegration of 

 the rock, so much lime is carried away in solution that 

 it constitutes less than 4 per cent, of the soil. J 



Since differences in chemical composition do not, as 

 a rule, explain the varying degrees of fertility possessed 

 by soils, to what then are they due ? They must be 

 ascribed to three principal causes hitherto very in- 

 completely studied. There is, in the first place, the 

 condition in which the food constituents are met with 

 in the soil. In a fertile soil they are presented to the 

 roots in sufficient quantity and in a soluble form in which 

 they are easily absorbed. On the other hand, they may 

 be so locked up that the plant is unable to extract as 

 much as it requires in the time at its disposal — that is, 

 during its growing period. This is often caused either 

 by the soil-particles being insufficiently comminuted or 

 by the insoluble nature of the compounds of which the 

 essential elements fonn a part. Roots are of course 

 made up of cells, and, as we have seen, plant cells are 

 so constructed that only fluid materials can enter them. 

 Plants, therefore, can obtain no mineral substances from 

 the soil which are not soluble in the soil water, containing 

 Carbon dioxide and other acids derived from living roots 

 and from decaying vegetable matter. § If the essential 

 mineral ingredients are present in the soil in such a 

 state that they can be neither decomposed nor dissolved 

 by the soil water, assisted by the acids it contains, they 

 are of no use to the plant. The bairenness of Granite 

 crags is well known to all who have visited them in 

 Scotland or elsewhere ; and yet Granite contains all the 

 mineral food that a plant needs. It is composed of throe 

 important minerals (see Fig.), Quartz (a). Felspar (b), 

 and Mica (c). 



Felspar and Mica contain the mineral constituents of 

 plant food, but in Granite these minerals are enclosed 

 in a network of Quartz, the insolubility of which has 

 already been noticed. As long as the network is entire 

 the nutritious constituents of the Felspai' and Mica ai-e 

 so locked up that plants cannot get at them. When the 



J "Eoclis, Rock-weathering and Soils." Merrill, p. 359. 



§ It was formerly stated that the solution of mineral substances in 

 the soil was largely due to tlie action of acids set free by living roots. 

 It is now believed, liowever, that CO, and, in a few cases, the acid 

 Phosphate of Potassium, are the only solvents which roots evolve. 



