SCIENCE-GOSSIP. 



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THE SOIL AND THE PLANT. 



By Dr. P. Q. Keegan. 



"TTTITH respect to the general treatment of 

 ^ * agricultural subjects in their relations 

 •with chemistry, it has been objected that every- 

 thing not directly connected with the chemistry 

 of the soil has been pushed into a secondary posi- 

 tion, the metabolism of the plant and the chemical 

 ■changes of its constituents being generally very 

 superficially treated and held of not very great 

 interest. While thoroughly agreeing with this 

 criticism, it may nevertheless be useful to read by 

 way of summary, some of the more interesting 

 results gained in recent years in connection with 

 -the chemistry of the soil and its bearing on the 

 physiology of the plant. 



There is no doubt, for example, that the condi- 

 tion of the soil, which is dependent very consider- 

 ably on its minei-al or chemical components, has a 

 greater influence upon the manner of the develop- 

 ment of the roots of a crop than has any inherent 

 peculiarity of the plants. In fact, the roots adapt 

 themselves to circumstances in a greater degree 

 -than is commonly imagined, and it happens very 

 ■often — nay, very generally — that just as the root is, 

 so is the plant. Again, the roots behave in a soil 

 rich in humus otherwise than in one poor in 

 humus ; they endeavour, as it were, to make the 

 •most of the humus in all directions, and there- 

 fore specially develop a very copiously ramifying 

 ■root-system to meet the circumstances of the case. 

 This being so, there is no absurdity in the remark 

 -that the constitution of the soil in which a plant 

 grows and flourishes' acts, or perhaps reacts, on its 

 anatomy as well as on its physiology ; on the latter 

 probably through the former. This reaction, how- 

 ■ever, seems to be mostly confined to temperate 

 climates, for in the tropics the rampant vegetable 

 igrowth there prevalent at certain seasons does not 

 depend on any inherent fertility of the ground. It 

 is the sun, moist air, and enormously rapid nitri- 

 fication that compensate for the barren sandy soil, 

 rains, heavy dews, and moist winds. Mould is not 

 there formed on the surface, and the ground to a 

 great depth never becomes thoroughly dried up, 

 Although no water ever collects on its upper layers 

 except about swampy places. 



This distinction between growth in the tropics 

 and growth in our temperate latitudes is a very 

 important one. It cannot be ignored by any 

 .student of agriculture. Another distinction is that 

 between the soil and the subsoil, the former being 

 -that which over a depth of four inches is found 

 directly influenced by the vegetable debris of 

 every kind, such as leaves, twigs, fruits, etc., which 

 icover it ; the latter is that which is not perceptibly 



coloured by organic matter, and where roots are 

 found only in small numbers. Sometimes, how- 

 ever, there is very little connection between the 

 soil and the subsoil — one foot deep of clay may 

 rest on a fundamental limestone rock ; or, occasion- 

 ally, a purely argillaceous rock may be covered by 

 a soil which encloses a greater or less quantity of 

 fragments of limestone. With regard to soils, the 

 best classification of them in relation to agri- 

 culture is perhaps that propounded by Professor 

 Deherain — namely, (1) free, (2) argillo-siliceous, 

 (3) argillo-calcareous, (4) argillo-humiferous, (5) 

 clayey, (6) sandy-argillaceous, (7) sandy-calcare- 

 ous, (8) sanjdy-humiferous, (9) sandy, (10) calcare- 

 ous. It will be observed that here the three chief 

 kinds of soils are taken to be clay, sand, and lime ; 

 but only the two former are regarded as modifiers 

 or diluents, so to speak, of the whole three, i.e. 

 lime never agriculturally modifies a clay or a 

 sandy soil. This is quite in accordance with what 

 is known as to the processes undergone in the 

 disintegration and weathering of the solid rocky 

 skeleton of the earth. In fact, some of the difficul- 

 ties which attend the artificial manuring of certain 

 lands would "be explained or obviated if this fact 

 was duly contemplated. In order to understand 

 the nature or capabilities of the soil which the 

 cultivator takes in hand, its origin in a chemical 

 and physical sense is by no means the least im- 

 portant consideration. 



Thus it is useful to be reminded that to the sand 

 springing from the mechanical disaggregation of 

 quartzy or calcareous rocks, to the clay arising 

 from the decomposition of felspathic rocks, to the 

 oxides of iron and manganese issued from basalts 

 or other basic volcanic rocks, there is super-added 

 the residue of the vegetation which follows in their 

 wake. Thus it comes to pass, according to Berthe- 

 lot, that " by means of vegetation and owing to 

 the fixation of free nitrogen by micro-organisms a 

 sandy soil has been converted into a true vegetable 

 soil ; natural clays (kaolins) also fix free nitrogen." 

 The sandy soils of Brittany, furnished with plenty of 

 organic nitrogen, yield immensely increased amounts 

 of root-crops when treated with chalk and lime, 

 with gypsum added, the object of which is to 

 render the nitrates of potass or lime more effec- 

 tive ; but in point of fact alTthat is necessary is to 

 neutralise the acidity of the soil to the depth of a 

 few inches, as a certain proportion of lime tends to 

 decrease the humus. All this is connected with 

 the influence of the nitrates on the physiology and 

 general habit, or facies, of the plant. 



There is no doubt whatever that it is actually 



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