160 



KNOWLEDGE. 



[July 2, 1900. 



and fewer, and therefore liave a smaller sui-face area. 

 An excess of sand therefore renders a soil dry, and its 

 vegetation suffers in a diy season ; if, on the other hand, 

 clay is too abundant the soil holds so much water that 

 it is rendered cold, and being badly aerated becomes 

 acidf and is not well suited to the growth of roots. A 

 soil well adaj)ted to support a thriving vegetation strikes 

 the happy mean between these two extremes, and may 

 have such a comjDosition as the following: — J 



Sand ... ... from 50 to 70 per cent. 



Clay „ 20 to 30 „ 



Lime ... ... „ 5 to 10 „ 



Humus ... ... ,, 5 to 10 ,, 



The water thus absorbed by the soil holds in solution 

 various acids formed during the decay of vegetable 

 remains and Cai'bon dioxide produced in the same pro- 

 cess and also given oflF by growing roots. § It is therefore 

 a slightly acid liquid w-hich dissolves such of the mineral 

 constituents of the soil as are soluble and renders them 

 accessible to the roots of plants. How great is its solvent 

 action may be imagined w'hen we consider that in a 

 cubic foot of soil it is in contact with 50,000 square feet 

 of mineral surface. 



A Vertical Section through Soil, showing the external cells of a 

 root (e) giving off root-hairs (A, A'). The dark angular masses are 

 soil-particles, each surrounded by a film of water (indicated by 

 concentric hues). The light patches (a, j^, y, etc.) are air-bubbles. 

 (After Sachs. Keproduced from Pfeffer's " Physiology of Plants " 

 — Eng. Trans. — by permission of the Clarendon Press.) 



The constituents of the soil then are inorganic and 

 organic particles, water and air. The particles are siu-- 

 rounded by films of water which sejjarate them from 

 their neighbours. The wat-er so held is in communication 

 over wide areas, and as its dissolved contents are re- 

 moved at one point by roots or other agents the 

 deficiency is made good from the suiTounding area. A 

 plant therefore does not necessarily obtain the whole of 

 its mineral food supply from the soil in contact with its 

 roots, but is able to draw supplies from a wide area, 

 the transport being eflFected by the soil-w-ater. Air- 

 bubbles entangled among the particles and their films 

 supply the Oxygen necessary for the respiration of the 

 roots : it is to be noted that this subterranean air differs 



t The normal decay of organic bodies is interrupted on account of 

 lack of oxygen ; instead, therefore, of the simpler ultimate products 

 of decomposition, there are produced complex vegetable acids. 



X Freani. " Soils and their Properties. " 



§ Knowibbgb, May, 1900, p. 102, footnote §. 



considerably in composition from that above ground in 

 that it contains a larger proportion of Carbon dioxide 

 derived from the decay of vegetable matter in the soil. 



The water which percolates through the soil is still 

 further affected by the remarkable power which the 

 latter possesses of withdrawing from solution certain 

 substances which ai-e dissolved in it. If some garden 

 soil be placed in a funnel and a water-solution of common 

 salt (sodium chloride) poured over it, the water which 

 runs away contains less salt than the original solution. 

 Some of it has been absorbed (or " fixed ") by the soil. 

 This property of the soil has been constantly used in 

 obtaining drinking water from impure sources. When 

 the Egyptian forces were besieging Ccesar in Alexandria 

 (b.c. 47), they fouled the wells of the city with sea water. 

 In the emergency, C<esar caused pits to be dug in the 

 sandy beach and the water which oozed into them from 

 the sea was " not altogether unfit for drinking. ''|| Bacon 

 relating this incident says,1[ " Caesar mistooke the Cause; 

 For he thought that all Sea-Sands had Xaturall Springs 

 of Fresh Water. But it is plaine, that it is the Sea- 

 Water; because the Pit filleth according to the Measure 

 of the Tide : and the Sea-Water passing or Straining 

 thorow the Sands, leauth the Saltnesse." This is a 

 classical example of the fixation of dissolved substances 

 by the soil, and is additionally interesting in that it 

 received the notice of the great Elizabethan philosopher. 

 Otherwise, however, it is not so good an illustration as 

 might be wished, for sand is less powerful in absorbing 

 substances from solution than almost any other soil ; 

 and of the substances which are removed from their 

 solutions by soil, common salt is affected to a much less 

 extent than are many other mineral compounds. Humus 

 and clay soils possess this property in a very high degree, 

 and in comparison with them the power of absorption 

 possessed by sand is very small indeed. Potash, Ammonia, 

 and Phosphoric Acid, and compounds containing them, 

 are removed from their solutions by the soil to a much 

 larger extent than are any other substances. Magnesia, 

 Soda and Lime are also absorbed to some extent, while 

 Sulphates, Chlorides and Nitrates are very slightly or 

 not at all affected. 



Nothing like a complete explanation of these interesting 

 facts is at present forthcoming. The absorption of a salt 

 from its solution by the soil is due to more or less compli 

 Gated chemical or physical changes the nature of which 

 is but little known, and indeed need not here be con- 

 sidered. The fact which is of importance as regards 

 the food supply of a plant is that certain substances, 

 in particular Potash, Ammonia and Phosphoric Acid, 

 important constituents of the mineral food of plants, 

 axe taken ujj by the soil from their solutions. In what 

 form they are stored in the soil is not known but it is 

 certain that in their " fixed " state they form a reserve 

 supply. The soil-water, as we have seen, is a weak 

 solution of the mineral food substances ; as a small 

 quantity of any of these is removed by the roots the 

 deficiency is made good by a corresponding amount of 

 the " fixed ' substance becoming again freely soluble 

 and in a condition to be taken up by the plant. 



Over other substances, notably Nitrates, the soil has 

 little or no control, and these are carried away by the 

 drainage water out of the sphere of influence of the roots 

 of land-plants. The existence of Nitrates in any quan- 

 tity in any but an exceedingly dry soil is therefore an 

 impossibility, a fact which leads to the consideration of 



II Merivale. " History of the Romans under the Empire," II. 

 (1873), 319. IT "'Sylva Sylvanmi " (1628), 1. 



