492 NOTES ON THE SOIL. 



(4) The chief physiographic factors of a habitat are alti- 

 tude, exposure, and slope. The two former act indirectly on 

 the water-content, by their influence on humidity, which has 

 just been discussed. The chief direct effect of slope is in 

 controlling the amount of water which runs off and that 

 which enters the soil, the amount of rainfall lost by run- off 

 increasing with the angle of the slope. 



To compare the absorption of water by different kinds of 

 soil, take 100 grammes of gravel, sand, rich loam (good garden or field 

 soil), leaf-mould, and dry, powdered leaves, and put each sample into 

 a wide tube e.g. a lamp-chimney closed at the lower end by a bored 

 cork ; a glass funnel will do instead. Hold each tube or funnel in 

 turn over a tumbler, and pour into it a litre of water ; then measure 

 the water that runs out at the bottom in each case. Which sample 

 absorbs and retains most water, and therefore allows least water to 

 run through ? In an experiment like this, the weight in grammes of the 

 water absorbed by ] 00 grammes of each dry sample was : gravel, 5 ; 

 coarse sand, 10 ; fine sand, 30 ; barren sandy soil, 35 ; clay, 55 ; rich 

 loam, 70 ; leaf -mould, 220 ; leaves, 500. These results show clearly 

 that the water-absorbing capacity of soils is greatly increased by the 

 presence of vegetable matter. 



To compare the evaporation of water from different kinds 



of soil, take 10 tumblers and fill them to within an inch of the top 

 with the following materials : 1, gravel ; 2, sand ; 3, sandy soil ; 4, 5, 6, 

 loam (garden soil) ; 7, gravel, covered by an inch of sand, with an inch 

 of loam on top ; 8, loam covered by an inch of powdered leaf -mould ; 

 9, powdered leaf-mould ; 10, water. The materials in tumblers 1 to 9 

 should be well dried, then an equal amount of water poured into each. 

 Keep the loam in 5 loose at the top by stirring it to a depth of about 

 an inch, and, after adding the water to it, place on the surface an inch 

 of dry, sifted soil. Stir the loam in 6 thoroughly after adding water, 

 and notice that the loam dries in hard lumps after this treatment. 

 Weigh each of the tumblers, and place them together in a position out 

 of direct sunlight. Weigh each day, and record the loss of weight of 

 each due to evaporation of water. 



Composition of Soil- Water. Soil-water contains 

 various dissolved substances e.g. carbonate of lime ; phos- 

 phate of lime ; sodium chloride (common salt) ; magnesium 

 sulphate (Epsom salt) ; sulphate of lime (gypsum) ; silicates 

 of lime, etc. ; nitrate of potash (saltpetre) ; compounds of 

 iron, etc. 



It has been calculated that no less than 100 tons of mineral 

 matter is annually removed in solution from every square 



