486 NOTES ON THE SOIL. 



if the clay is packed tightly enough, come out at the bottom without 

 any smell of ammonia. The ammonia has been absorbed by the silicate 

 of alumina present in the soil. 



(/) Repeat (e) with (1) dilute solution of caustic potash, (2) dilute 

 solution of phosphoric acid . In each case test the solution with litmus 

 before adding it, and also test the water that comes out at the bottom. 



Limestone (carbonate of lime) may be present as soil-grains or 

 merely as a cementing material to quartz-grains. It supplies plant- 

 food in the form of lime, magnesia, and phosphoric acid. Its action on 

 clay has been mentioned. It also acts as a base with which acids, 

 formed by decay of organic matter in the soil, can unite and thus be 

 made harmless. If such basic material is absent, the soil becomes 

 "sour," through the accumulation of organic acids. Even more im- 

 portant is the part played by carbonate of lime in the process by which 

 certain Bacteria convert into nitric acid the nitrogen existing in 

 organic matter or ammonia compounds ; this process only occurs in a 

 weakly alkaline solution, and the nitric acid unites with the lime. 



Calcareous soils, in which limestone or chalk predominates, are 

 usually greyish in colour. They readily harden and crack when 

 warmed. They are greatly improved by heavy dressings of decayed 

 organic matter, nitrogenous substances, decomposed turf, and dark 

 coloured soil rich in humus. 



Humus, the decaying organic matter of the soil, is of great 

 importance both from its physical and chemical properties. It is a 

 light, bulky substance, with high specific heat, great capacity for 

 holding water, and dark colour. The presence of humus gives the 

 soil a loose, open texture, and makes it able to absorb and retain 

 water. In woods, humus often accumulates to a considerable depth, 

 but in ordinary soils it is only present to a depth of about a yard 

 (below which the roots of few plants penetrate), and this part of the 

 soil has a looser texture and darker colour than the underlying subsoil, 

 which contains no humus. Humus contains from 4 to 9 per cent, of 

 nitrogen, far more than is present in the vegetable matter from which 

 the humus is produced. The conversion of vegetable matter into 

 humus is largely caused by Bacteria and Moulds. 



Microscopic Examination of Soils. Examine some sand, 

 mounted in water, (a) in transmitted light with the mirror, (b) in 

 reflected light without the mirror ; sketch some of the grains. Ex- 

 amine in the same way some clay, and try to distinguish between the 

 grains of quartz (clear), felspar (cloudy), mica (brown), hornblende 

 (black). Examine some leaf-mould, teased out in water, and make out 

 the remains of leaves, twigs, roots, etc. , and the fungus-threads which 

 branch through the decaying vegetable matter. 



