126 WATER- ABSORPTION AND TRANSPIRATION. 



available phosphoric acid, for example, is to treat the soil 

 with a 1 per cent, solution of citric acid, which represents 

 roughly the average acidity of the sap of roots and root-hairs. 



(a) Grow seedlings with their roots resting on wetted blue litmus 

 paper, or dipping into blue litmus solution, and notice the change of 

 colour, due to the acid substances excreted by the root-hairs. 



(b) Grow seedlings in a layer of sawdust or soil resting on a slab of 

 polished limestone. After a week or so, when the roots have reached 

 the slab, remove the latter and examine the surface closely for the 

 tracks eaten into it by the roots. 



(c) To show that roots give out carbon dioxide, it is only necessary 

 to grow seedlings for a short time with their roots dipping into lime- 

 water ; set up a control experiment, with a jar containing lime-water 

 but no plant. 



161. The Soil is the medium in which land-plants may 

 place their roots in such a manner as to enable them to 

 stand erect in the light and air, and it is a storehouse of 

 moisture for the use of plants. The productiveness of a soil 

 depends largely on the amount of water it can hold, and on 

 the readiness and completeness with which plants growing in 

 it can withdraw the water for their use as required. The soil 

 is also a storehouse from which plants get the necessary ash 

 ingredients of their food, the lime, potash, phosphoric acid, 

 etc., which are formed by the breaking-down and solution of 

 the soil-grains. The soil is also a laboratory in which various 

 lowly plants (fungi and bacteria) are at work breaking down 

 dead organic matter, and converting it into nitric acid and other 

 forms in which it becomes available for the use of higher plants. 



Find out all you can, from books on Geology, Physiography, Agri- 

 culture, and Gardening, about the origin of soil ; the work of running 

 water, freezing and thawing, wind, earthworms, and other agencies by 

 whicli soil is formed ; sedentary and transported soils ; the structure 

 of soil ; sand, clay, humus, chalk, loams, marls, and other kinds of soil ; 

 the free, capillary, and hygroscopic water of the soil ; how the water- 

 content of soils is influenced by the nature of the soil, by the rainfall, 

 by the humidity of the air, and by the physiographic factors (altitude, 

 slope, exposure, covering of vegetation) ; the composition of soil-water 

 and of soil-air ; how soil-temperature is influenced ; the specific heats 

 of different soils ; how soil is improved physically (ploughing, harrow- 

 ing, digging, rolling, draining, etc.), and chemically (by manures and 

 fertilisers) ; tilth, tillage, mulching; nitrogenous, phosphatic, potassic, 

 and other fertilisers, natural and "artificial"; the meaning of 

 "rotation of crops" ; the work done by nitrifying and nitrogen-fixing 

 Bacteria. 



