THE WATER-RELATION 



of humus. It is important to realise that the soil-water is mobile, 

 and tends to distribute itself uniformly through a mass of soil 

 that if water is drawn off at any point in the soil there will be a certain 

 compensatory flow of water towards that point. The consequence 

 of this will be that a root can to some extent draw on the whole 

 reservoir of water which the soil in its neighbourhood contains. 

 Dissolved in the soil-water are small quantities of salts such as 

 nitrates, carbonates, phosphates and sulphates of sodium, potassium, 

 calcium, etc., the total concentration being usually less than o-i per 

 cent. 



The study of soils is a science in itself. Only a few general features can 

 be mentioned here. The constituents of a soil fall into several groups. 

 The first group includes those which together form the general framework 

 of the soil. This is composed firstly of the coarser particles of sand and 

 gravel, consisting for the most part of pure silica : secondly, of finer 

 particles forming clay, which consists of silica combined with oxides of iron, 

 aluminium, and other metals. There is no sharp line to be drawn between 

 the former and the latter, and the name " silt " is sometimes applied 

 with varying meaning to material that is intermediate between them, while 

 the name " loam " is used for soils containing more fine clay than sand. 

 These components of the soil all owe their origin to the weathering of rocks 

 of inorganic origin. 



A second group of soil-constituents includes those derived ultimately from 

 organic life. The most important is calcium carbonate, produced by the 

 weathering of chalk and limestone ; this is a regular constituent of loams. 

 It has important effects on the condition of soils, preventing sourness, and 

 making a heavy clay soil more- workable. Lime (calcium oxide) has a similar 

 effect. Another substance of this type is calcium phosphate, which is partly 

 of organic origin, but partly derived from the decay of rock. 



A third group consists of those organic substances collectively called humus 

 (see above), which represent the intermediate products of the decay of plants 

 which have previously grown on the soil. As these die their leaves and other 

 parts are carried down by earth-worms into the soil, and there the materials 

 which compose them are gradually broken down into simpler compounds. 

 This is largely the result of the activities of the micro-organisms of the soil 

 (see below). These simple compounds are then available for absorption by 

 existing plants. There is thus a circulation of material between the soil and 

 the vegetation which it carries : food is extracted from it, but restoration 

 of the substances is made on the decomposition of the plant-body. 



Any ordinary soil contains water in more or less quantity, and being porous, 

 it may be permeated by atmospheric gases. The supply of water comes from 

 rain, or from the sub-soil. It is diminished by evaporation and by drainage, 

 and the water-content of the soil at any moment represents the balance 

 between gain and loss, being liable to constant change. The constitution of 

 the soil, and the nature of its surface are factors which affect it. For instance, 

 sand retains water less than clay : a fine surface broken by rake or hoe checks 



B.B. G 



