116 



as the wetness of the soil increases. Bearing this fact in mind it will 

 be more easily understood how it is that the fineness of the soil influ- 

 encfs the capacity of a soil for retaining moisture. In other words, the 

 finer the soil the more particles, and the more particles the more films 

 of water. This is what is meant by saying that the amonnt of water 

 permanently held by a soil is determined by the amount of surface ex- 

 posed by the particles, i.e.y the extent of the internal surface. It follows 

 then thbt gravel and coarse sand will be the soils that retain least water, 

 while most vrill be retained by fine silts and clay. 



There is however one other factor influenting this property of the 

 soil and that is the porosity of the individual particles. It is quite 

 evident that a soil will be able to hold all the more water if its parti- 

 cles are themselves pore us, as in the case of chalk and humus. Here 

 then we have one of the many reasons why the addition of animal or 

 yegetable matter improves the soil Soils rich in humus are much 

 better able to withstand prolonged drought than those poor in this 

 constituent, because they are able both t ) absorb more water and also 

 to hold it more firmly. A soil that has long been cropped without any 

 return in the form of" humus will be found to have lost much of its 

 water-holding capacity. The fertility of a poor sandy soil can be appre- 

 ciably improved by ploughing in green crops, such as th- velvet bean or 

 other leguminous plants, or by a liberal application of stable manure. 



As it is our aim to see how the agriculturist can keep the moisture 

 in his soil, it is necessary to consider the question of the movements of 

 Trater in soils which result in the loss of soil water. There are three 

 chief ways in which the soil L ses water (i) by percolation (ii) by eva- 

 poiation (iii) by ^he transpiration of ph nts. 



(i.) hercolalicn. This process is the natural sinking of waiter in the 

 soil due to the porosity of the latter. The surface soil is perfectly 

 saturated only after h- avy rain ; for as soon as the rain ceases, water 

 ccmmences to sink owing to the ordinary action of gravity. The films 

 of water around the particles at the surface get thinner and thinner, 

 while an increasing number of the diy particles below becomes sur- 

 rounded by the films, the water distributing itself by passing 

 from one particle to the next. The water that sinks down in this way 

 may be peimanently lost to the crop by draining right away through, 

 cracks and crevic( s in the rock to the streams and rivers, or it may 

 simply sink down till it meets an impervious bed in the subsoil and 

 there lemain to be brought up again for the use of the crop in a man- 

 ner that will be discussed later. The former case needs serious consi- 

 deration, for rot only dees it rob the soil of water but also of a large 

 amount of soluble and active plant food. Every film of water will 

 have dissolved a little plant food from the soil particle with which it 

 was in contact, consequently each drop of water that escapes by drain- 

 age will take away with it a little soluble plant food. Let us consider 

 the case of the sinkirg of lime as an istnance. 



Many intelligent practical agriculturists are inclined to scofF, 

 -when told their lands want liming, end exclaim that that is absurd 

 because the soil rests on a limestone formation. They do not realise 

 that it is quite possible to have a soil deficient in lime resting on a 

 limestone foimation. Qhis is often un&tisfactorily explained by the 

 statement that lime sinks. But this movement is not peculiar to the 



