SOILS. 



135 



nitric acid formed; and in leaf -moulds rich in 

 humus the nitrification is sometimes rendered 

 difficult by the lack of such a constituent, which 

 may be either lime, magnesia, or potash. In a 

 great many fertile soils the conveyance of cal- 

 careous matters by the frequent waterings suffices 

 to keep a sufficient richness in lime ; but in leaf- 

 moulds containing an excess of organic matter 

 the bases rendered soluble by nitrification get 

 rapidly used up, and the moulds in consequence 

 become overcharged with acidity to the injury 

 of the growing plants. It is needful in such 

 cases to apply some potash salt, carbonate of 

 potash being preferred (a constituent of wood 

 ashes), which tends to accelerate the nitrification 

 in an extraordinary manner. In some experi- 

 ments carried out in Paris by Georges Truffaut 

 in May and June, 1892, carbonate of potash 

 applied as a manure to leaf-mould was found 

 to increase the nitric acid formed by 9 '15 per 

 cent in one case and by 5*14 per cent in another; 

 while the addition of lime raised the nitrifying 

 power scarcely a quarter per cent. 



In order further to test the nitrifying action, 

 a series of investigations were set on foot with 

 various descriptions of moulds employed in horti- 

 culture. In these experiments nitrification and 

 oxidation were found to be more active and 

 regular in leaf-mould than in peat-mould. 



The Soil and Water. — Since a certain amount 

 of water is necessary for plant life, it is of great 

 consequence that the soil has the power not 

 only of absorbing moisture from the atmosphere, 

 either in the state of dew, rain, snow, or other 

 aqueous deposits, but of retaining this moisture. 

 This power is, however, possessed by some soils 

 to a much greater extent than by others, and 

 in the most fertile soils it is always found to be 

 greatest. 



Schiibler, to whom we owe much important 

 information regarding soils, made an elaborate 

 series of experiments on the absorptive and 

 retentive power of soils and their constituents. 

 He found that after seventy-two hours' exposure 

 to moist air humus had taken up nearly two 

 and a half times as much water as clay, and 

 forty times more than sand, which, under the 

 same circumstances, took up sixteen times less 

 than pure clay. In a second series of experi- 

 ments he placed weighed quantities of the dried 

 soils in funnels, made them perfectly wet by the 

 gradual addition of water, and then left them 

 to drain. As soon as the water ceased to drop 

 from them, the wet masses were carefully 

 weighed, The difference in weight between 

 the dry and wet soils was taken to represent 



the amount of water that they would hold after 

 thorough saturation by long-continued rain. 



The retentive power of the different soils 

 was then determined by exposing the saturated 

 masses for about four hours to a dry atmosphere, 

 having a temperature of 66° Fahr. 



The greater the loss of water experienced 

 under these conditions, the less retentive the 

 soils were shown to be. 



The following are some of the results obtained 

 by Schiibler: — 



Sand, 



Loamy clay, 

 Heavy clay, 

 Pure clay, 

 Eich garden soil, 

 Peaty mould, 



Water Absorbed 



by 100 parts of 



Soil. 



per cent. 

 25 

 40 

 61 

 70 

 96 

 190 



Of 100 lbs. of Water 

 Absorbed, there 



Evaporated in four- 

 hours, at 66 F\ 



lbs. 

 88 

 52 

 35 

 31 

 25 

 21 



These differences are mainly dependent on 

 the mechanical texture or porosity of the soil 

 material. In a soil consisting of solid particles 

 of fairly uniform size, the interspaces are about 

 40 per cent of the volume, whether the particles 

 are large or small; but if the particles are a 

 mixture of large and small (such as gravel 

 and sand), the volume of the interspaces is 

 much diminished. On the other hand, if the 

 particles themselves are porous, as in the case 

 of chalk, loam, and especially of humus, the 

 volume of the interspaces is much increased. 

 It is this volume of the interspaces which 

 determines the amount of water which a soil 

 will contain when perfectly saturated, or the 

 amount of air which it will contain when dry. 



The influence of humus on the capacity of a 

 soil for water is remarkable. The surface soil 

 of the experimental wheat-field at Rothamsted 

 was sampled in January, 1869, when saturated 

 with water; the unmanured land contained 

 32-4 of water per 100 of dry soil: the land 

 manured with farmyard manure for twenty-six 

 years contained 65 '8 of water per 100 of soil. 



Capillar i) Power of Soils. — A series of investi- 

 gations by Zenger show conclusively that the 

 capillary power of soils is greater in proportion 

 as their pores are finer; but fineness of pores 

 must not be confounded with fineness of par- 

 ticles. It is true enough that up to a certain 

 point a soil will have more capillary power in 

 proportion as its particles are more finely 

 divided; but the moment this limit is passed 

 fineness is disadvantageous for capillarity, since 

 the minute particles of earth are apt to cohere 



