678 



gravity in its downward distribution. The 

 water of the soil holds in solution the food 

 of the plant — those portions at least which 

 are absorbed by the roots. From the 

 leaves of growing plants there is perpetu- 

 ally going on an enormous evaporation. 

 Calculations founded on experiments of 

 Hales and Saussure demonstrate that from 

 an acre of sunflowers, each plant occupy- 

 ing four square feet of ground, there oc- 

 curs during four months growth, the evap- 

 oration of four and a half millions pounds 

 of water. This water con.es from the 

 soil and passes through the plant. All the 

 mineral matters and a portion of the or- 

 ganic bodies, which feed the plant, are 

 carried into it by this water. So long as 

 evaporation goes on from the surface of 

 the soil, so long there is a constant up- 

 ward flow of saline matters. Those por- 

 tions which do not enter vegetation accu- 

 mulate on or near the surface of the 

 ground ; when a rain falls they are washed 

 down again to a certain depth, and thus 

 are kept constantly changing their place 

 with the Waaler which is the vehicle of 

 their distribution. In regions where rain 

 falls periodically or not at all, thi,- upward 

 flow of the soil water often causes an 

 accumulation of salts on the surface of the 

 ground. Thus in Bengal, many soils which 

 in the wet season produce the most luxu- 

 riant crops, during the rainless portion of 

 the year become covered with white crusts 

 of saltpetre. Doubtless the beds of ni- 

 trate of soda that are found in Peru have 

 accumulated in the same manner. So in 

 our w^estern caves, the earth sheltered 

 from rains, is saturated with salts — epsom 

 salts, glauber salts and saltpetre or mix- 

 tures of these. Often the rich soil of 

 gardens is slightly incrusted in this man- 

 ner in our summer weather ; but the sa- 

 line matters are carried into the soil with 

 the next rain. 



It is easy to see how, in a good soil, ca- 

 pillarity thus acts in keeping the roots of 

 plants constantly immersed in a stream of 

 water or moisture that is now ascending, 

 now descending, but never at rest, and 

 how the food of the plant is thus made to 

 circulate around the organs fitted for ab- 

 sorbing it. 



The same causes that maintain this per- 

 petual supply of water and food to the 

 plant, are also efficacious in constantly 

 preparing new^ supplies of food. The ma- 



terials of the soil are always undergoing 

 decomposition, whereby the silica, lime, 

 phosphoric acid, potash, Sec, of the insol- 

 uble fragments of rock, become soluble in 

 water and accessible to the plant. Water 

 charged v/itli carbonic acid and oxygen, is 

 the chief agent in these cheinical changes. 

 The more extensive and rapid the circula- 

 tion of water in the soil, the more matters 

 will be rendered soluble in a given time, 

 and other things being eqlial, the less will 

 the soil be dependent on manures, to keep 

 up its fertility. 



No matter how favorable the structure 

 of the soil may be to the circulation of 

 water in it, no continuous upward move- 

 ment can take place without evaporation. 

 The rapidity of evaporation depends upon 

 several causes, which will be individually 

 noticed. One of the most important is : 



IV. The retentive powers of the soil for 

 water. — The following tables by Schiibler 

 illustrate the peculiarities of different soils 

 in this respect. The first column gives the 

 per cents of water absorbed by the com- 

 pletely dry soil. In these experiments the 

 soils were thoroughly wet with water, the 

 excess allowed to drip off", and the increase 

 of weight determined. In the second col- 

 umn are given the per cents of water that 

 evaporated dating the space of one hour, 

 from the saturated soil spread over a given 

 surface. 



Quartz sand, ... 25 88.4 

 Gvpsum, .... 27 71.7 

 Lime sand, ... 29 75.9 

 Slaty marl, ... 34 68.0 

 Clay soil (sixty per cent clay,) 40 52.0 

 Loam, .... 51 45.7 

 Plough land, ... 52 32.0 

 Heavy clay, (80 per cent clay,) 61 34,9 

 Pure gray clay, ... 70 31.9 

 Fine carbonate of lime, . 85 28.0 

 Garden mould, . . - 89 24.3 

 Humus, . . . . 181 25.5 

 Fine carbonate of magnesia, 256 10.8 



It is obvious that these two columns ex- 

 press nearly the same thing in different 

 ways. The amount of water retained in- 

 creases from quartz sand to magnesia. 

 The rapidity of drying in the air diminish- 

 es in the same direction. 



The want of retentive power for water 

 in the case of coarse sand, is undeniably 

 one of the chief reasons of its unfruitful- 

 ness. The best soils possess a medium 

 retentive power. In them, therefore, are 



