154 LECTURES ON 



than we can discover with the best optical aids, because the pores of 

 the roots of plants are not discernible by any microscope. The 

 mineral matters of the soil must be dissolved or diffused in water. 

 The rapidity of their solution is in direct proportion to the extent of 

 their surface. The finer the particles, the more abundantly will the 

 plant be supplied with its necessary nourishment. In the Scioto 

 valley soils the water which is transpired by the crops comes in con- 

 tact with such an extent of surface that it is able to dissolve the soil- 

 ingredients in as large quantity and as rapidly as the crop requires. 

 In the coarse-grained soils this is not the case. Soluble matters 

 (manures) must be applied to them by the farmer, or his crops refuse 

 to yield handsomely. 



It is furthermore obvious, that, other things being equal, the 

 finer the particles of the soil the more space the growing roots have 

 in which to expand themselves, and the more numerously are they 

 able to present their absorbent surfaces to the supplies which the 

 soil contains. 



It will presently appear that other very important properties of the 

 soil are more or less related to its state of mechanical division. 



The soil has, secondly, a 'power of ivithdr uiving from the air vapor of 

 water and condensing the same in its pores. It is, in other words, 

 hygroscopic. 



This property of a soil is of the utmost agricultural importance, 

 because, 1st, it is connected with the permanent moisture which is 

 necessary to vegetable existence, and, 2d, since the absorption of 

 water-vapor determines the absorption of other vapors and gases. 



In the following table from Schiibler we have the results of a 

 series of experiments carried out by that philosopher for the pur- 

 pose of determining the absorptive power of different kinds of earths 

 and soils. 



The column of figures gives in thousandths the quantity of moisture 

 absorbed by the previously dried soil, under the same circumstances, 

 in twenty-four hours: 



Quartz sand, coarse 



Gypsum 1 



Lime sand 3 



Plough land 23 



Clay soil, (60 per cent, clay) 28 



Slaty marl 33 



Loam 35 



Fine carbonate of lime 35 



Heavy clay soil, (80 per cent, clay) 41 



Garden mould, (7 per cent, humus) 52 



Pure clay 49 



Carbonate of magnesia, (fine powder) 82 



Humus 120 







An obvious practical result follows from the facts expressed in the 

 above table, viz : that sandy soils which have little attractive force 

 for watery vapor, and are therefore dry and arid, may be meliorated 

 in this respect by admixture with clay, or better with humus, as 



