AGRICULTURAL TEXT-BOOK. 29 



101. On these principles, in practical farming, is based the 

 utility of fallowing;, and the rotation of crops. 



102. The mechanical texture of the soil has a strong influence 

 upon its practical fertility; very heavy clays, and very light 

 sands, being both, for opposite reasons, apt to produce badly. 

 The soil in which the particles are the finest, so that the air can 

 enter, and the roots spread without difficulty, is, other things 

 being equal, the best. In clay soils this division of the particles 

 must be produced by the plow and other mechanical means; 

 while in loose sands it is too great, and must be amended by an 

 admixture of clay and other substances. The great fertility of 

 the bottom lands of the Scioto River, Ohio, is attributed to the 

 remarkable comminution of the particles of these soils, 



103. The sub-soil also produces a sensible effect on the con 

 dition of the soil above it. If the soil is clay, it is impervious 

 to water, and if the sub-soil is clay also, it also is impervious to 

 water. The immediate effect of this arrangement is to render 

 both soil and sub-soil habitually wet, and therefore cold, or per 

 haps barren, until evaporation dries first the one and then the 

 other. A retentive sub-soil may render even a porous, sandy or 

 gravelly soil above it habitually wet. A gravelly sub-soil, which 

 is always porous, greatly assists in keeping a clay soil diy. When 

 a porous soil rests on a sandy sub-soil, water can do no injury, 

 while in dry weather, the evaporation from below may do great 

 good. On the other hand, a clay sub-soil retains manure, while 

 a sandy one allows it to leach away, and in practice, renders the 

 soil incapable of improvement. Rock may act either as a re 

 tentive or porous sub-soil, according to its structure. In many 

 clay sub-soils, draining is absolutely necessary. In sandy and 

 gravelly sub-soils, very rarely so. 



103*. There is an important element which must always be 

 taken into account in estimating the value of soils their depth 

 or thickness. In miming & deepish furrow in a cultivated field, 

 we generally distinguish, at a glance, the depth of the super- 



