190 



On the Physical Properties of Soil. 



Diminution of the consistency of soil by the penetration of frost. 

 —When soil in its wet state is exposed to the effects of cold in 

 winter, so as to be thoroughly frozen, this circumstance is found 

 to exercise a considerable influence on its consistency ; on being 

 completely dried after this exposure, and submitted to the exami- 

 nation already mentioned for the trial of consistence, the degree 

 of that consistence will be found considerably diminished ; this is 

 more particularly the case with clays and soils of great consist- 

 ence : their firmness becomes diminished nearly one-half by expo- 

 sure to frost : with loamy clay the consistence is reduced from 

 69 to 45 of the scale previously employed, with an ordinary arable 

 soil from 33 to 20. The presence of moisture is essential for 

 the production of this effect, as completely dry earths suffer no 

 change by frost. This phenomenon is to be explained by 

 the crystallization of the water in the interstices of the soil, occa- 

 sioned by freezing, in consequence of which, the several particles 

 of earth become forced from their position, and their points of 

 contact are thus rendered fewer in number. 



The beneficial influence of breaking up the earth<before winter 

 sets in, in order to make it more easy for the frost to penetrate 

 the broken clods, depends on this diminution of consistency oc- 

 casioned by the frost : but if a soil that has thus been rendered 

 lighter by frost is worked in too wet a state in the early part 

 of the spring, the beneficial loosening which had taken place is 

 again lost, since by such working the earthy particles are once 

 more brought into intimate contact : this is the reason why it is 

 of such lasting injury for a soil to be worked while the weather 

 is too wet. The throwing-out, as it is called, of many plants from 

 the ground in changeable winters, when but little snow falls, as so 

 often occurs, in consequence of alternate freezing and thawing, 

 receives its explanation also in this enlargement of bulk occasioned 

 by the frost in the soil — the smaller plants being thus gradually 

 raised up out of the soil, and their upper roots in consequence 

 very often wholly laid bare of earth, and the whole plant on 

 that account destroyed ; plants having the advantages of stronger 

 and more deeply penetrating roots, are consequently far less ex- 

 posed to be thrown out by frost. 



IV. Capability of soils to become more or less speedily dry, or 

 their power of retaining water. — It is a question of considerable 

 importance in vegetation, whether a soil gives up its acquired 

 moisture again to the atmospheric air quickly, or retains possession 

 of it for a long time in its pores. By the following process, this 

 property may be subjected to a comparative examination. We 

 place on a round surface of tin plate, having a raised border, a 

 given quantity of the earth to be examined ; having previously 

 saturated this fully with water, we spread it out evenly, and 



