154 THE TEMPERATURE OF THE SOIL [chap. 



The accompanying curves (Fig. 14) show the daily 

 maxima and minima of the air temperatures of 4 feet 

 from the ground for a few days before and after the 

 occurrence of a disastrous late spring frost in May 1902. 

 One station, A, was in a river meadow about 100 feet 

 above sea-level, the second, B, was on a terrace about 

 50 feet higher and half a mile away ; the third, C, was on 

 the flat summit of the down, 500 feet above sea-level 

 and about \\ mile from the first station. It will be 

 seen that the river-side station gave always the highest 

 maxima during the period and the lowest minima, 

 showing on the one occasion n of frost. 



Early and Late Soils. 



From all the considerations developed above it will 

 be seen that an early soil is essentially a coarse-textured 

 and well-drained one. Such a soil retains little water, 

 thus possessing a low specific heat, and is easily warmed; 

 at the same time the low water content gives rise to 

 less evaporation at the surface, and this great cause of 

 cooling is minimised. The dryness of the soil permits 

 of early cultivation, which, by cutting off the access of 

 subsoil water and diminishing the conduction of heat 

 from the surface, quickens the warming up of the 

 seed bed. The early aeration and warming of the soil 

 promotes the nitrification which is also necessary to 

 growth. If, further, the soil be stony, the conduction of 

 heat from the surface layer into the soil is more rapid, 

 solids being better conductors than powders. Such 

 soils, again, are generally dark coloured, because on the 

 comparatively small surface exposed by the coarse 

 grains the same proportion of humus has a greater 

 colouring effect. 



These conditions are generally fulfilled by the 

 alluvial soils bordering the larger rivers ; in the 



