66 PLANT PRODUCTS 



air. When air rises from the plains to the hills it expands, 

 and in expanding loses heat. The wind, therefore, rising 

 from the plains to the hills, cools the tops of the hills. 



In cold climates the removal of superfluous water by 

 drainage is of great value in maintaining the temperature of 

 the soil. Hoeing and harrowing also assist in this direction, 

 and the use of any kind of mulch eifects the same purpose. 

 In hot climates irrigation not merely supplies water, but also 

 lowers the temperature. Very shallow ploughing, harrowing, 

 and hoeing make the surface a relatively bad conductor of 

 heat, and, therefoie, prevent the penetration of solar heat. 



Colour of Soils. — Dark-coloured soils absorb and radiate 

 more heat than light-coloured soils. In hot climates some 

 of the black soils show very striking variations between the 

 temperatures at 2 p.m. and 4 a.m., as is well known to those 

 who camp out on them. In damper climates the black soils 

 are oiten visited by mist and fog. On the general average 

 the black soils will have a higher temperature than light 

 soils, since at night they will protect themselves from cooling 

 by a local blanket of fog. Dark soils will accumulate more 

 dew than the light soils, and are generally regarded with 

 favour. The origin of the dark colour may be somewhat 

 varied. It is most frequently due to organic matter, either 

 produced by natural accumulations or by deliberate addition 

 of organic manures. In gardens, in the vicinity of towns, 

 black colour is often due partly to soot and cinders. The 

 real source of the colour of the Indian black cotton soils has 

 been much disputed. A red colour is generally due to ferric 

 hydrate, a blue colour to iron in a lower stage of oxidation. 



Conduction of Heat. — Air is a bad conductor, and, 

 although silica is not a particularly good one, it is relatively 

 better than air. Compact soils conduct heat best, and will 

 vary in temperature most. Superficial tillage is, therefore, 

 advantageous. Observations under experimental conditions 

 at Cockle Park for very many years prove that cultivated 

 soils show less variation in temperature than untilled land. 

 The best conductors of all are moist gravels, which type of 

 soil produces the earliest crops. Deep down in the subsoil 



