ii6 THE MOVEMENTS OF SOIL WATER [CHAP. 



the quantity of water they retain. Only a light, coarse- 

 grained soil, possessing a good natural drainage, can free 

 itself quickly from the winter rainfall, and therefore can 

 warm up quickly ; but whether such a soil will also be an 

 early one, depends also upon its situation, aspect, and a 

 few minor factors like colour. The heating of a soil 

 is practically wholly due to the sun ; although, if a soil is 

 made very rich in fresh organic matter, like farmyard 

 manure, there will be a little heating of the soil owing 

 to the rapid decomposition of organic matter that ensues. 

 In this case the soil has been to some degree converted 

 into a hot bed ; but putting this rare factor aside, the 

 colour of the soil has a certain influence upon the com- 

 pleteness with which the sun's rays are absorbed and 

 converted into heat within the soil. Black soils will be 

 the most effective, red soils come next. One practical 

 application of this fact lies in the use of soot as a top- 

 dressing for wheat in the early spring ; the soot contains 

 a little ammonia which acts as manure, it also checks 

 the attacks of the slugs and small snails which often do 

 damage to the young corn, but it has a further beneficial 

 action, because its colour enables the soil to absorb more 

 heat, enough in fact to raise the temperature by one or 

 two degrees on a sunshiny day. A very simple experi- 

 ment will demonstrate this : a small plot of ground, a few 

 yards square will suffice, must be brought into a state 

 of good uniform tilth in March or April, and one-half 

 should be lightly dusted over with soot, so as to blacken 

 the surface, without protecting it by any such coating as 

 would act as a mulch. Two ordinary thin-stemmed 

 thermometers (which should be checked against one 

 another beforehand), are then plunged in the soil, one 

 in each portion, care being taken to bury the bulbs at 

 exactly the same depth of 3 inches and to make the 

 soil firm round them. On a bright day of continuous 



