530 THE PHYSICAL CONDITION OF SOIL IMPORTANT. 



moisture. When soil is tenacious, or plastic, it absorbs heat 

 slowly, and it parts with its water with great difficulty, as is 

 the case in the London clay ; the number of cultivated plants 

 to which this is suitable is so small that it is almost expelled 

 from gardens, where the object is to expose the cultivated 

 species to conditions more favourable than those afforded 

 them by nature. The small amount of bottom-heat afforded 

 by clay, and the difficulty of draining it, sufficiently explain 

 the badness of its quality for gardening purposes, even without 

 taking into account the resistance experienced by plants in 

 passing their spongioles through so compact a substance. On 

 the other hand, loose sand, whose piirticles have no cohesion, 

 although it imbibes water with great facility, parts with it as 

 readily, and, being easily heated by the sun's rays, becomes so 

 Boon dried up as to be for that reason as unsuitable to most 

 plants as the worst plastic kinds of clay. It is by obtaining 

 a mean between these extremes that the soU is formed most 

 favourable to the growth of plants' in general. 



As has been already stated, the artificial soil of the gardener 

 is for the most part a combination, in various proportions, of 

 loam, sand, and peat. The loam is a source of fertility, and 

 parts with water reluctantly; sand increases porosity; and 

 peat, in addition to any manuring value it may have, at once 

 prevents consolidation, secures free drainage, and enables 

 delicate roots to spread without hindrance. If chalk is seldom 

 or never employed by gardeners, except as a manure, it is 

 because that substance is abundant in garden loam. They 

 know by experience the best loam to be what is called cal- 

 careous, and that they always obtain, if possible. 



It is by no means meant by the author that the whole use 

 of such substances is dependent upon physical qualities ; it 

 is only suggested that their principal mode of action is such 

 as has been described, and that the precise chemical condition 

 of loam, sand, and peat, is of comparative unimportance in 

 horticulture. With agriculture it is otherwise. 



For example, it was once a general belief, and stiU is in some places, 

 that a fruit-tree border could only be made by paring off the surface of 

 an old pasture, and employing the sods after having been laid in a 



