98 THE BOOK OF NATURE STUDY 



reaching the ground, while their roots, which extend sometimes 

 to a distance horizontally of twenty or thirty feet, abstract 

 large quantities of water and plant food from the area embraced 

 by them. The garden should therefore be so situated that it is 

 not nearer than the distance mentioned to forest trees of con- 

 siderable size. 



The best soil for a garden is a medium loam, that is to say, 

 a soil containing about 50 per cent, of soil particles intermediate 

 in size between the large grains (sand) and the very fine ones 

 (clay), which possess a diameter of only from one-hundredth 

 to one-thousandth of a millimetre. A sandy soil is characterised 

 by its warmth, porosity, feeble power of holding capillary water, 

 inability to " bind/' and deficiency of mineral salts. While, 

 therefore, on the one hand it is easy to work and produces early 

 crops, it tends on the other hand to become parched, and soluble 

 plant food is readily washed through it into the subsoil. A 

 clay soil is cold, tenacious, and heavy in working ; water passes 

 through it only very slowly. Owing to the fineness of its con- 

 stituent particles it is always moist, at any rate just below the 

 surface. It absorbs and retains certain mineral matter from 

 aqueous solutions. Roughly speaking, very light soils are satis- 

 factory only in moist seasons, and very heavy soils only in fairly 

 dry ones. To improve the water-holding capacity of a light soil, 

 plenty of decayed vegetable matter should be worked into it, 

 and the same substance only slightly decayed will, if worked into 

 a heavy soil, tend to provide air spaces and thus make it more 

 porous. Heavy soils are also as a rule improved by liming. 



The same considerations as those mentioned apply to the 

 underlying rock. Soils situated on sand, gravel, or chalk are 

 warm and well drained, those resting on clay are cold and apt 

 to become waterlogged. Hence where the underlying soil is 

 composed of clay the garden must be drained. The main drain, 

 at a depth of three and a half feet, should run down the centre 

 of the garden and into a water course ; the subsidiary drains 

 should be at a depth of three feet. The accompanying sketch 

 (Fig. 46) shows the arrangement. If the expense is considered 

 too great a main drain at a depth of two and a half feet, with 

 subsidiary drains at a depth of two feet may be laid, but this 



