156 INSECT PESTS 



ground healthy and the plants vigorous in growth and 

 capable of resisting the attacks of pests or diseases. On 

 light land little drainage is required, and its texture will 

 be much improved by the tipping of clay on it, an opera- 

 tion, however, which is not a financial proposition on a 

 large scale except in special circumstances. Plentiful 

 dressings of dung will be wanted on all sandy soils, whereas 

 in the case of clay, soot and lime should be given the 

 preference, the former as a plant food, and the latter 

 (ordinary quicklime) being used as a disintegrator to 

 lighten the groimd. 



We have now seen how variety of soil is given to a 

 country by reason of the many changes in its geological 

 history, and how soil itself is gradually formed under 

 the action of wind and weather and the accumulation in 

 it of the organic remains of the countless generations of 

 plants and animals living on its surface. Soils generally 

 may be classified into five main divisions, viz. sand, clay, 

 loam, peat and chalk. Peat is almost entirely decayed 

 vegetable matter. (See Plate 39.) 



Sand is the least fertile of all soils, if left to itself, 

 and consist of the heavier water-washed particles of 

 alluvial deposits. These particles, rounded as they are 

 like so many pebbles on a small scale, are too loosely held 

 together to retain moisture or afford firm root-hold for 

 the plants. 



Clay consists of the minutest particles of former rocks 

 which have been carried away in suspension and deposited 

 as mud at the mouths of old rivers. The smallness of 

 these particles may be easily demonstrated by putting 

 a Uttle clay into a glass of water and noticing how long 

 it remains discoloured. Had we tried clean sand it 

 would have become transparent again at once. 



This failure to precipitate readily in the case of china 

 clay is made use of at a kaohn works, where the clay and 

 water, termed " milk," is run through pipes to a central 

 group of tanks, whence after the process of solidification, 



