322 ECOLOGY 



therefore, for vigorous plant-growth, must contain these 

 necessary constituents, and it is from plant-remains in the 

 soil that much of this food is derived ; hence the greater 

 fertility of the surface soil. But surface soils vary consider- 

 ably both in physical properties and chemical constitution, 

 according to the proportions of sand, clay, lime, humus, or 

 organic matter they contain. 



Samples of different soils should be obtained and the 

 properties of the different constituents studied. By means 

 of a few simple experiments many important facts may be 

 discovered. 



Take a little garden-soil and weigh out 10 grammes. 

 Spread it out to dry for a few days at the temperature of 

 the room, and weigh again. 1 How much has been lost in 

 drying ? Now place the soil on a tin lid, heat it for a short 

 time at iooC, and weigh again. Has more been lost ? 

 Finally, burn the soil and weigh again. How much has 

 been lost by burning ? 



The part that burns away is organic matter, chiefly de- 

 caying remains of plants. Note the change in colour after 

 burning ; the dark soil has become ' terra cotta '. Ten 

 grammes of garden soil when air-dried lost 2-56 grammes 

 of water, and a further 0-33 when heated to ioo C, and 

 lost o-88 of organic matter on burning. What is the pro- 

 portion of water to the humus in the soil ? Determine 

 this proportion in different soils. 



Put some garden -soil into a jar of water and stir 

 thoroughly. Note the floating fragments of humus. Pour 

 off the muddy water into a large vessel and repeat the 

 washing until the water clears quickly. Allow the muddy 

 water in the large vessel to stand for a few days and notice 

 how long it takes to settle. Pour off the clear water and 



1 The water-content will vary even in the same soil, being greater 

 on a wet than on a fine day. 



