488 NOTES ON THE SOIL, 



diffusion ; expansion and contraction of soil-air due to changes 

 in temperature and in barometric pressure. 



The air contained in the soil is not enclosed, but is con- 

 stantly being renewed by diffusion from the air above. Since 

 the processes going on in soil consist largely of oxidation, or 

 are accompanied by oxidation, the soil-air is poorer in oxygen 

 than the air above. In general, the percentages of oxygen 

 and carbon dioxide together equal 21, but that of oxygen 

 varies from 10 to 20 per cent., that of carbon dioxide from 1 

 to 10 per cent. 



(a) To find out the best percentage of water and of air in the soil for 

 any plant, take five tumblers filled with soil. Add to A 16 c.cs. of water 

 each day ; to B, 8 c.cs. ; to C, 4 c.cs. ; to D, 2 c.cs. ; to E, 1 c.c. Plant 

 the same number of seeds (small seeds like Wheat or Cress are best) 

 in each tumbler. After a few weeks notice that those receiving too 

 much water do not grow well because of lack of air in the soil ; those 

 receiving too little water will suffer from drought. Select the tumbler 

 which gives the best result, and find out how much water the soil con- 

 tains by weighing a sample of the soil (after stirring- it up), then 

 drying it and weighing it again. To find out how much air this soil 

 contains, put the tube of a thistle -headed funnel to the bottom of the 

 soil in the tumbler, and from a graduated vessel pour in water till it 

 stands level with the surface of the soil. Since the water displaces the 

 air in the soil, we may consider the volume of water poured into the 

 soil as representing roughly the volume of air it contained ; compare 

 this with the volume of the soil. 



(b) Get two glass jars of the same inside diameter. Pour water into 

 one jar until it is rather less than half full, and mark its level. Mark a 

 corresponding level on the other jar, which is to be kept quite dry 

 inside, and put dry soil into this jar until it reaches the mark. We 

 have now equal volumes of water and of soil: Pour the soil slowly 

 into the water ; if the soil contains no air, it will make the level of 

 the water rise to twice its original height, hence the amount by 

 which it fails to do this measures the volume of air, in the soil. 

 Try this simple experiment with different soils (sand, chalk, clay, 

 loam), and find the percentage volume of air in each soil. 



(c) Tie a piece of muslin over one end of a lamp-chimney, fill the 

 chimney with soil, and set it in a basin. Pour water into the basin, 

 and notice its rise in the soil. Are the spaces between the soil-grains 

 filled with air or with water at the beginning of the experiment ? Try 

 different soils, comparing the rise of water in them. 



(d) Spread out some moist soil or fine gravel in a thin layer on a 

 sheet of paper ; notice the glistening appearance of the grains. As 

 the soil dries the grains become dull in appearance ; why ? In moist 

 soil the water is in the form of films around the soil-grains, and the 

 spaces between the grains are filled with air ; only in quite wet soil are 

 these spaces filled with water. 



