NOTES ON THE BOIL, 487 



Classification of Soils. The different kinds of 

 soils may be grouped under certain generalised headings 

 according to the amounts of chalk, sand, clay, and humus 

 they contain. Loams contain 40 to 70 per cent, of clay 

 mixed mainly with sand. If only from 10 to 40 per cent, of 

 clay is present a sandy loam results, and below 10 per cent, 

 of clay the soil is merely sandy. A loam containing 70 to 85 

 per cent, of clay is a clayey loam, and above this is a strong 

 clay soil. The latter when containing up to 20 per cent, of 

 chalk are termed marl, and if more than this they are termed 

 calcareous marls. Similarly we have calcareous sand, gravelly 

 loam, chalk loam, etc., while any soil containing over 20 per 

 cent, of organic matter may be termed a humus soil inde- 

 pendently of its other ingredients. 



Soil-Air. The spaces left between the soil-grains 

 form capillary tubes extending in all directions. These 

 capillary spaces are occupied largely by air, if the soil is in 

 good physical condition and not water-logged. Great harm 

 is done to plants by over- watering ; house-plants are more 

 often killed by this than by anything else, and plants in pots 

 should be watered from below (why ?). In general, the soil 

 should not contain more than half its total water- holding 

 capacity, i.e. about half of the capillary spaces in the soil 

 should be occupied by air. 



The presence of air, containing oxygen, in the soil is 

 obviously essential if healthy growth of seedlings and roots is 

 to take place ; moreover, the air in the soil must be kept in 

 circulation. Good ventilation is as necessary for germinating 

 seeds, and for the roots of plants, as it is for the life and 

 health of human beings. Not only is free oxygen in the soil 

 required by germinating seeds and for roots, which must 

 breathe, but many processes occur in the soil that depend on 

 the presence of oxygen. Nitrogen is required for the use of 

 Bacteria, which fix free nitrogen gas and make nitrates, and 

 these Bacteria, as well as others which convert the nitrogen 

 of humus, manure, and other decaying organic matter into 

 nitric acid, require large amounts of oxygen in order to do 

 this work. 



The exchanges of gases between soil and atmosphere are 

 brought about in several ways e.g. the slow process of 



