SAND, CLAY, AND HUMUS 103 
much older than the Ice Age, often of a gravelly or 
clayey nature, occupy the ground, and from these the 
present soils are derived. 
There is another constituent of soils of primary 
importance for vegetable life, which results from the 
decay of the generations of plants which have gone 
before. When plants die, their bodies are decomposed 
by the agency of bacteria. Some of the constituents 
pass off as gas or water, but there remains an amount 
of solid matter (humus) which mixes with the soil 
and is of the utmost importance for plant growth. 
Nitrogen, which forms the greater part of the atmo- 
sphere, cannot in the gaseous state be absorbed by 
plants, although they spend their lives surrounded by 
it. It is a necessary substance in the plant’s economy, 
and through the action of soil bacteria, which change 
the nitrogenous matter in humus into soluble nitrates, 
plants are able to utilize this store. 
The ordinary soils of our fields may be defined as a 
mixture of sand, clay, and humus. A soil which is too 
rich, or too poor, in any one of the three will support 
plant life with difficulty. 
The roots of plants require also a due amount of 
both water and air if they are to fulfil their functions 
adequately. An examination of the minute structure 
of the soil shows that it consists of angular particles 
of very various size—the larger ones classed as sand 
and consisting largely of silica; the smaller, which 
decrease in size beyond the limits of microscopic 
vision, mainly of clay (silicates) and humus. A film of 
water clings round each particle, and between the 
particles the chinks are filled with air. For healthy 
plant growth a nice balance between these constitu- 
