A perfect soil would be such a blend of sand, clay, chalk and humus as would contain sufficient clay and humus to prevent drought, enough sand to render it pervious to fresh air and prevent waterlogging, chalk enough to correct the tendency to acidity of the humus present, and would have within it various substances which would serve as food-materials to the crops.
Hymenomycetes are a very large group containing over 11,000 species, most of which live in soil rich in humus or on fallen wood or stems, a few only being parasites.
This tendency to destroy organic matter makes the repeated application of lime a pernicious practice, especially on land which contains little humus to begin with.
This is especially true of the flood plains where the annual inundations prevent the formation of humus and retard forest growth.
Mineral salts, especially calcium carbonate, often rich in acidic humous compounds, and characterized by oak and birch woods, siliceous pasture, and heaths with much acidic humus in the sandy soil.
It is best adapted for application to clays and fen lands and should not be practised on shallow light sands or gravelly soils, since the humus so necessary for the fertility of such areas is reduced too much and the soil rendered too porous and liable to suffer from drought.
If the burningup of humus and the leaching of the soil could be prevented, there is no reason whyia cotton soil should not produce good crops continuously for an indefinite time.
Some two hundred species of flowering plants struggle for a precarious existence in the tundra region, the frozen ground and the want of humus militating against them more than the want of warmth.
The general surface of the interior highland consists of bare rolling moor-like country, with a great amount of red claylike soil, while the valleys have a rich humus of bluish-black alluvium.
For the carrying on of their functions they all need to be supplied with carbohydrates or other carbon compounds which they obtain ordinarily from humus and plant residues in the soil, or possibly in some instances from carbohydrates manufactured by minute green algae with which they live in close union.