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Clay soil. A mixture of a large proportion of clay with sand, 
silt, and humus. Likely to be supplied with potash but lacking 
in phosphoric acid. Heavy, sticky, difficult. Suitable for wheat 
and corn. 
Muck. Large amount of humus mixed with sand and clay ; 
dark brown or black. 
III. Soil Fertility 
A fertile soil will provide for roots three things : 
{a) The right conditions of moisture, heat, and air. These will 
depend largely on the texture of the soil. 
(^) Opportunity for the growth of certain living organisms in 
the soil. The most necessary are the nitrogen-fixing bacteria. 
{C) Plant food. 
IV. Soil Feeding 
Plants require seven elements : nitrogen, phosphorus, potas- 
sium, magnesium, calcium, sulphur, and iron. 
The last four are in every soil sufficiently abundant, but nitro- 
gen, phosphorus, and potassium need to be made available or sup- 
plied artificially. 
V. Sources of Food 
There are two sources of food : manures and commercial fertil- 
izers. Farm manures include barnyard manure and green manure. 
Barnyard manures are "complete manures.” They contain all the 
necessary elements of plant foods ; they improve the texture ; they 
yield heat. Green manures furnish humus. They return to the soil 
food that has been incorporated into the plant through its roots 
from the depths of the subsoil. A cowpea root can be traced to 
the depth of sixty-one inches. Nitrogen is supplied by certain 
green crops, such as cowpeas, beans, clover, and other legumes. 
Investigations at the Louisiana Experiment Station have shown 
that one acre of cowpeas turned under gives to the soil nearly 
