eee ROOT 189 
so wilting ensues. Therefore not all the water that is in the 
soil is available. The percentage of water that is left in a soil 
when a plant wilts is called the wilting coefficient of that soil. 
This wilting coefficient varies with different soils, being higher 
with fine soil than with coarse. soils. It also varies with the rate 
of evaporation, as a plant will wilt with more water in a soil 
when the rate of transpiration is high than when it is low. 
Effects of cultivation. Cultivation destroys weeds and renders 
the soil more porous, so that it is better aérated, water is ab- 
sorbed readily, and roots can penetrate it easily. As previously 
pointed out, cultivation of the surface layers may decrease the 
amount of water lost from the soil through evaporation. 
Humus. Soils contain varying quantities of dark-colored de- 
caying organic matter known as humus. It is owing to the 
presence of humus in soil that the soil is usually darker colored 
than the subsoil. As a source of nitrogen, humus is a very 
important constituent of soils. It is also valuable in other 
ways. Humus has great capacity for absorbing water and so 
may have great value in increasing the water-holding capacity 
of a soil. Clay soil is rendered more porous by the presence 
of humus. 
Organisms in the soil. The decay of the organic matter in 
the soil is due to the action of living organisms, chiefly bac- 
teria and fungi, including the vegetative parts of such forms as 
mushrooms. 
Bacteria are very important in changing organic nitrogen 
into a form that can be absorbed and used by green plants. 
By bacterial action ammonia is produced from organic com- 
pounds of nitrogen, and the ammonia is then converted into 
nitrates. 
An important class of bacteria is that which has the property 
of absorbing free nitrogen and uniting it with other elements to 
form compounds. Such bacteria add nitrogen to the soil and are 
of great advantage in the maintenance of soil fertility. 
