52 VEGETABLE FORCING 
Color.—Black soils are usually more fertile than light- 
colored soils, although there are many exceptions. The 
color of the soil is of greater importance in the forcing of 
vegetables than it is in the production of crops in the open 
ground. This is due to the great power of dark soils to 
absorb the heat rays of the sun, thus reducing the amount 
of fuel required to maintain proper temperatures. Black 
soils are also good radiators; the heat absorbed during the 
day radiates throughout the night. The advantage of 
heat gained in this way is particularly noticeable in the 
management of coldframes. How much ofa factor it is in 
the heating of greenhouses has not been determined, but 
it must be of considerable importance, especially when a 
large proportion of the ground is not shaded by plants. 
The absorption of heat accelerates chemical activities in 
the soil and also has some influence upon the soil’s 
physical properties. 
Organic content.—All classes of cultivators have long 
recognized the value of a liberal quantity of soil organic 
matter. Of the various factors which contribute to plant 
growth, this, with the exception of water, is unquestion- 
ably the most important. The organic matter furnishes 
plant food; secures better aeration; promotes chemical 
activities; improves physical properties; darkens the 
color; increases the water-holding power; supplies the 
best conditions for the work of friendly bacteria; in- 
creases the rapidity of water absorption; favors root 
penetration; and reduces the cost of tillage operations. 
No class of soils, except the mucks, contains such large 
amounts of organic matter as do greenhouse soils which 
have been used for many years in producigg vegetables 
for commercial purposes. 
Water content.—Greenhouse soils are generally quite 
constant in moisture content because water is applied 
whenever needed. See page 149, which relates to watering. 
Chemical composition—As previously stated in this 
