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 of a 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 organ'c 

 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 producing 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 



