SOIL FOR GREENHOUSES 181 



decaying leaf tissue is present are most valuable from the 

 plant-grower's viewpoint, for such soils are not only rich 

 in various food elements which stimulate plant growth, 

 but they render the soil porous and at the same time in- 

 crease the water-holding capacity. 



151. Value of soils from sod. Sod land, especially 

 pasture sod, is filled with fine, fibrous roots of grasses. 

 Continual cropping by animals develops the area of root 

 surface, and because of its large fiber contents, pasture 

 sod is considered preferable to sod from mowings. Prob- 

 ably no other agent gives greater life or " vitality " to the 

 soil than does a large amount of root fiber in various stages 

 of decomposition. 



152. Muck and peat soils. Many low lands have 

 been the dwelling place of mosses and other low forms 

 of plant life. These have died, and gradually the soil 

 has become filled with their decaying materials. These 

 soils the flower-grower knows as " peat soils," and they 

 are most valuable for certain types of florists' plants, 

 particularly orchids. Peat is not particularly rich in 

 available plant food, but because of its porosity, it makes 

 an excellent medium for plants which require a thor- 

 oughly aerated soil. Muck, wiiich represents a more ad- 

 vanced stage of decay, is less commonly used by florists. 

 Often it is filled with organic acids which render it " sour " 

 and unsuited for plant growth. If it is thoroughly com- 

 posted with other soils, however, it becomes a most valu- 

 able soil medium for greenhouse plants. 



The organic matter in soils varies much in its chemical 

 composition. It is composed of many carbon compounds, 

 into which enter nitrogen and all the mineral elements 

 which are necessary for plant growth. 



153. Chemical compounds in soils. In discussing 



