44 TEXTBOOK OF BOTANY 



grasses may still grow well. In cool temperate climates other species of 

 bluegrass ( Poa trivialis and Poa annua ) are frequently planted in shaded 

 parts of lawns. These two species of bluegrass do not survive on hot dry 

 soils during the summer months. Kentucky bluegrass is also limited in 

 this respect. In the Southern States one may expect to find Kentucky 

 bluegrass as a dominant plant in lawns only at high elevations or where 

 the surface temperature of the soil is reduced by partial shade. The 

 characteristic lawn grasses of these states are Bermuda grass during the 

 summer months and Italian rye grass during the winter months. Near 

 the Gulf borders carpet grass, centipede grass, and St. Augustine grass 

 dominate the lawns. 



The above facts about lawns and pastures sufficiently demonstrate 

 how the composition of a plant community is influenced by the climatic 

 and soil factors, and also by the effects of plants on each other. They 

 also illustrate that certain biological facts and principles may be ap- 

 plicable in theory and practice in any part of the world. On the other 

 hand, there are many facts and principles that cannot be generally 

 applied because of local and regional diversity in soil, climate, and avail- 

 able species. 



Owing to the great diversity in cultivated plants and in environments, 

 at least one agricultural experiment station with several substations is 

 maintained in every state of the Union, and the effects of local differ- 

 ences are continuously investigated. Anyone who wishes to supplement 

 his own observations on lawns or other economic plant communities will 

 find valuable assistance in the weed manuals and other publications of 

 the nearest experiment station. 



Bulletins on lawn-making frequently contain information about cer- 

 tain animals that destroy lawn plants or bring about changes in the 

 composition of plant communities. Recent publications are the most 

 desirable because some of the earlier recommendations for the care of 

 lawns have been revised as new facts were discovered. For instance, field 

 tests have shown that the traditional method of applying lime to all grass- 

 lands not only is unnecessary but may even bring about undesirable 

 results in certain types of soils. Many owners enjoy making their own 

 tests by applying various fertilizers to small strips in the lawn. Such tests 

 are reliable, inexpensive, and a source of valuable information from 

 year to year. Salts valuable as lawn fertilizers^ are often mixed with 



^ In experiment station records the amounts and kinds of these salts tliat should be 

 added to the soil are usually indicated by a brief statement such as 1 lb. of 10-6-4 fer- 

 tihzer mixture per 100 sq. ft. The abbre\iation 10-6-4 refers to the percentages of 

 nitrogen, phosphoric acid, and potash respecti\ ely in a fertilizer mixture. 



