152 CALIFORNIA FRUITS: HOW TO GROW THEM 



2. By its light, yet bulky mass, it prevents the extremely fine par- 

 ticles of clay soils from running together and becoming cemented, and 

 improves the essential feature of aeration in heavy soils. 



3. Organic matter contains most of the nitrogen found in soils 

 and all plants except the legumes are absolutely dependent on it for 

 their supply of that essential element. 



The greater ease of tillage of soils enriched with organic matter is 

 a matter of universal experience. Besides such a contribution, cover 

 crops may be claimed to confer such specific benefits as these : 



4. Organic matter is the source of energy and the laboratory in 

 which the important beneficial soil bacteria manufacture available plant 

 food and bring about other changes of importance to plants. 



5. Through its decomposition by soil micro-organisms, organic 

 matter acts as a source of carbonic acid and other weak acids which 

 help to dissolve and make available the mineral matter necessary to 

 the growth of plants. 



While growing they probably assist in preventing the radiation of 

 heat from the ground in time of a freeze. They do away with the 

 cultivation of the ground during the winter months. 



Each of these reasons suggests a chapter of discussion and explana- 

 tion which can not be indulged in. Every reader should keep himself 

 up to date in this progressive subject by thoughtful reading of our 

 horticultural journals, proceedings of our horticultural assemblies, 

 and the publications of the University of California Experiment Station. 



