18 CIRCULAR 2 51, U. S. DEPARTMENT OF AGRICULTURE 



meager evidence, and for every grower who is of the opinion that 

 manure is benefited by the addition of a certain material there are 

 others who claim to have tried the material without success. This 

 difference in opinion easily might be due to differences in the source 

 of manure used by the different growers or to variation in the yield 

 obtained in different houses because of different combinations of cir- 

 cumstances. Therefore, it would seem to be sound practice for the 

 commercial grower to confine his activities along this line to small 

 experiments until he has convinced himself of the value of the prac- 

 tice or until State experiment stations or the Department of Agri- 

 culture have published convincing experimental evidence. 



SPAWNING AND CASING 



The spawn is inserted in the beds as soon as the temperature in the 

 compost has receded to 75° F. Most spawn makers advocate using 

 spawn pieces about one-half as large as a hen's egg and spacing them 

 about 8 inches to 1 foot apart in the bed. An imperial quart bottle of 

 spawn supplies about 35 to 40 such spawn pieces. The spawn is 

 removed from the bottle by breaking the bottle and the pieces are in- 

 serted in the bed about iy 2 inches from the surface. After the spawn 

 is inserted the temperature of the bed is usually maintained between 

 70° and 75° for a week or 10 days to insure a good " catch " of spawn. 

 It is then dropped slowly until the desired cropping temperature is 

 reached. The mycelium undoubtedly grows most vigorously at about 

 75° in both sterilized and unsterilized compost (fig. 11), but in most 

 cases it is not desirable to maintain a temperature as high as this for 

 more than 10 days because of the increased insect activity at the 

 higher temperature and the tendency for the beds to dry out 

 excessively. 



The casing of the beds consists simply of spreading soil about 1 inch 

 thick over the beds from two weeks to a month after spawning. It 

 is evident from the widespread distribution of the industry that 

 suitable casing soil may be found in most sections of the United 

 States. One of the primary characteristics of soil for casing is its 

 water-holding capacity. A good casing soil will tenaciously retain 

 water and dry out uniformly without cracking or forming a crust 

 on the surface. Successful growers believe that neutral clay loam 

 soils with a fair amount of organic matter make the best casing 

 soil. Subsoil and acid sandy soils are avoided and the practice of 

 neutralizing soil with lime has gained favor in recent years. More 

 specific recommendations than these must await systematic experi- 

 ments under controlled conditions. 



Many growers purchase and maintain small fields especially for 

 casing soil. After stripping the upper 6 inches of a part of the field 

 for casing soil they pass on to other parts in succeeding years. Mean- 

 while the subsoil of the stripped portion is built up by liming if 

 necessary and by growing a green cover crop to add humus to the 

 soil. A rotation of this kind is usually planned so that an interval 

 of four or five years elapses before soil is taken a second time from 

 any part of the field. It is important to make sure a field for casing 

 soil is not contaminated with the fungus causing the " Mycogone " 

 or " bubbles " disease of mushrooms. The accepted practice is to 

 avoid all fields that have been previously fertilized with spent mush- 



