30 MASS. EXPERIMENT STATION BULLETIN 347 



Soil Temperature as an Important Ecological Factor in Greenhouses. (L. H. 



Jones.) Investigations indicate that the real value of air temperature control 

 in greenhouses may lie in its effect on soil temperature, for soil temperature has a 

 greater influence on plant development than has air temperature. Knowledge of 

 factors affecting soil temperature would lead to changes in practice that should 

 maintain the desired soil temperature. There is a lag in the rise and fall of soil 

 temperature as the air temperature is maintained high or low, which may be as 

 much as four hours in benches and considerably longer in ground beds, and 

 depends not only on the depth of soil, but also on the amount of moisture in the 

 soil. There are two available instruments for measuring soil temperature. One 

 is a soil thermometer similar to ordinar\' air thermometers, except that the bulb 

 is protected by a pointed, perforated metal cap which is thrust into the soil. 

 The other is a recording soil thermometer. The first gives the reading only at the 

 time it is observed. The recording type reproduces all values on a chart for 

 the duration of a week. An effort is being made to construct a maximum-minimum 

 thermometer for soil work. Such an instrument will be of greater value than the 

 type which gives no record of changes and will be less expensive than the record- 

 ing type. 



Root-System Development of Hardened Plants. (L. H. Jones.) Potted gardenia 

 and rose plants which had become hardened by lack of nitrogen and frequent 

 drouth conditions were slow to develop new roots and shoots when transplanted 

 to a rich soil that was kept moist. However, when the hardened plants were 

 watered with a dilute solution of ammonium sulfate (one ounce per gallon) five 

 days before transplanting, there was a much greater increase in the number of 

 roots and length of shoot development than when the plants did not have the 

 nitrogen applied before transplanting. The results obtained from these tests 

 indicate that there is an advantage for reestablishment of plants in applying 

 nitrogen before the root system is disturbed. 



Study of Diseases of Ornamental Herbaceous Plants Caused by Soil-Infesting 

 Organisms, with Particular Attention to Control Measures. (VV. L. Doran.) 

 Work under this project has involved a* search for better methods for using old 

 soil disinfectants as well as other chemicals, and for possible new soil disinfectants, 

 which may have more nearly ideal qualities. The soil used was either naturally 

 infested or artificially inoculated with Pythium and Rhizoctonia. Rhizoctonia 

 solani Kuhn. used for test purposes during the past season was isolated from 

 Mentha Reqiiienii Benth. and Cerastium tomentosum L. 



Undiluted vinegar, 200 to 235 cc. per sq. ft., has continued to give satisfactory 

 control of damping-off. Worked into or mixed with the soil immediately before 

 seeding, this has usually been safe enough although crucifers seem to be less 

 tolerant of acetic acid (of which vinegar contains about 4.0 percent) than are the 

 other plants used. Crucifers are less tolerant of formaldehyde also and the point 

 is of some importance, for the family includes a number of the most popular 

 ornamental plants: for example, species of Arabis, Cheiranthus, Hesperis, Mathiola, 

 Aubrietia, Draba, Alyssum and Iberis. Treatments especially adapted to them 

 are now being developed. 



Other things being equal, the species most subject to injury by acetic acid or 

 formaldehyde, applied to soil immediately before seeding, were those the seeds 

 of which germinate most promptly. 



About 8 cc. of acetic acid (80 percent), diluted to 300 cc. (per sq. ft. of soil), 

 did not injure even crucifers when seeds were sown immediately after it was 

 worked into the soil. By the prevention of some pre-emergence damping-off, 



