36 



MISCELLANEOUS PUBLICATION 1271, U.S. DEPARTMENT OF AGRICULTURE 



ment is most difficult as pointed out and discussed 

 by Thomas and Ronningen (69). 



In order to observe plants in the field while 

 maintaining some control over the soil environ- 

 ment, researchers have installed controlled tem- 

 perature tanks outside (14, 28). Mack and Evans 

 (42) and MacLeod and Decker (43) have used 

 heat exchangers to modify soil temperatures in 

 the field. This paper deals with use of the Mary- 

 land facility to study soil temperature effects on 

 perennial forage species (43). 



Materials And Methods 



The Maryland plant-climate facility is located 

 in an open field on a Sassafras silt loam soil. 

 A schematic is shown in figure 1. It consists of 

 a chilled water source, three tempering tanks 

 equipped with electric heaters, zone valves, and 

 circulating pumps. Tempered water circulates 



from tempering tanks to heat exchangers placed 

 at a soil depth of 9 cm. to provide three replica- 

 tions for each temperature plot (1.83 by 4.57 m.). 

 In and out temperature changes were usually less 

 than 3° C, and controls were set to bracket target 

 temperatures of 10°, 21°, and 32°. A fourth t?eat- 

 ment was ambient temperature. Shading was used 

 in some studies to vary light intensity, and indi- 

 vidual plot irrigators were available. Field lay- 

 out with shades and irrigators is shown in figure 

 2. 



Average soil temperature 8 cm. under a sod at 

 College -Park, Md., April 1, approximates 10°C. 

 This temperature was essentially maintained 

 throughout the growing season at the 10° C treat- 

 ment. It took about one week to reach an equi- 

 librium temperature of 21° or 32° in the spring, 

 and a similar time to cool the soil to 10° in 

 midsummer. Typical temperature differentials 



SYSTEM 



— e — 



P^CKAGE 

 CHILLER 



1 "- 



1 



" 



1 





< 



, 1 





1 1 





1 









~* M 









1 . ' 







1 





1 4 1 



IWSERT 



CHILLED W»TER PUMP 

 (TkCO T5) 



Figure 1. — Schematic of field plot soil temperature control system (43) 



