68 



The two horizon sequences were established by placing either 

 './ashed, quartz sand or the dried, screened, subsurface soil in the 

 bottom of the pipe. Distilled water was added to settle the soil and 

 moisten it to near field capacity (1/3 bar suction) . The amount of 

 soil or sand added was predetermined so that a final depth of approxi- 

 mately 24 cm was achieved. The moistened soil was allowed to 

 equilibrate for 3 days to ensure uniform moisture distribution. 



The treated, surface soil was weighed and placed above the sub- 

 surface layer so that the final depth, after wetting, would also be 

 approximately 24 cm. Distilled water was added to moisten the surface 

 soil slightly beyond field capacity. The soil-filled tubes were 

 arranged on a center, greenhouse bench and randomized within soil 

 series. A schematic of one experimental unit is shown in Fig. 6. 



After 1 week of equilibration, all tubes were planted on 5 Febru- 

 ary 1980 to a sorghum x sudangrass hybrid (Sorghum Sudanese (Piper) 

 stapf 'Dekalb SX16A'). After emergence, seedlings were thinned to 

 allow eight per pot (tube) to grow. Two weeks after emergence, all 

 pots were fertilized with 25 ppm N and 25 ppm K as NH ; N0_ and KC1 

 based on the v?eight of the surface soil. Plants were watered as needed 

 with distilled water in an attempt to maintain a reasonably uniform 

 moisture level throughout the tube. Greenhouse temperatures were ad- 

 justed for a maximum of 30 C and a minimum of 16 C. 



Plants were harvested every 4 weeks for a total of 4 harvests. 

 Plants were clipped 7.5 cm above the soil line, placed in paper bags, 

 and dried in a forced-air oven at 70 C. Samples were then weighed, 

 ground to pass a 1-mm screen, ashed, and analyzed for N and S. Details 

 of tissue analyses will be given in a later section. Relative dry 



