EVAPOTRANSPIRATION ETC., WITH IRRIGATED GRAIN SORGHUM 



PROCEDURE 



Experimental Design 



The experimental design was a randomized 

 complete block with split plots. Four replica- 

 tions of six soil moisture levels as complete blocks 

 and six fertilizer treatments as split plots were 

 used. Each moisture level was included in a 

 level basin diked on all sides with level area 

 dimensions of 30 by 165 feet. Depth of irriga- 

 tion water applied was based on dimensions from 

 center to center of the dikes, 33.3 by 168 feet. 

 Fertilizer plots were 15 by 50 feet. The treat- 

 ments were maintained on the same plots for the 

 four seasons. (Farmers frequently grow sorghum 

 on the same field 3 to 5 years in succession.) 



Moisture Levels 



A preplanting irrigation was given all moisture 

 treatments each year several weeks before plant- 

 ing to wet the soil to a depth of about 6 feet. 

 Moisture levels are described below. 



Code No. Moisture level 



M] Preplanting irrigation only. 



M 2 One 4-inch irrigation 1 week prior to boot 



stage. 



M 3 Irrigated when the weighted mean soil 



moisture tension approached 9 atmos- 

 pheres. 



M 4 Irrigated when the weighted mean soil 



moisture tension approached 4 atmos- 

 pheres. 



M 5 Irrigated when the weighted mean soil 



moisture tension approached V-/% at- 

 mospheres. 



M 6 Irrigated the same as M3, for the first 



irrigation, and irrigated the second time 

 before the soil moisture tension ap- 

 proached 4 atmospheres in 1957, 1958, 

 and 1959. Irrigated the same as M 4 in 

 1956, except the third irrigation was not 

 given. 



The weighted mean soil moisture tension was 

 obtained by weighting tensions in successive 

 quarters of the moisture depletion zone by 4, 3, 

 2, and 1. The weighting procedure was based on 

 typical soil moisture extraction patterns. Soil 

 moisture tension was measured indirectly by cured 

 plaster of paris moisture blocks (2) calibrated in a 

 pressure membrane apparatus. Calibration con- 

 sisted of placing six blocks selected at random in 

 a special-built pressure membrane apparatus with 

 1 cm. of soil above and below the block. Indi- 

 vidual leads for each block in the pressure mem- 

 brane were used. The calibration curve was 

 adjusted to 70° F. and used without further 

 correction. The curing process consisted of at 

 least two 24-hour soaking and drying cycles. The 

 standard deviation of the resistance of individual 

 cured blocks immersed in tapwater was about 25 

 ohms. The curing process removed most of the 

 drift in calibration that normally would have 



occurred in the field and some of the variability 

 between blocks. New moisture blocks were 

 installed each summer at depths of 4, 9, 16, 29, 

 and 42 inches in the F 3 and F 5 fertility subplots 

 of each moisture treatment. Readings were 

 made three times a week during the main part of 

 the crop season. 



A summary of dates, depths of water applied, 

 and stage of growth at each irrigation is given in 

 table 10 in the appendix. Water from a well was 

 delivered and measured to each moisture plot by 

 the use of gated aluminum pipe and a flowmeter. 

 A summary of rainfall by storms received during 

 the growing seasons as recorded in a gage near the 

 plots is presented in table 11 in the appendix. 

 The sums of these values by months differ some- 

 what from those presented in table 3 because of 

 location. 



Fertilizer Treatments 



Nitrogen in the form of ammonium sulfate 

 (20.6 percent N) and phosphorus in the form of 

 concentrated superphosphate (45 percent P 2 O s ) 

 were used at the rates given below each year 

 except in 1957. No fertilizer was applied in 1957, 

 because response to applied fertilizer did not 

 occur in 1956. the first year of irrigation. 



Nitrogen 



Fertilizer treatment No. (Lb. /acre) 



F x 240 



F 2 



F 3 60 



F 4 120 



F 6 240 



F» 240 



Phosphorus 

 (45 percent 



P 2 O s ) 

 (Lb. /acre) 





 30 

 30 

 30 

 30 

 60 



In 1956, nitrogen was broadcast on the surface 

 of the soil and worked into the top 2% inches of 

 soil. In 1956, phosphorus placed with the seed 

 restricted the germination to some extent, owing 

 to limited soil moisture conditions. Consequently, 

 all plots were irrigated on June 26 and June 27 

 to improve the stand. In 1958 and 1959 nitrogen 

 was broadcast just ahead of each furrow opener 

 and phosphorus placed ahead of the press wheel 

 following the furrow opener (just above the seed). 



Cultural Practices 



Tillage 



In 1956 and 1957 the plots were sweep-plowed 

 to a depth of 6 to 8 inches after harvest. Other 

 tillage operations varied from year to year accord- 

 ing to weed and volunteer sorghum growth. 

 After the preplanting irrigation, usually given 

 early in June or late in May, the plots were 

 sweep-plowed to a depth of 2 to 3 inches and 



