8 



CONSERVATION RESEARCH REPORT 5, U.S. DEPT. OF AGRICULTURE 



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MEAN P 



OTENTIAL E T 



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JUNE 



SEPTEMBER OCTOBER 



Figure 3. — Rate of evapotranspiration (E t ) for grain sor- 

 ghum at Bushland, Tex., with optimum soil moisture 

 conditions and estimated mean potential E t . 



decrease. The E t rate was much less than the 

 potential in June and early July because of limited 

 transpiring surface area. As transpiring surface 

 area increased in July, the mean Et rate increased 

 rapidly until the potential rate was reached during 

 the first few days in August. The decrease in 

 mean E t rate during August follows the decrease 

 in potential E t . The effects of crop maturation and 

 lower soil moisture levels appear to have reduced 

 E t below the potential during the latter part of 

 September and in October. The mean rate of 

 evapotranspiration shown in figure 3 should be 

 applicable to irrigated fields in the area planted to 

 grain sorghum about June 15. For sorghums 

 planted 2 or 3 weeks earlier, the peak rate of E t 

 will be reached about 10 days earlier. 



A summary of total water in the 0- to 4-foot 

 depth for all moisture levels by sampling dates is 

 presented in table 13 in the appendix. These 

 values are the average of the F 4 and F 5 plots. 

 Total water in the 4- to 6-foot depth increment is 

 presented in table 14. Soil moisture extraction 

 from the 4- to 6-foot depth was small except on 

 the dry treatments. Therefore, this depth was 

 not sampled each time the 0- to 4-foot depth was 

 sampled. 



irain 



Yield 



Grain yields for each combination of moisture 

 level and fertilizer treatment and a summary of 

 the analysis of variance are presented in table 4. 

 Yields were greatly affected by nitrogen fertilizer, 

 especially where soil moisture did not restrict 

 yields. No significant response to phosphorus 

 occurred in the 4 years, with average yields of the 

 same treatment as high as 7,200 lb. /acre. 



Nitrogen fertilizer did not increase yields in 

 1956. Yields ranged from less than 1,000 lb./acre 



with a preplanting irrigation only to over 6,000 

 lb./acre on the M 4 moisture level plots. Severe 

 lodging occurred on the M 2 and M 6 moisture levels 

 due to charcoal rot (Macrophomma phaseoli). 

 Moderate lodging occurred on the M 3 and M 5 and 

 some lodging occurred on the M 4 plots. Lodging 

 was more severe on the plots fertilized at the 240- 

 lb./acre nitrogen rate. The Mi moisture level 

 plots had very little lodging. 



A response to nitrogen applied in 1956 occurred 

 on the higher soil moisture levels in 1957. The 

 yield without applied nitrogen was less on the 

 optimum moisture level than on the medium 

 level, apparently because about 1,600 lb./acre 

 more grain was produced on this treatment in 

 1956. The Mi moisture level plots lodged, and 

 some lodging occurred on the M 3 level. 



In 1958, the third year of irrigation, yields 

 dropped about 50 percent without nitrogen 

 fertilizer on the medium and optimum soil moisture 

 levels. With a preplanting irrigation only, the 

 yield was largest without applied nitrogen. 



In 1959, the fourth year of irrigation, yield 

 without applied nitrogen decreased to about 

 3,000 lb./acre on the optimum soil moisture 

 level. The largest increase in yield occurred with 

 the first increment of nitrogen (60 lb./acre) 

 applied in 1956, 1958, and 1959. 



The 4-year average yield with a preplanting 

 irrigation only was about 2,500 lb./acre. On this 

 treatment a slight response to applied nitrogen 

 occurred in 1959. Similar results were obtained 

 on the M 2 moisture level (preplanting plus one 

 seasonal irrigation), except a substantial response 

 to nitrogen fertilizer occurred during the fourth 

 year. In this case, the 4-year average yield with- 

 out applied nitrogen was about 4,080 lb./acre, 

 whereas the average yield with applied nitrogen 

 was about 4,400. Most of this difference occurred 

 in 1959. In 1959, the plots without applied nitro- 

 gen yielded only about 3,300 lb./acre compared 

 with the fertilized plots that yielded as high as 

 4,590 lb./acre. 



On adequately fertilized plots of the medium 

 moisture level (M 3 ), which averaged about 5,800 

 lb./acre, a response to residual nitrogen occurred 

 during the second year under irrigation. By the 

 fourth year, yields increased from 3,200 to 5,700 

 lb./acre with applied nitrogen. These results 

 indicate that, when irrigating for a yield potential 

 of 6,000 lb./acre, no nitrogen fertilizer may be 

 necessary the first 2 years this soil is irrigated 

 but about 60 lb. of N will be needed the third 

 year and between 60 and 120 lb. the fourth year. 



With average yields of 7,000 to 7,500 lb./acre, a 

 response to nitrogen occurred during the second 

 year under irrigation. By the fourth year, nitro- 

 gen increased yields from about 3,000 to 7,800 

 lb. per acre. Continuous production of 7,000 lb. 

 per acre, or more, would require at least 120 pounds 

 or more of nitrogen annually. 



