Evapotranspiration and Soil Moisture-Fertilizer Interrelations with Irrigated 



Grain Sorghum in the Southern Great Plains 



By Marvin E. Jensen and Willis H. Sletten, Soil 

 and Water Conservation Research Division, Agricultural 

 Research Service ' 



In 1956 and 1957 grain sorghum represented 

 37 percent of the harvested crop acreage in Texas 

 (1/+). 2 The largest concentrated area of sorghum 

 is in the High Plains where the proportion of 

 sorghum irrigated increased greatly during the 

 drought years of the 1950's. In 1959 the mone- 

 tary value of irrigated grain sorghum was esti- 

 mated to be about $100 million. 



As reported by the U.S. Census of Agriculture, 

 the acreages of irrigated grain sorghum harvested 

 in the 42-county High Plains area in 1950, 1954, 

 and 1959 were 387,000, 1,006,000, and 1,224,000 

 acres, respectively. The irrigated grain sorghum 

 acreage in the eight counties — Castro, Deaf 

 Smith, Floyd, Hale, Lamb, Lubbock, Parmer, 

 and Swisher — represented over 80 percent of the 

 total in the High Plains in 1954 and 1959. 



Development 'of high-yielding hybrids and a 

 large increase in the number of irrigation wells 

 in the area have been major factors in the three- 

 fold increase in irrigated grain sorghum. The 

 number of wells increased from about 8,000 in 

 1948 to 45,000 in 1958 and 52,000 in 1963. Ex- 

 pansion since 1959 has been mainly north of the 

 Canadian River. 



Ideal topography was instrumental in the rapid 

 development of irrigation in the area. It is not 

 uncommon for farmers to irrigate one-half-mile 

 rows without any land smoothing. 



The predominant soils in the area, except for 

 Lamb and Lubbock Counties, consist mostly of 

 clay loams and silty clay loams (10, 15). The 

 High Plains is perhaps one of the largest irrigated 

 areas having a single predominant soil. 



The source of water used for irrigation is an 

 aquifer that underlies most of the area. The 



total water supply is extensive, but the rate of 

 recharge by rainfall is very small compared to 

 the current rate of pumping. The pumping lift 

 ranges from about 100 to 400 feet in different 

 areas of the High Plains. The cost of pumping 

 water and the growing awareness that the current 

 pumping rates greatly exceed natural recharge 

 created a need for maximizing economic returns 

 per unit of irrigation water and precipitation. 



Without irrigation, inadequate precipitation is 

 the major factor limiting crop production in the 

 area. Under nonirrigated conditions, the common 

 cropping sequences involving grain sorghum are 

 continuous sorghum and sorghum after wheat. 

 Bond and coworkers evaluated the frequency of 

 obtaining various yields of grain sorghum under 

 dryland conditions from 1907 to 1919 and 1943 to 

 1958 on the "hardland" soils in the Texas High 

 Plains (1). They found that the yield of con- 

 tinuous grain sorghum was less than 2,000 pounds 

 per acre about 80 percent of the time. With 

 sorghum after wheat (fallow from harvesting of 

 wheat until the next season), the yield was less 

 than 2,000 pounds 70 percent of the time. The 

 yields were less than 800 pounds 20 percent of the 

 time with continuous sorghum and 15 percent of 

 the time with sorghum after wheat. Yields 

 exceeded 2,500 pounds about 5 percent of the 

 time with continuous sorghum and about 15 

 percent of the time with sorghum after wheat. 

 Commercial fertilizers currently are not needed in 

 dryland sorghum production. 



Yields were doubled or tripled during the first 

 1 or 2 years of irrigation. Yields decreased in 

 subsequent years if nitrogen fertilizer was not used. 

 The purpose of this study was to combine moisture 

 and fertilizer levels in an experiment to provide 

 irrigation and fertilizer recommendations for use 

 by irrigation farmers in the area. 



STUDY AREA 



Locat 



ion 



The experiment was located on the USDA 

 Southwestern Great Plains Research Center near 

 Bushland, Tex., 14 miles west of Amarillo (latitude 



1 The authors gratefully acknowledge the computer 

 services provided by the Data Processing Center, Texas 

 A&M University, and the assistance of Frank O. Wood, 

 Southwestern Great Plains Research Center, Bushland, Tex., 



35°15' N., elevation 3,825 feet). The station is 

 located near the northern edge of the irrigated 

 grain sorghum counties previously mentioned. 

 The soil on the experiment station is representa- 

 tive of the irrigated area to the south as well as to 



in carrying out field operations and processing the volu- 

 minous data. 



2 Italic numbers in parentheses refer to Literature 

 Cited, p. 17. 



