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Agronomy Journal 94:936-943 (2002)
© 2002 American Society of Agronomy

DRYLAND CROPPING SYSTEMS

Water Depletion Depth of Grain Sorghum and Sunflower in the Central High Plains

Loyd R. Stone*,a, Dwayne E. Goodruma, Alan J. Schlegelb, Mahmad Nor Jaafarc and Akhter H. Khand

a Dep. of Agron., Throckmorton PSC, Kansas State Univ., Manhattan, KS 66506-5501
b Southwest Res.-Ext. Cent., Tribune, KS 67879
c Malaysian Agric. Res. and Dev. Inst., P.O. Box 203, 13200 Penang, Malaysia
d Dep. of Soil Sci., Univ. of Dhaka, Dhaka-1000, Bangladesh

* Corresponding author (lrstone{at}ksu.edu)

Received for publication June 6, 2001.

   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY
 REFERENCES
 
In dryland agriculture of the central High Plains, water is often the primary factor influencing selection of crops and cropping systems. For improved water management, a greater percentage of precipitation during fallow must be stored and used in crop production. More efficient water use can be promoted via agronomic management such as extending the root zone by use of deep-rooted crops. While sunflower (Helianthus annuus L.) has a reputation for deep rooting, grain sorghum [Sorghum bicolor (L.) Moench] is the dominant dryland row crop in western Kansas. Our objective was to contrast the depth of soil water depletion and end-of-season rooting depth of sorghum and sunflower. Rooting depth at end of season was measured by the core-break method during a 3-yr study near Tribune, KS, on a Ulysses silt loam soil (fine-silty, mixed, superactive, mesic Aridic Haplustoll). Water content was measured to the 3.2-m soil depth by neutron thermalization. The water depletion front advanced downward at greater rates and to deeper depths with sunflower (3.1 m) than with sorghum (2.5 m). Water depletion in the 2.2- to 3.3-m soil depth zone was significantly more for sunflower (48 mm) than for sorghum (14 mm). End-of-season rooting depth was significantly greater for sunflower (3.03 m) than for sorghum (2.54 m). The faster advance of the water depletion front and greater depth of rooting of sunflower compared with sorghum are factors contributing to drought avoidance in sunflower and its ability to deplete water from deeper soil depths.


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY
 REFERENCES
 
IN DRYLAND AGRICULTURE of semiarid regions, such as the central High Plains of the USA, plant-available water is the factor most limiting to yield potential. Therefore, water is often the primary factor influencing management decisions about selection of crops and cropping systems. In western Kansas (west of 100° W long), corn (Zea mays L.) is the dominant irrigated crop, and winter wheat (Triticum aestivum L.) is the dominant dryland crop. Of 0.76 million ha of irrigated corn, grain sorghum, soybean [Glycine max (L.) Merr.], and wheat harvested for grain in 1998, 63% was in corn (Kansas Agric. Stat., 1999). Of 1.92 million ha of dryland corn, sorghum, soybean, and wheat harvested for grain in 1998, 72% was in wheat (Kansas Agric. Stat., 1999). Changes in the ratio of irrigated to dryland cropped areas in the central High Plains and in the relative mix of crops within cropping systems will be brought about primarily through changes in the amount of water available for irrigation, precipitation use efficiency, and economics (crop prices, production costs, and government programs).

Water supply for irrigation in western Kansas largely depends on ground water, with the Ogallala Formation of the High Plains Aquifer being the main source (McGrath and Dugan, 1993). Withdrawal from the Ogallala exceeds recharge in most areas where extensive irrigation development has occurred, and as a result, water tables have declined. With ground water depletion, well yields are reduced and pumping costs increased by additional lift. Irrigated farmland will eventually convert to dryland production when pumping from depleted ground water supplies is no longer economical. To aid this transition, cropping systems that efficiently use both irrigation water and precipitation are needed.

Rotating irrigated and dryland crops within cropping systems improves the efficiency of use of limited water supplies and is an alternative to continuously irrigated cropping. The irrigated crop provides more residue that can be used to increase water storage (Unger and Phillips, 1973) and also provides increased water at deeper soil depths (Aronovici and Schneider, 1972), both of which increase yield of the dryland crop in rotation compared with continuous dryland. Unger (1977) reported that with a system of alternating irrigated and dryland winter wheat crops on the same plots, mean grain yield over 4 yr was 10% greater than the overall mean for continuous irrigated and continuous dryland wheat plots. Other plans such as irrigated wheat–dryland sorghum (Unger and Wiese, 1979; Norwood, 1995), irrigated wheat–dryland sunflower (Unger, 1981), irrigated sorghum–dryland wheat (Baumhardt et al., 1985; Norwood, 1995), and irrigated sorghum–dryland sunflower (Jones and Johnson, 1983) also have been explored. With two-crop systems, one crop should perform well with irrigation, and the other should produce satisfactorily on dryland (Unger, 1977).

In dryland agriculture of the central High Plains, wheat is the dominant crop, and fallow traditionally has been used to store water for the following crop. The wheat–fallow rotation yields one crop in 2 yr, with 14.5 mo of fallow between crops. Alternatives are wheat–summer crop–fallow rotations, which yield two crops in 3 yr with 11-mo fallow periods between wheat and the summer crop and then between the summer crop and wheat. For these more intensive systems to succeed, a greater percentage of precipitation between crops must be stored in the soil, and the stored water and growing season rainfall must be used more efficiently in crop production. Compared with wheat–fallow, more intensive cropping systems improve precipitation use efficiency when coupled with decreased or no tillage, good weed control, and the maintaining of crop residue on soil (Peterson et al., 1996; Norwood and Currie, 1997). No-tillage systems result in deeper water movement and increased water stored at greater soil depths compared with conventional tillage (Smika, 1990; Eck and Jones, 1992; Norwood, 1994). Optimization of a cropping system's water use efficiency requires selecting crops with the highest potential water use efficiency for the specific environment (Peterson et al., 1996).

Opportunities exist for promoting more efficient water use via agronomic management such as extending the root zone by planting deep-rooted crops (Sharpley et al., 1998). Sunflower has an extensive root system that can extract water to soil depths of 3 m, which suggests it can be included in crop sequences to use water that has amassed below the normal rooting depth of most crops (Jones and Johnson, 1983). Robinson (1978) stated that sunflower is not highly drought tolerant but commonly is grown as a dryland crop and often produces satisfactorily when other crops are damaged seriously. Sunflower's ability to produce under low-rainfall conditions is aided by its relatively deep root system (Connor and Hall, 1997) and relatively low water use requirement for initial seed yield (Nielsen, 1998).

In the central High Plains, dryland crops for use in rotations are selected with consideration given to water located at deeper soil depths because of fallow, no-tillage systems, or irrigation. Information is needed on root zone distribution and water depletion depth of the principal crop candidates. Of the harvested area of dryland row crops in the western one-third of Kansas in 1998, 62% was in sorghum, 8% in sunflower, and 28% in corn (Kansas Agric. Stat., 1999). Our purpose was to evaluate sorghum and sunflower root distribution and water depletion with depth in the loessial soils of the central High Plains. Our specific objective was to measure depth of soil water depletion and end-of-season rooting depth of grain sorghum and sunflower.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 SUMMARY
 REFERENCES
 
This 3-yr field study was near Tribune, KS, USA (38°30' N, 101°47' W; 1086 m above sea level), a region with a semiarid, continental climate. Mean date of the last frost (-1°C) in spring is 1 May, and mean date of the first frost (-1°C) in fall is 7 October (1913–1994). The soil is mapped as Ulysses silt loam, which has moderate permeability, high available water capacity, and no root-restricting features (McBee et al., 1961). The Ulysses series consists of deep soils formed in loess, and similar soils occupy about 2.34 million ha of the central High Plains (Aandahl, 1982). Particle size distribution, bulk density, and water content vs. matric potential of the study soil were presented by Stone et al. (1987). Limits of plant-available soil water to the 1.6-m depth were field-determined through gravimetric sampling of soil water content following complete profile wetting and a 3-d drainage period (upper limit of 560 mm) and following harvest after dry seasons (lower limit of 250 mm) (Stone et al., 1987). The gravimetric water contents (dry mass basis) were multiplied by bulk density to convert to volumetric water contents. The 1.6-m depth was selected to reflect water conditions in that portion of the root zone normally monitored for management of irrigation water and in evaluation of crop water stress conditions.

The previous crop in the test area (37 by 80 m) was winter wheat, which was harvested in June 1984. The area was fertilized (152 and 20 kg ha-1 N and P, respectively) in March 1985, February 1986, and April 1987. The area was divided into eight 17.4- by 16.3-m research basins as four blocks of two basins each that were constructed with earthen berms, with 2.2-m separation between basins. Within each of the four blocks, one basin received off-season irrigation designed to wet the soil profile to the 3-m depth (wet treatment), and one basin received no or limited off-season irrigation (dry treatment). Each of the eight basins was split longitudinally at planting, with one-half planted to sorghum and one-half planted to sunflower. Each individual plot unit was 8.7 by 16.3 m with 11 crop rows spaced 0.76 m apart.

Irrigation water was delivered to individual basins by timed delivery through gated pipe from a metered source. The wet basins were irrigated in November 1984 (10 cm on each of three dates), January 1986 (10 cm on each of four dates), and November 1986 (10 cm on each of four dates). The dry basins received no irrigation before the 1985 and 1986 growing seasons. Because of extreme dryness in fall 1986 and concern for the upcoming 1987 crops in the dry basins, all dry basins received a 10-cm irrigation in November 1986. All basins received a 7.5-cm irrigation on 15 June 1987 because of variable plant emergence.

Grain sorghum (‘Triumph TWO 54-YG’) was planted on 24 May 1985, 23 May 1986, and 27 May 1987, and sunflower (‘Triumph 585’) was planted on 24 May 1985, 26 May 1986, and 26 May 1987. Seeding rates were 12 and 4 seeds m-1 row in sorghum and sunflower, respectively. After maturity, plants were hand-harvested from 22.5 m of row (7.5-m lengths of three rows) from each plot. Seed yield is reported at water contents on a wet mass basis of 125 g kg-1 for sorghum and 100 g kg-1 for sunflower.

Soil water content was determined about every 2 wk during the growing seasons. Two 3.66-m-long aluminum tubes for neutron probe (Model 1257, Troxler Electronic Lab., Raleigh, NC) access were installed in the central area of each of the 16 plots. Volumetric water content was obtained for soil depths of 0.15 to 3.20 m in 0.15-m-depth increments by converting neutron probe readings using a field-specific calibration equation (calibration: n = 27, r2 = 0.76, and root mean square error = 0.014 m3 m-3). The 3.2-m depth was selected in an attempt to capture the full depth of root activity and water depletion. Seasonal soil water depletion for each water measurement depth was calculated by subtracting the final water content (m3 m-3, measured in September) from the initial water content (m3 m-3, measured in June). The accuracy in measurement of water content change at fixed positions was discussed by Gardner (1986). With our neutron probe, water content change (net depletion) is known to the nearest 0.020 m3 m-3 at the 95% confidence level, calculated by the procedure of Gardner (1986). Water depletion at a soil depth was considered meaningful if >=0.020 m3 m-3. The water depletion rate at each measurement depth for selected time intervals during the seasons was calculated as the final soil water content (m3 m-3) subtracted from the initial water content (m3 m-3) and the difference divided by the number of days in the time interval. Water content (dry mass basis) of the surface 0.08 m was determined by gravimetric sampling, with the gravimetric water contents multiplied by surface bulk density (1.32 Mg m-3) to convert to volumetric water contents. Rain was measured 0.6 km from the plots.

Rooting depth at end of season was determined by the core-break method (Böhm, 1979) in October of 1985, 1986, and 1987. Soil cores (76 mm diam.) were collected using a hydraulic probe mounted on a tractor. The core sampler was placed directly over a plant whose aboveground parts had been removed at the soil surface. Soil cores were broken and examined for roots visible to the unaided eye until no roots were observed in a vertical distance of 0.6 m. Depth of the deepest root was recorded for each core, with two cores taken in each of the 16 plots.

Analyses of variance were performed using PROC ANOVA of SAS (SAS Inst., 1987). The format of the analyses of variance was that of a split-plot design with four blocks, two main-plot treatments (water level), and two subplot treatments (crop) patterned after Gomez and Gomez (1984). Following the analyses of variance, standard error of the mean difference for each of two types of pair comparison with a split-plot design was calculated (pair comparison types 3 and 4 given by Gomez and Gomez, 1984). The LSD values for comparing the two crops at the same water level (type 3) and for comparing the two water levels for the same crop (type 4) were calculated at an a priori significance level of 0.10. Standard errors of the mean were calculated using PROC MEANS (SAS Inst., 1987).


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
SUMMARY
 REFERENCES
 
Precipitation during the study years is given in Fig. 1 . Total precipitation was 427 mm in 1985, 391 mm in 1986, and 484 mm in 1987 compared with the 1900 to 1994 mean of 421 mm. A relatively dry period occurred in early 1986 (1 January to 31 May) when total precipitation was 58 mm compared with the long-term mean of 147 mm. A relatively wet period occurred in the spring of 1987 (15 March to 15 April) when 112 mm of precipitation was received compared with the long-term mean of 29 mm. Crop development during the three growing seasons is summarized in Table 1, with growth stage descriptions patterned after those given by Vanderlip and Reeves (1972) for sorghum and by Schneiter and Miller (1981) for sunflower. The dates listed were when 75% of the crop population had reached the indicated stage. Crop development dates were similar among the 3 yr, with emergence in early June for both crops. Flowering in both crops started in early August and was completed by mid-August. Physiological maturity was reached about 1 wk into September for sunflower and about 3 wk into September for grain sorghum.



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  		Fig. 1. Cumulative precipitation during 1985, 1986, and 1987, and the long-term mean (1900–1994).

 

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  		Table 1. Summaries of growth stages for grain sorghum and sunflower near Tribune, KS.

 
Soil water content by depth at the initial sampling each year is presented in Fig. 2 . Water content distribution in the wet treatment was similar for the three seasons and relatively uniform with depth in the range of 0.30 to 0.35 m3 m-3. Soil profile water content distributions in the dry treatment varied greatly among the 3 yr and from those in the wet treatment. In 1985, water distribution in the dry treatment showed an increase during fallow mainly in the top 1 m. In 1986, the dry treatment's water content data showed water recharge after the 1985 crop was primarily in the top 0.6 m. At the 1- to 2-m depths in 1986, soil water content was near that at matric potential of -1.5 MPa (0.15 m3 m-3). In 1987, because of the 175 mm of irrigation and above-average rain in April, the upper 1 m of soil in the dry treatment had relatively high water contents (0.30–0.35 m3 m-3).



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  		Fig. 2. Soil water content vs. soil depth for the four treatments in June of each year. Bars indicating ±standard error (SE) are shown if sufficiently large for illustration (SE >= 0.0075 m3 m-3; n = 4). GS, grain sorghum; SF, sunflower; DR, dry; WT, wet; 85, 15 June 1985; 86, 20 June 1986; 87, 28 June 1987.

 
Total water to the 3.27-m soil depth and available water to the 1.60-m depth at the initial water samplings and available water to the 1.60-m depth in early to mid-September are presented in Table 2. Plant-available soil water at the June samplings of 1985, 1986, and 1987 were 78, 82, and 92% of maximum (241, 254, and 287 mm), respectively, in the wet treatment and 50, 20, and 78% of maximum (155, 62, and 243 mm), respectively, in the dry treatment. In September, available soil water in the dry treatment was not significantly different between crops: 15, 9, and 13% of maximum (48, 28, and 41 mm) in 1985, 1986, and 1987, respectively. In the wet treatment, available soil water was greater for sorghum than for sunflower in September of 1985 and 1987 but did not differ significantly in 1986. At soil water measurements of early to mid-September, sunflower was at physiological maturity, but sorghum was not (Table 1). That is, in early to mid-September, sunflower had lost its green leaf area and was essentially through using water while sorghum, with its green leaf area remaining until frost, was still depleting soil water.


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  		Table 2. Total water to 3.27-m soil depth in June and available water to 1.60-m soil depth in June and September during 3-yr study near Tribune, KS.

 
Seasonal soil water depletion by depth is presented in Fig. 3 . Water depletion in the dry treatment of 1986 was only to the 0.6-m soil depth, which paralleled the initial soil water content pattern (Fig. 2). Meaningful water depletion in the combinations of water levels and years (dry 1986 excluded) reached mean depths of 2.5 m for sorghum (n = 5 and SE = 0.23 m) and 3.1 m for sunflower (n = 5 and SE = 0.09 m). Seasonal water depletion amounts for three soil depth zones are presented in Table 3. In the 0- to 1.14-m depth zone, neither crop had consistently more or less water depletion. End-of-season water sampling dates were different for the two crops, and differences between crops in depletion amount in the top 1.1 m often were caused by end-of-season rain patterns. Bremner et al. (1986) found that sunflower and sorghum depleted similar amounts of water from the top 1 m of soil that was protected by automatic rain shelters. In the 1.14- to 2.21-m depth zone in our study, no significant differences in depletion occurred between crops in 1985 and 1986 or in the dry treatment of 1987. Only in the wet treatment of 1987 did a significant difference in depletion occur, with sunflower depleting more water than sorghum. In the 2.21- to 3.27-m depth zone, net soil water depletion was significantly greater in sunflower than sorghum for five of six water level–year combinations, the exception being the dry treatment of 1986. Excluding that water level–year combination, the net depletion means for the 2.21- to 3.27-m depth zone were 48 mm for sunflower and 14 mm for sorghum.



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  		Fig. 3. Seasonal water depletions by sorghum and sunflower vs. soil depth in each of the 3 yr. Bars indicating ±standard error (SE) are shown if sufficiently large for illustration (SE >= 0.005 m3 m-3; n = 4). Time intervals were 15 June to 27 Sept. 1985 (85), 20 June to 25 Sept. 1986 (86), and 28 June to 24 Sept. 1987 (87) in sorghum and 15 June to 14 Sept. 1985 (85), 20 June to 13 Sept. 1986 (86), and 28 June to 6 Sept. 1987 (87) in sunflower. DR, dry; WT, wet.

 

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  		Table 3. Seasonal water depletion in soil profile depth intervals during 3-yr study near Tribune, KS.

 
Other studies also have reported water depletion from greater soil depths with sunflower. Water depletion by sunflower to a soil depth of 2.9 m was measured by Dardanelli et al. (1997). Unger (1984) stated that with favorable water conditions in the Pullman clay loam soil (fine, mixed, superactive, thermic Torrertic Paleustolls), sunflower depleted water to the 3-m depth. Other studies comparing water depletion by sorghum and sunflower also have reported that sunflower depletes more water from greater depths than sorghum. For example, seasonal soil water depletion from the 1.75- to 2.0-m depths was 0.054 m3 m-3 for sunflower and 0.020 m3 m-3 for sorghum; soil water below 2.0 m was not measured (Bremner et al., 1986). Sunflower and sorghum both depleted water from the 1.5- to 1.8-m soil depth zone, but sunflower depleted more water and probably removed water from depths below 1.8 m (Norwood, 1999).

Water depletion rate by soil depth is presented in Fig. 4, 5, and 6 for 1985, 1986, and 1987, respectively. Sunflower seasons were divided into three time periods for presentation. Sorghum seasons were divided into the same three time periods as sunflower, plus an additional fourth period to account for the 2-wk longer time for sorghum to reach physiological maturity. Water depletion patterns result from a mixture of water loss by evaporation, loss from uptake by plant roots, and loss or gain from water movement in response to potential energy differences. If evaporation loss and water movement are zero, then water depletion equals water uptake by plant roots (water extraction). We did not measure water flux; therefore, the contribution of water flux in forming the water depletion patterns of Fig. 3 through 6 cannot be quantified. Some depletion at the deeper depths may have been due to upward water flux in response to upward-acting matric potential energy differences as a consequence of soil drying in the upper layers. Times of downward water flux may have occurred in response to gravitational potential energy and matric potential energy differences as a consequence of relatively wet soil overlying relatively dry soil. In general, the water depletion front descended more rapidly and to greater depths with sunflower than with sorghum in the three matching time frames. Bremner et al. (1986) also found water depletion by sunflower started sooner in each successively deeper soil layer compared with depletion by sorghum.



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  		Fig. 4. Water depletion rates by sorghum and sunflower vs. soil depth in four time periods of 1985. Bars indicating ±standard error (SE) x 103 are shown if sufficiently large for illustration (SE >= 0.00010 m3 m-3 d-1; n = 4). DR, dry; WT, wet; 1st, 15 June to 12 July; 2nd, 13 July to 16 August; 3rd, 17 August to 14 September; 4th, 15 to 27 September.

 


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  		Fig. 5. Water depletion rates by sorghum and sunflower vs. soil depth in four time periods of 1986. Bars indicating ±standard error (SE) x 103 are shown if sufficiently large for illustration (SE >= 0.00014 m3 m-3 d-1; n = 4). DR, dry; WT, wet; 1st, 20 June to 26 July; 2nd, 27 July to 20 August; 3rd, 21 August to 13 September; 4th, 14 to 25 September.

 


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  		Fig. 6. Water depletion rates by sorghum and sunflower vs. soil depth in four time periods of 1987. Bars indicating ±standard error (SE) x 103 are shown if sufficiently large for illustration (SE >= 0.00016 m3 m-3 d-1; n = 4). DR, dry; WT; wet, 1st, 28 June to 21 July; 2nd, 22 July to 15 August; 3rd, 16 August to 6 September; 4th, 7 to 24 September.

 
In the dry treatment of 1986, available soil water was 20% of maximum (62 mm) in mid-June and 9% of maximum (28 mm) in mid-September (Table 2). Rooting depth in that treatment was not significantly different between crops (Table 4). Lack of root growth and water depletion activity in dry soil, as in the dry treatment of 1986 (Fig. 3 and 5), has been illustrated in other studies. Where drought and water use by the previous crop rendered the subsoil dry, the result was a relatively shallow pattern of root penetration (Merrill et al., 1996). Excluding the dry treatment of 1986, the end-of-season rooting depths in Table 4 had mean values of 2.54 m for sorghum (n = 5 and SE = 0.15 m) and 3.03 m for sunflower (n = 5 and SE = 0.12 m). Sunflower in 1986 was the only crop–year combination that had a significant difference in rooting depth between water levels. The relatively low seed yields in the dry treatment of 1986 (Table 4) are indicative of the poor soil water conditions.


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  		Table 4. Rooting depth and seed yield during 3-yr study near Tribune, KS.

 
An examination of soil water content, soil water depletion patterns, and rooting depth data reveal interrelationships among the three. Seasonal water depletion in the upper 2 m of soil in the dry treatment of 1985 (Fig. 3) matched the distribution of early season soil water (Fig. 2). Both crops had similar patterns of water depletion in the upper 2 m of soil, with sorghum having somewhat more depletion than sunflower (Table 3 and Fig. 3), probably due to more early season soil water in the sorghum plots (Table 2 and Fig. 2). At soil depths of 2.2 to 3.3 m, sunflower depleted more water than sorghum (dry treatment of Table 3 and Fig. 3). In the wet treatment of 1985, both crops depleted similar amounts of water in the upper 2 m, and at depths below 2 m, sunflower depleted more water than sorghum (Table 3 and Fig. 3). In the first three time intervals of 1985, and in both water level treatments, water depletion by sunflower was deeper than by sorghum (Fig. 4), consistent with sunflower's deeper root system (Table 4) in both dry and wet treatments.

Seasonal water depletion in the dry treatment of 1986 was confined to the upper 0.6 m (Fig. 3), with essentially no net seasonal water depletion in the 1- to 2-m zone (Table 3 and Fig. 3). However, there was water depletion within the season to the 1.7-m soil depth (Fig. 5). Due to rain (146 mm) during 20 June to 27 July 1986 (Fig. 1), there was a net gain of water in the upper 1.7 m of soil during the first time interval in the dry treatment, with more gain in sorghum due to less water use by sorghum. Then, in the second time interval, there was water depletion by both crops to the 1.7-m depth, with more depletion by sorghum because of more water gain in sorghum plots in the first time interval. After water gained during the first time interval to the 1.7-m depth was depleted in the second time interval, there was essentially no water depletion during later time intervals in the dry treatment (Fig. 5). End-of-season root depths of sunflower and sorghum in the dry treatment of 1986 were not different (Table 4).

Early season soil water content in the dry treatment of 1987 was relatively high in the surface 1 m and relatively low from 1.5 to 2.5 m (Fig. 2). During the first time interval (dry treatment of 1987), there was a net gain of water in the 1.5- to 2.5-m soil depths (Fig. 6) as a result of downward water movement, consistent with the water content distribution of Fig. 2. In the second and third time intervals of 1987, and in both water level treatments, water depletion with sunflower was deeper than with sorghum (Fig. 6). Deeper water depletion by sunflower compared with sorghum in 1987 was consistent with sunflower's deeper root system in both dry and wet treatments (Table 4).


   SUMMARY
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY
REFERENCES
 
We compared water depletion and end-of-season rooting depth of field-grown grain sorghum and sunflower in a randomized and replicated experiment to provide similar soil and environmental conditions for the two crops. Water depletion reached mean depths of 2.5 m for sorghum and 3.1 m for sunflower. Water depletion in the 2.2- to 3.3-m depth zone was significantly more in sunflower (48 mm) than in sorghum (14 mm). Water depletion fronts descended more rapidly and to deeper soil depths with sunflower than with sorghum in corresponding time periods. End-of-season rooting depth was significantly greater in sunflower (3.03 m) than in sorghum (2.54 m). The faster front advance and the deeper depth of water depletion compared with sorghum are factors contributing to drought avoidance in sunflower and its ability to deplete water from greater soil depths.

The qualities of a relatively fast-descending and deep root system give sunflower an advantage over many crops in the exploration of soil for water and in the recovery of NO3 ions from deeper soil depths (Sharpley et al., 1998; Halvorson et al., 2001). However, roots do not grow in dry soil (Merrill et al., 1996; Connor and Hall, 1997). Therefore, the potential for deeper rooting with sunflower will be an asset in rotations only if replenishment of subsoil water is achieved. Precipitation and within-profile water movement can replenish subsoil water during noncrop periods. The rotating of irrigated and dryland crops within cropping systems provides increased subsoil water for the dryland crop (Aronovici and Schneider, 1972). No-tillage cropping systems result in deeper water movement and more subsoil water than do conventional (sweep) tillage systems (Norwood, 1994). If increased amounts of water are stored at deeper soil depths, selection of a deep-rooted crop such as sunflower will more completely tap the water for crop production.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY
 REFERENCES
 
Contrib. 01-167-J, Kansas Agric. Exp. Stn.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY
 REFERENCES
 




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