Soil Water Storage During Fallow in the Central Great Plains as Influenced by Tillageand Herbicide Treatments1

D. E. SMIKA AND G. A. WICKS2

ABSTRACT

Soil water storage during the fallow period of winter wheat-sorghum-fallow (Triticum aestivum and Sorghum vulgare) andalternate winter wheat-fallow rotations was measured duringthree fallow periods. Storage was greater when herbicidesrather than conventional tillage practices were used to controlweeds. The storage increase of nearly 4 cm of available wateras a result of treatments was distributed throughout the profilein the 3-year rotation, but the greatest increase occurred in theupper 60 cm of soil. In the 2-year rotation the nearly 14-cmincrease in available water due to treatment was also presentin the entire profile, but most of the storage increase occurredto a depth of 210 cm, with soil water content to a depth of180 cm being at or near field capacity.

Soil water storage for the total fallow period of the 3-yearrotation ranged from 18.6 to 22.3 cm, having correspondingstorage efficiencies of 35.4 and 42.4%, for conventional tillageand complete herbicide treatments, respectively. Water storagefor the complete fallow period of the 2-year rotation rangedfrom 18.6 cm with spring plowing (bare soil) to 23.8 cm withtillage stubble mulch to 32.5 cm with complete use of herbi-cides, and had storage efficiences of 25.0, 32.0, and 43.7%,respectively.

Additional Key Words for Indexing: storage efficiency, stub-ble mulch, rotations.

IN THE winter wheat-producing semiarid Great Plains, theprimary purpose of fallowing is to store water in the soil

for the subsequent wheat crop. One of the historic criti-cisms of the practice of fallow is the relatively low storageefficiency. Storage efficiency being = (soil water gain duringfallow/Total fallow period precipitation) X 100. Previousworkers reported fallow efficiencies of 26% or less (7, 8,12), but recent workers using stubble mulch tillage havereported fallow efficiencies of 34% (3, 5). This is an 8%increase in storage efficiency, and improvement is believedpossible.

One approach to increasing storage efficiency during thefallow period is the use of herbicides instead of tillage forweed control. Army et al. (1) reported a higher water con-tent in the surface 6 cm of soil where herbicides were usedthan where conventional stubble mulch tillage practiceswere used during the fallow period. Wiese and Army (11)reported greater amounts of residue remained on the soil

592 SOIL SCI. SOC. AMER. PROC., VOL. 32, 1968

surface at the end of the fallow period when herbicidesrather than when conventional stubble mulch tillage prac-tices were used. Greb et al. (6) showed that soil water stor-age increased with increasing surface mulch rates. AndGreb (5) also showed that soil water losses by solar distil-lation decreased with increasing straw mulch rates. Theabove reports imply that the elimination of tillage opera-tions by the use of herbicides maintains more of the initialresidue on the soil surface, which increases water storage.Infiltration of water into the soil during a 2-hour periodwas found to be greater where herbicides were used forweed control than where conventional stubble mulch andother tillage practices were used (2).

The objectives of this study were to determine (i) if andwhen differences in soil water storage occurred duringthe fallow period as a result of tillage practices or herbicidetreatment; and (i i) if differences occurred, the location ofthe storage differences in the soil profile. The two predomi-nant rotations practiced in the Central Great Plains, win-ter wheat-sorghum-fallow and winter wheat-fallow (Triti-curn aestivum L. and Sorghum vulgare L.), were used inthis study.

METHODS AND MATERIALS

Data reported in this study were collected during the periodJuly 3, 1963 to September 17, 1966. All treatments had beenapplied 1 year prior to the data collection period reportedherein. Before initial treatment application, the area had beencontinuously cropped to sorghum for 2 years. Average lengthof the fallow period was 11 months for the wheat-sorghum-fal-low (3-year) rotation and 14.5 months for the wheat-fallow(2-year) rotation. Average annual precipitation for NorthPlatte, Nebraska is 49.3 cm. Precipitation during the period ofstudy was 49.8, 47.5, 71.2, and 44.7 cm for 1963, 1964, 1965,and 1966 respectively.

Soil of the experimental site is classified as Holdrege silt loamwhich was developed on Peorian loess and is 61 m deep at thislocation. The slope of the experimental area was < 0.5%. Indi-vidual plots were 4.2 m wide by 15.0 m long and were separatedby a 0.9-m buffer area between plots within each replication.Treatments were randomized in 3 replications in both rotations.In the 3-year rotation there were 3 blo'cks of treatments to pro-vide 1 block for each phase of the rotation each year. In the2-year rotation there were 2 blocks of treatments to provide 1block for each phase of the rotation each year.

Soil water was determined in all replications with a neutronprobe at 30-cm increments to a depth of 300 cm with the sur-face 30 cm determined gravimetrically. Downward water move-ment below the 300-cm sampling depth may have occurred incertain treatments in both rotations as no measurements weremade below 300 cm. However, upward water movement wouldnot be a factor because the water table is 61 m below the soilsurface. The standard error of the mean for sampling in theloess soil on the North Platte Station is 0.8 cm in a 300-cm pro-file. In the 3-year rotation, determinations were made at sor-ghum harvest in October, early spring (mid-April), early sum-mer (near July 1), and at the end of fallow (mid-September).In the 2-year rotation, determinations were made at wheatharvest (early July), late fall (mid-November), early spring(mid-April), early summer (near July 1), and at the end offallow (mid-September). These sampling dates divided the fal-low period of each rotation into soil water storage periods. Soilwater content for field capacity was determined at 1A bar bythe pressure plate method (9). Available soil water was deter-mined by subtracting the minimum point of exhaustion (4)from the quantity of water present in the soil.

Treatments used during the 3-year rotation are presented in

Table 1. Immediately following wheat harvest, atrazine granuleswere broadcast at the rate of 2.2 kg/ha. In the spring, atrazinewas sprayed at the rate of 2.2 kg/ha in the form of a wettablepowder. Treatments in the 2-year rotation are given in Table 2.Atrazine applications in this rotation were all at the rate of 1.7kg/ha and broadcast in granular form.

The contact herbicide used varied depending on the weedspecies to be killed (grassy or broadleaf), portion of the fal-low period, and climatic conditions at the time of application.All applications were made on weeds from emergence and5 cm in height. Chemicals and rates used were: amitrole at 0.6to 2.2 kg/ha, paraquat at 1.1 kg/ha, and 2, 4-D at 1.1 kg/ha.Tillage equipment mounted on tool bars were: five 76-cmsweeps, three 152-cm sweeps, and a 3.6-m rodweeder withshovel points attached immediately in front of the rod. Herbi-cides and tillage were used only as needed to control weedgrowth.

RESULTS AND DISCUSSION

3-Year Rotation

During the period of study all plots started the fallowperiod with an average of 1,800 kg/ha of sorghum residueon the soil surface as measured by standardized procedures(10). Treatments D3 and E3 had an average of 900 kg/haof wheat residue on the surface in addition to the sorghumresidue. At the end of the fallow period treatments A3, B3,and C3 had an average of 560 kg/ha of sorghum residueand treatments D3 and E3 had an average of 1,000 kg/haof mixed wheat and sorghum residue on the soil surface.At the beginning of the fallow period stored soil water wassimilar for all treatments (Table 3); therefore, water stor-age during the fallow period of this rotation was not affectedby an initial water differential. Soil water change by stor-age period and for the entire fallow period are shown inTable 3. Treatments are ranked from lowest to higheststorage for the total fallow period.

Water storage during the overwinter period ranged froma low of 10.9 cm with treatment A3 (tillage stubble mulch)to a high of 14.2 cm with treatment D3 (herbicides only).From early spring to early summer there was no statisti-cal difference in water storage between treatments buttreatments A3 and D3 again had the lowest and higheststorage of 9.7 and 11.9 cm, respectively. During the sum-mer period all treatments lost water—treatment D3 hadthe greatest loss of 3.8 cm—but there was no significant

Table 1—Tillage treatments used in 3-year rotation

Treatmentsymbol

cjE!

Treatment from when harvestto sorghum planting

Fall

SubtillageSubtillageAtrazineAtrazineSubtillage

Spring

DiskAtrazineAtrazineAtrazineAtrazine

Treatment from sorghumharvest to wheat seeding*

Subtillage (5)fSubtillage (4)Subtillage (4)Contact herbicide (4-6)Contact herbicide (4-6)

* No treatments were made until spring following sorghum harvest.•j- Values in parentheses denote number of operations.

Table 2—Tillage treatments used in 2-year rotationTreatment

symbol

clD,

Initial fallow operationfollowing when harvest

Plow*SubtillageAtrazine followed by subtillageAtrazineAtrazine

Subsequent fallowoperations

Subtillage (5)tSubtillage (5)Subtillage (5)Subtillage (4)Contact herbicide (4-6)

October plow 1964, April plow 1965 and 1966.Values in parentheses denote number of operations.

SMIKA AND WICKS! SOIL WATER STORAGE DURING FALLOW 593

Table 3—Soil water storage by fallow periods and total fallow storage and efficiency as affected by rotation treatment in 3- and2-year rotations at North Platte, Nebraska. Average of 3 fallow periods, 1963-66

Soil water gain during fallow periods

Rotationtreatment

Initialsoil

water

Harvestto

fall*

Early spring Early summerOver- to

wintert early summerto end of

fallow

Fallowperiodtotal}

Fallowstorage

efficiency?centimeters of soil water in 300-cm soil orofile

A,

Bj

DJAvg precipitation

A,

c\ ' -EI

Avg precipitation* Period from July 3 to Nov, 15.

23.423.423. 123.923.6

25.425.425.125.324. 9

t

__

——_ _ _ _- ——

-6. 6 a-1. Ob3.0 be0. 8b5. Sc

23.9

3-Year Rotation10. 9aH10. 7 a12. 7ab12. 7ab14. 2 b13.2

2-Year Rotation12. 2 a12. 4 a11. 2 a13. 2 a15. 2 a11.2

Period from Oct. 6 to April 18 in 3-yr rotation and from

9. 7a11. 7a10. 4 a11. 2a11. 9a22.6

7. 9a8. la

11. 2 a11. 2 a11. 2a22.6

-2.0 a-1. 3a-I.Sa-2. 3 a-3. 8 a16.8

5. Ib4. 3b1. Sab2. Sab0.3 a

16.7

18. 6 a21. lab21. Sab21. 6 ab22. 3 b52.6

18. 6 a23. 8 b27. 2b27. 5b32.5074.4

35.440.140.541.142.4

25.032.036.637.043.7

Nov. 15 to April 18 in 2-yr rotation.t Eleven months In 3-yr rotation and 14.5 months in 2-yr rotation.K fc« , or Fallow period storage total „ -..8 Efficiency % = Fallow grlod precipitation total" 10°-IT Values In the same period accompanied by the same letter or letters are not significantly different at the 5% level.

Table 4—Average available soil water content of profile to adepth of 300 cm at the end of fallow for the period

1963 through 1966 for various treatments in awinter wheat-sorghum-fallow rotation and soil

water content at V6 bar (field capacity)Treatment

Depth

306090

120150180210240270300

Total

(A,)till.S.M.

5. 9 a*5. 9a7 . 0 a6.4a4 .6a3. 5a3.0a2. Sa1.5a1.4a

42. Oa

till' - till. -herb. S .M. herb. S.M.

6. la5. 7 a7. la7. Ob5. 2 b3. 6 a3.0 a2. 9a1.9aI .Sa

44.3ab

——— cm ——5. Oa6. Oa7. la6. 5 a5. Oa3. Sab3. 6 b2. Sa1. 9a1. Sa

herb.S.M.

6 . 7 b6. 5 b7.0a6.4a4. Sab4.1 b3. 3ab3. laI .SaI .Sa

44. Sab 45. Sab* Treatment values within the same depth accompanied by the

not significantly different at the 5 % level. There is no mea;between depths.

(IheS.

6.6.6.6.5.3.3.3.1.1.

46.

'A'.M.

6b6b9a7ab7c7ab5bOaSa6aOb

same letter orsure of signific;

Fieldcapacity

7. 38.08.28. 28.46 .75.65.65.65.7

69. 3letters areince

differences between treatments. Water storage during theentire fallow period was significantly greater where onlyherbicides were used (treatment D3) when compared toconventional tillage (treatment A3) 22.3 vs. 18.6 cm, re-spectively. However, there was no significant differencebetween any of the tillage treatments receiving herbicides.The above water storage trends were consistent each yearof the study.

Storage efficiencies ranged from a low of 35.4% for treat-ment A3 to a high of 42.4% for treatment D3. These stor-age efficiencies are considerably above those reported inrecent work (3, 6). However, this rotation involves only11 months, whereas previous reports are for 14 months.Therefore, in this area a 3-year rotation that involves ashorter fallow period may be one approach to increasingwater storage efficiency even with conventional tillage. Theuse of herbicides throughout the rotation to provide maxi-mum weed control and maximum residue preservationcould be utilized for greater storage efficiency.

Available water content of the soil to a depth of 300 cmat seeding (Table 4) shows no difference between treat-ments below a depth of 240 cm. However, where only her-bicides were used throughout the entire rotation, more

Table 5—Grain yields obtained in 3-year and 2-year rotationsfor various treatments. Three-year average.

North Platte, Nebraska

Treatment

A,B,c,D.E,

A,B,c,°>E,

CropWheat

kg/ha3-Year Rotation

3, 490 a*3,760a3, 630 a3,490a3,630a

2-Year Rotation3, 090 a3,360ab3,290ab3,360ab3,560b

Sorghum

4,080a4, 200 a4,580ab4, 890 b5,020b

----- _ _ _

——

* Values for each crop within each rotation accompanied by the same letter or lettersare not statistically different at . 05 level.

available water was present in the upper 60 cm of soilthan where tillage operations were used. Water contentof the upper 30 cm approached field capacity at the end ofthe fallow period. The presence of this water near the soilsurface may also account for the greater water loss fromthese treatments during the early summer to end of fallowperiod.

The water near the soil surface may be important in theestablishment of winter wheat stands in certain years. Thecrop yields obtained in this rotation are presented inTable 5. These yields varied between years; however, theaverage wheat yields were not reduced where herbicideswere used, and average sorghum yields were significantlyincreased.

2-Year Rotation

During this study all treatments started the fallow periodwith an average of 6,500 kg/ha wheat residue as determinedby standardized procedures (10). At the end of the fallowperiod, treatment A2 was bare, treatments B2, C2, and D2averaged 1,700 kg/ha of residue and treatment E2 had3,100 kg/ha of residue. Soil water content at the beginningof this rotation was also similar between treatments (Table3), eliminating any initial water differential effect on waterstorage. Changes in soil water content by storage periodand for the complete fallow period are shown in Table 3.

594 SOIL SCI. SOC. AMER. PROC., VOL. 32, 1968

Treatments are ranked from lowest to highest storage forthe complete fallow period.

From harvest to fall freezeup, treatments A2 and B2lost water while the other treatments gained soil water(Table 3). Complete weed control by herbicides (treat-ment E2) resulted in the largest soil water gain of 5.8 cm,though the gain was only 24.5% of- the precipitation re-ceived during this period. Storage during this period isimportant because over 30% of the total fallow precipita-tion is received at this time. The present study suggests thatthe cultural practices commonly used in the Central GreatPlains (treatments A, and B2 no fall tillage and one fallsubtillage, respectively) are not storing water during thelate summer through fall fallow period in years when initialsoil water content is high as occurred during this study.

Overwinter water storage by the treatments was notstatistically different, but storage was 3 cm higher on treat-ment E., than with treatment A2 (bare soil). From earlyspring to early summer, there was also no statistical differ-ence in storage between treatments, even though all treat-ments receiving herbicides (treatments C2, D2, and E2)stored 3.1 cm more water than conventional tillage stubblemulch. The use of herbicides during the first three storageperiods of the fallow period resulted in greater waterstorage. This greater water storage is attributed to nearlycomplete weed control and greater preservation of residues.Weed control with tillage alone is often difficult to obtainduring this portion of the fallow period.

From early summer to end of fallow, treatments A2 andB2 stored 5.1 and 4.3 cm of water, respectively, whiletreatment E2 stored only 0.3 cm of water. This might beexpected because at the beginning of this period treat-ments A2 and B2 were lower in water and would be morereceptive to storage ificreases.

Water storage for the entire fallow period was lowestwith treatment A, (18.6 cm) where no weed control wasdone until the spring following wheat harvest and highestwith treatment E2 (32.5 cm) where near complete weedcontrol was practiced during the entire fallow period.Water storage in treatment E2 was significantly greater thanall other treatments. Treatment B2 (tillage stubble mulch)was significantly better than treatment A2 (spring plow)but there was no statistically significant difference betweentreatments B2, C2, and D2. Water storage in each of thesetreatments was consistent and ranked the same each yearof the study.

Storage efficiencies ranged from 25.0 to 43.7% for treat-ments A2 through E2. This high storage efficiency is largelydue to the increased storage during the harvest to fallperiod. Nearly complete weed control by herbicides duringthis time and maximum preservation of residues during theentire fallow period have been shown to increase waterstorage (6). During the 14 month fallow period there wereperiods in which water storage was very low; nevertheless,the total storage efficiency where only herbicides wereused was very good. In this study the all herbicide treat-ment (E2) had the highest storage efficiency of all treat-ments with 43.7% which is nearly 20% greater than thebest reports for work in the 1930's and 1940's (7, 8, 12)

Table 6—Average available soil water content of profile to adepth of 300 cm at the end of fallow for the period

1963 through 1966 for various treatments in awinter wheat-fallow rotation and soil water

content at Vs bar (field capacity)

Depth

306090

120150180210240270300

Total

AzPlow

6.5.7.7.6.4.2.1.0.0.

7b*6a7ab2alaSa8a8a9a4a

44. Oa

B2till.

S.M.

5.6.7.7.7.5.3.3.1.1.

9a2b5alaOb5b6bOb9b5b

49. 2b

TreatmentC2

till. -herb. S. M.

6 .2ab6. lab7.9ab7. la7. 3b5. 8b3. 8b3. 3b2 .6b2. 2 ed

52. 3b

D2till. -

herb. S. M.

6.5.7.7.7.6.4.3.2.2.

52.

2ab9ab9ab2a3bObcObc3b6b4d8b

E2herb.S.M.

6. 8b7 . 2 e8. 2 b7. 9b8 . 4 C6. 6c4. 6c3. 5b2. 5b1. 7bc

5 7 . 4 0

Fieldcapacity

7.38.08.28.28.46. 75.65.65.65.7

69. 31 Treatment values within the same depth accompanied by the same letter or letters

are not significantly different at the 5% level. No measure of significance betweendepths has been made.

and approximately 10% greater than with present stubblemulch equipment (3, 6, and treatment B2). At Sidney,Montana (3) during a fallow period comparable in lengthto this study, water storage efficiency where only herbi-cides were used ranked 6 out of a total of 10 treatmentswhich included various tillage alone and tillage-herbicidecombinations.

At the end of fallow treatment, E2 had the highestwater content of all treatments to a depth of 240 cm (Table6). Available water content of the soil profile under thistreatment was at or near field capacity to a depth of 180cm. This high water content in the surface soil mightaccount for the low storage during the early summer toseeding storage period.

The high available water content of the surface soil inthis rotation is also very important in the establishment ofwinter wheat. The winter wheat yields obtained followingfallow in this rotation (Table 5) show the lowest yieldswhere the residue was plowed under (treatment A2) andthe highest yields where only herbicides were used (treat-ment E2). This yield pattern is identical to the soil waterstorage pattern at the end of the fallow period.

In the two rotations studied the use of herbicides insteadof tillage was of definite value in increasing storage of soilwater during the fallow period.

NOTES 595