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Fallow method influences on soil water and precipitation storage efficiency
Soil & Tillage Research, 9 (1987)307-316
307
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
F a l l o w M e t h o d I n f l u e n c e s on Soil W a t e r a n d Precipitation Storage Efficiency I D.L. TANAKA and J.K. AASE
United States Department of Agriculture, Agriculture Research Service, P.O. Box 1109, Sidney, MT59270 (U.S.A.) (Accepted for publication 14 November 1986)
ABSTRACT Tanaka, D.L. and Aase, J.K., 1987. Fallow method influences on soil water and precipitation storage efficiency. Soil Tillage Res., 9: 307-316. Summer fallowing is practiced in the Great Plains of the U.S.A. in order to store soil water, control weeds, make nutrients available and stabilize crop yields. Soil water storage and precipitation storage efficiencies on chemical and stubble-mulch fallow plots were compared for three 14-month fallow periods for a winter-wheat-fallow rotation ( Triticum aestivum L. ) and three 21month fallow periods for spring-wheat-fallow rotation in the northern Great Plains to determine during which seasonal segment the fallow method might influence soil water storage. The experiment was conducted on a glacial till Williams loam (fine-loamy mixed, Typic Argiboroll) from July 1981 to April 1985. Soil water contents to a depth of 1.70 m were measured, using the neutron scatter technique, for seasonal segments during the fallow period. Soil water storage was similar from harvest to spring on chemical and stubble-mulch fallow plots. The over-winter to spring segment resulted in the most consistent precipitation storage efficiencies (33.3-71.1% ). Soil water storage differences as a result of a fallow method are most likely to occur during summer fallow for 14-month winter-wheat-fallow rotations and during the second overwinter for 21-month springwheat-fallow rotations. Soil water storage for the entire fallow period was greater on chemical fallow than on stubble-mulch fallow in two out of three 14-month winter-wheat fallow periods and in one out of three 21-month spring-wheat fallow periods.
INTRODUCTION S u m m e r f a l l o w i n g is p r a c t i c e d i n t h e s e m i - a r i d G r e a t P l a i n s o f t h e U . S . A . t o s t o r e soil w a t e r , c o n t r o l w e e d s , m a k e n u t r i e n t s a v a i l a b l e a n d s t a b i l i z e c r o p y i e l d s . A m o u n t o f soil w a t e r s t o r e d d e p e n d s o n c l i m a t e , soils, t i l l a g e , p r e v i o u s c r o p a n d q u a n t i t y o f s u r f a c e r e s i d u e m a i n t a i n e d d u r i n g fallow. Soil w a t e r e v a p oration makes summer fallowing very inefficient in storing summer precipi~Contribution from the United States Department of Agriculture, Agriculture Research Service, in cooperation with the Montana Agricultural Experiment Station, Journal Series No. J-1864.
0167-1987/87/$03.50
© 1987 Elsevier Science Publishers B.V.
308 tation (Black et al., 1974). Surface residue during fallow provides physical protection for soils to control wind and water erosion. Residue can reduce evaporation by decreasing air movement immediately above the soil, changing albedo and insulating the soil surface. How surface residue is managed can be a factor in minimizing evaporation and increasing soil water storage ( Bond and Willis, 1969; Smika et al., 1969; Hammel et al., 1981; Fenster and Wicks, 1982). Chemical and stubble-mulch fallow maintained 70-80% and 25-45%, respectively, of the residue present after harvest during a 14-month fallow period ( Fenster and Peterson, 1979; Tanaka, 1986 ). Greater quantities of surface residue during fallow should suppress evaporation and increase potential for chemical fallow to store more soil water than stubble-mulch fallow. Chemical fallow has stored more soil water than stubble-mulch fallow during 14 months of fallow in the central Great Plains (Good and Smika, 1978; Fenster and Peterson, 1979; Greb and Zimdahl, 1980). In the northern Great Plains, differences in soil water storage caused by increased quantities of residue during 21-month fallow periods have not been evident (Black and Power, 1965; French and Riveland, 1980; Deibert et al., 1986). In the central Great Plains, conserving crop residues by reducing or eliminating mechanical tillage has increased precipitation storage efficiency (percentage of precipitation stored in the soil profile ) from 20-24% for bare fallow to 40-55% for no-till or chemical fallow. Increased efficiencies for chemical fallow result from greater quantities of surface residue, minimum residue disturbance and better weed control ( Greb, 1983 ). In the northern Great Plains, precipitation storage efficiencies for 14- and 21-month fallow periods range from 14 to 40% (Black and Power, 1965; Haas et al., 1974). Precipitation storage efficiencies are usually the highest after harvest and through the first winter, with a range of 35-154% and averaging 71%. Summer fallow precipitation storage efficiencies are low due to evaporation, and range from a loss of 11 to a gain of 25%, with an average gain of 5.9% (Black et al., 1974). Small gains, and in most cases loss of soil water, occurred over the second winter during a 21-month fallow period. Precipitation storage efficiency data for seasonal fallow segments have been studied for stubble-mulch tillage with little or no data for chemical fallow. The purpose of our study was to compare soil water storage and precipitation storage efficiencies on chemical and stubblemulch fallow plots for 14- and 21-month fallow periods in the northern Great Plains of the U.S.A., and to determine during which seasonal segment fallow methods might influence soil water storage. MATERIALSAND METHODS The study began in April 1980 and was conducted on a glacial till Williams loam (fine-loamy mixed, Typic Argiboroll) in the northern Great Plains 11 km northwest of Sidney, Montana. The Williams loam in our study contained
309
28.8, 39.7 and 31.5%, sand, silt and clay, respectively. Four replications each of chemical and stubble-mulch fallow plots, with a plot size of 7 × 40 m were arranged in a randomized complete-block design for winter- ( T r i t i c u m aestiv u m L. ) and spring-wheat-fallow rotations. Chemical fallow plots were sprayed 3-5 times with glyphosate (N- (phosphon-methyl) glycine), a non-selective translocating herbicide, for weed control during fallow. Residue and soil were undisturbed throughout fallow. Herbicide application rates ranged from 0.4 to 0.8 kg ha 1depending on weed species and climatic conditions. Stubble-mulch fallow plots were tilled with 0.50-m sweeps to a depth of about 0.10 m in late May, followed by 3-5 tillage operations with a rod weeder, as needed, to control weeds during fallow. The quantity of surface residue on chemical and stubble-mulch fallow plots was measured after harvesting winter and spring wheat, and again before seeding winter and spring wheat, by sampling two 1-m 2 samples per plot according to methods described by Whitfield et al. (1962). The neutron scatter technique was used to determine soil water storage to a depth of 1.70 m in 0.20-m increments. The first measurement was at the 0.20m depth and accounted for the surface 0.30 m of soil water. Soil water storage was determined for the 1981-1982, 1982-1983 and 1983-1984 14-month winter-wheat fallow periods and the 1981-1983, 1982-1984 and 1983-1985 21month spring-wheat fallow periods. The 14-month winter-wheat fallow period began in late July after winter-wheat harvest, and continued through the winter and summer months until winter-wheat seeding time in mid-September; the 21-month spring-wheat fallow period began in mid-August after springwheat harvest and continued through two winters until spring-wheat seeding time in mid-April. Calculations of soil water storage and precipitation storage efficiencies began at harvest and ended with the seeding of the next crop. Each fallow period was divided into seasonal segments which consisted of harvest to soil freeze-up (harvest to October), over-winter to spring ( November to April ), summer fallow ( May to October, or until seeding winter wheat in mid-September) and second overwinter (from November until seeding spring wheat in mid-April). Precipitation storage efficiency was calculated by dividing the increase in soil water content to a depth of 1.70 m for each seasonal segment or fallow period by the precipitation received during the seasonal segment or fallow period. Precipitation adjacent to the study was measured using a standard 203-mm weighing rain gauge. Average long-term precipitation since 1949 was from 11 km southeast of the study area. Seasonal segment precipitation probabilities were calculated from long-term averages. Single degree of freedom (1 df) comparisons were used to determine residue and soil water content differences ( Steel and Torrie, 1960).
310 TABLE I Average long-term monthly precipitation (mm) and precipitation adjacent to the study (mm) from January 1981 to April 1985 Month
Average 1949-1984
1981
1982
1983
1984
1985
January February March April May June July August September October November December
l0 9 13 29 50 73 44 41 33 21 11 11
1 5 5 27 25 91 41 40 15 20 10 15
27 12 39 25 55 61 24 20 22 67 3 17
8 3 12 1 19 40 47 14 15 15 8 13
12 1 12 17 5 75 5 15 26 4 13 4
2 2 14 33
RESULTS AND DISCUSSION
Precipitation varied considerably from one fallow period to the next and for each seasonal segment. Total precipitation for the 1981-1982, 1982-1983 and 1983-1984 14-month winter-wheat fallow periods was 95, 67 and 54%, respectively, of the average 414 mm (Table I). Total precipitation for the 1981-1983, 1982-1984 and 1983-1985 21-month spring-wheat fallow periods was 90, 70 and 56%, respectively, of the average 523 mm. Seasonal segment precipitation varied from 42% of the average 95 mm from harvest to soil freeze-up in 1983-1984 and 1983-1985 fallow periods to 154% of the average 83 mm for overwinter to spring in 1981-1982 and 1981-1983 fallow periods. The quantity of surface residue after harvest and before seeding are shown in Table II. The quantity of surface residue did not differ between chemical and stubble-mulch fallow plots after harvest, but about twice as much surface residue was present on chemical fallow plots as compared with stubble-mulch fallow plots at the end of the fallow period. Soil water content changes to a depth of 1.70 m for the harvest to soil freezeup segment ranged from a loss of 14 mm to a gain of 100 mm. Precipitation storage efficiencies ranged from - 3 2 . 5 to 91.7% (Tables III-VIII). Moist soil conditions at harvest, high evaporative demand and small amounts of precipitation during the first fallow segment resulted in the loss of soil water and the negative precipitation storage efficiencies for the 1981-1983 21-month springwheat fallow period (Table IV). The chance of receiving average or aboveaverage precipitation from long-term data was about 60% for winter-wheat fallow, compared to about 40% for spring-wheat fallow during the first segment. This difference was due to a later spring-wheat harvest.
311 TABLE II The quantities of winter and spring wheat residue (kg ha 1) after harvest and prior to seeding a crop on chemical (CF) and stubble-mulch (SM) fallow plots CF
SM
1 df comparisons (CF vs. SM) ~
Winter wheat (14-month) 1981-1982 After harvest Before seeding
3700 2800
3700 1000
NS *
Spring wheat (21-month) 1981-1983 After harvest Before seeding
2400 1100
2400 300
NS *
Winter wheat (14-month) 1982-1983 After harvest Before seeding
5300 3100
4500 1500
NS *
Spring wheat (21-month) 1982-1984 After harvest Before seeding
2700 1400
2400 800
NS *
Winter wheat (14-month) 1983-1984 After harvest Before seeding
4200 3400
4600 2200
NS *
Spring wheat (21-month) 1983-1985 After harvest Before seeding
3800 1300
3900 500
NS *
~*, Significantly different at 0.05 probability level; NS, not significantly different. D u r i n g t h e s e c o n d fallow s e g m e n t , soil w a t e r gain r a n g e d f r o m 16 to 91 m m . T h i s s e g m e n t h a d t h e m o s t c o n s i s t e n t p r e c i p i t a t i o n s t o r a g e efficiencies (33.3 to 71.1% ) of a n y fallow s e g m e n t a n d were similar to w h a t G r e b (1983) r e p o r t e d for t h e s a m e p e r i o d in t h e c e n t r a l G r e a t Plains. T h e c h a n c e for receiving average or a b o v e - a v e r a g e p r e c i p i t a t i o n for this s e g m e n t was a b o u t 60% for w i n t e r a n d s p r i n g - w h e a t fallow periods. In general, p r e c i p i t a t i o n s t o r a g e efficiencies a n d soil w a t e r s t o r a g e i n c r e a s e d as s n o w a c c u m u l a t i o n increased. Soil w a t e r s t o r e d d u r i n g t h e first two fallow s e g m e n t s a v e r a g e d 70 a n d 60% of t h e t o t a l soil w a t e r s t o r e d in t h r e e s e g m e n t s of a 1 4 - m o n t h a n d four s e g m e n t s of a 21m o n t h fallow period, respectively. S p r i n g - w h e a t fallow (21 m o n t h s ) s t o r e d less soil w a t e r t h a n w i n t e r - w h e a t fallow (14 m o n t h s ) b e c a u s e of l a t e r h a r v e s t date, less s u r f a c e residue a n d longer fallow period. D u r i n g y e a r s of b e l o w - a v e r a g e p r e c i p i t a t i o n for t h e first two fallow segm e n t s , w h i c h f r o m l o n g - t e r m d a t a o c c u r r e d a b o u t 40% of t h e t i m e for w i n t e r w h e a t fallow a n d 50% of t h e t i m e for s p r i n g - w h e a t fallow, soil w a t e r s t o r a g e
312 T A B L E III The 1981-1982 14-month w i n t e r - w h e a t fallow soil water gain or loss to a depth of 1.70 m and p r e c i p i t a t i o n storage efficiency based on stored soil water and p r e c i p i t a t i o n on chemical (CF) and stubble-mulch ( S M ) fallow plots Seasonal segments
Soil water gain ( + ) or loss( ) ( r a m ) CF
Harvest to soil freeze-up Over-winter to spring S u m m e r fallow Total
SM
1 df comparisons ( CF vs. SM) ~
P r e c i p i t a t i o n storage efficiency (To) CF
SM
Precipitation (mm)
+ 16
+ 17
NS
15.2
16.2
105
+ 69 + 33
+ 74 + 19
NS NS
53.9 20.5
57.8 11.8
128 161
+ 118
+ 110
NS
29.9
27.9
394
~NS, not significantly different.
during summer fallow (third segment) can be important (Tables III, IV, VII and VIII). Soil water gain depends on previous soil water content, amount, frequency and distribution of precipitation, and quantity of surface residue to suppress evaporation. Evaporative demand during the summer fallow segment is high, limiting soil water storage. Stored soil water for the first two fallow segments during 1981-1982 (Table III) and 1982-1983 14-month winter-wheat fallow ( Table V) and during 1982-1984 21-month spring-wheat fallow (Table VI) was relatively high, and resulted in small soil water gains and low precipitation storage efficiencies during the summer fallow segment of 1982 and 1983. In contrast, stored soil water was low for the first two fallow segments during 1983-1984 (Table VII), resulting in high precipitation storage efficiencies T A B L E IV The 1981-1983 21-month spring-wheat fallow soil water gain or loss to a depth of 1.70 m and p r e c i p i t a t i o n storage efficiency based on stored soil water and p r e c i p i t a t i o n on chemical (CF) and stuble-mulch ( S M ) fallow plots Seasonal segments
Soil water gain ( + ) or loss( - ) ( m m ) CF
Harvest to soil freeze-up Over-winter to spring S u m m e r fallow Second overwinter Total
1 df comparisons (CF vs. S M ) '
SM
P r e c i p i t a t i o n storage efficiency (%) CF
SM
Precipitation (mm)
- 14
- 13
NS
-35.0
-32.5
40
+ 89 + 70 0
+91 + 53 +3
NS NS NS
69.5 28.1 0.0
71.1 21.3 7.0
128 249 43
+ 145
+ 134
NS
31.5
29.1
460
~NS, not significantly different.
313
TABLE V
The 1982-1983 14-month w i n t e r - w h e a t fallow soil water gain or loss to a d e p t h of 1.70 m and precipitation storage efficiency based on stored soil w a t e r and p r e c i p i t a t i o n on chemical (CF) and stubble-mulch ( S M ) fallow plots Seasonal segments
Soil water gain ( + ) or
loss( - ) ( m m ) CF Harvest to soil freeze-up Over-winter to spring S u m m e r fallow Total
1 df comparisons (CF vs. S M ) 1
SM
P r e c i p i t a t i o n storage efficiency (%) CF
SM
Precipitation (mm)
+ 98
÷ 88
NS
89.9
80.7
109
+ 23 + 18
+ 16 + 16
NS NS
52.3 14.3
36.4 12.7
44 126
+ 139
+ 120
*
49.8
43.0
279
L., Significantly different at 0.05 probability; NS, not significantly different.
during the 1984 summer fallow segment. Chemical fallow stored significantly more soil water than stubble-mulch fallow during the 1982-1983 (Table V) and 1983-1984 (Table VII) 14-month winter-wheat fallow periods. This was because the greater quantities of winter-wheat residue on chemical than on stubble-mulch fallow plots (Table II) more effectively suppressed evaporation. The quantities of spring-wheat residue on chemical fallow plots were too small to cause significantly more soil water storage on them than on stubblemulch fallow plots during the summer fallow segment. The second overwinter segment for 21-month spring-wheat fallow resulted in small gains and, in some instances, losses of soil water because (1) soils were near field capacity, (2) there were small quantities of surface residue, T A B L E VI
The 1982-1984 2 1 - m o n t h s p r i n g - w h e a t fallow soil water gain or loss to a d e p t h of 1.70 m and precipitation storage efficiency based on stored soil water and p r e c i p i t a t i o n on chemical ( C F ) and stubble-mulch ( S M ) fallow plots Seasonal segments
Soil water gain ( ÷ ) or loss( - ) ( m m ) CF
H a r v e s t to soil freeze-up Over-winter to spring S u m m e r fallow Second o v e r w i n t e r Total
1 df comparison (CF vs. S M ),
SM
P r e c i p i t a t i o n storage efficiency (%) CF
Precipitation (mm)
SM
+ 100
+ 88
NS
91.7
80.7
109
+ 25 + 15 - 5
+ 23 + 19 - 7
NS NS NS
56.8 10.0 - 10.6
52.3 12.7 - 14.9
44 150 47
+ 135
+ 123
NS
37.5
34.3
358
'*, Significantly different at 0.05 probability; NS, not significantly different.
314 T A B L E VII The 1983-1984 14-month w i n t e r - w h e a t fallow soil water gain or loss to a depth of 1.70 m and p r e c i p i t a t i o n storage efficiency based on stored soil water and p r e c i p i t a t i o n on chemical (CF) and stubble-mulch ( S M ) fallow plots Seasonal segments
Soil water gain ( + ) or loss(- ) (mm) CF
1 df comparisons (CF vs. S M ) I
SM
P r e c i p i t a t i o n storage efficiency (%) CF
SM
Precipitation (mm)
Harvest to soil freeze-up Over-winter to spring S u m m e r fallow
+7
+5
NS
15.9
11.4
44
+ 21 +56
+ 33 +28
NS *
33.3 48.3
52.4 24.1
63 116
Total
+ 84
+ 66
*
37.7
29.6
223
~*, Significantly different at 0.05 probability; NS, not significantly different.
and (3) frozen wet soils reduced infiltration (Tables IV and VI). If dry climatic conditions existed during previous fallow segments, soil water gains over the second winter segment can be significant, since from long-term data a 60% chance occurred for average or above-average precipitation. For example, during the 1983-1985 21-month spring-wheat fallow period, chemical fallow plots stored twice as much soil water than stubble-mulch fallow plots for the second overwinter segment (Table VIII). Twice as much surface residue, with 25% positioned vertically, held snow from late-winter storms to double precipitation storage efficiencies. As a consequence, chemical fallow plots gained significantly more soil water than stubble-mulch fallow plots at 0.20- and 0.40-m soil depths (Fig. 1 ). Only once during the three 21-month spring-wheat fallow T A B L E VIII The 1983 1985 21-month spring w h e a t fallow soil water gain or loss to a depth of 1.70 m and precipitation storage efficiency based on stored soil water and p r e c i p i t a t i o n on chemical (CF) and stubble-mulch ( S M ) fallow plots Seasonal segments
Soil water gain ( + ) or loss{ - ) ( m m ) CF
Harvest to soil freeze-up Over-winter to spring S u m m e r fallow Second overwinter Total
1 df comparisons (CF vs. SM)
SM
P r e c i p i t a t i o n storage efficiency (%) CF
SM
Precipitation (mm)
+7
+6
NS
15.9
13.6
44
+ 32 +38 +33
+ 22 +30 + 15
NS NS *
50.8 29.7 67.3
34.9 23.4 30.6
63 130 49
+ 110
+ 73
*
38.5
25.5
286
'*, Significantly different at 0.05 probability; NS, not significantly different.
315 SOIL
0
0
WATER
CONTENT
O. 10 O. 2 0 I
O. 3 0
i
0
(m 3 /m ~ )
0. i0
i
LSO X
=
CF
0
=
SM
O. 3 0
0.20
i
i
I
(0.
10) H
t
AO. 4 E v ]Z
~0.8 [D /
ml.2
1.6 23
OCT
984
12
APR
1985
Fig. 1. Profile soil water contents during the second overwinter segment on chemical (CF) and stubble-mulch (SM) fallow plots for the 1983-1985 21-month spring-wheat fallow period.
periods was there a difference in total profile soil water storage between the two fallow treatments. The extra soil water on chemical fallow plots near the soil surface can be important in seed germination and seedling establishment. Soil water storage on chemical and stubble-mulch fallow plots was similar for the first two fallow segments. Differences between chemical and stubblemulch fallow plots would only be likely to occur if plot residue orientation were altered by mechanical tillage after harvest or if weed growth caused soil water losses. One would expect soil water storage differences between chemical and stubble-mulch fallow plots to begin to appear after the first tillage operation in late May on stubble-mulch fallow plots and for them to continue throughout the remainder of fallow. The data suggest that during dry years, chemical fallow plots can store more soil water than stubble-mulch plots during 14-month winter-wheat fallow periods, but spring wheat may not produce sufficient quantities of residue for differences to occur. Soil water differences because of the fallow method are most likely to occur during the summer fallow segment for 14-month winter-wheat fallow periods. During the second overwinter of a 21-month spring-wheat fallow period, vertical and prone residue on chemical fallow plots held snow, and resulted in significantly greater soil water gains than on stubble-mulch fallow in one out of three 21-month fallow periods. Chemical fallow maintained greater quantities of surface residue, with a higher proportion of residue remaining vertical, than stubble-mulch fallow. Chemical fallow increased the chance of storing additional soil water during years with erratic precipitation patterns. Low quantities of residue and/or flattened stubble due to tillage on stubble mulch plots decreased the chance of storing additional soil water.
316 ACKNOWLEDGEMENTS
We thank Mr" R:L. Kolberg for his assistance with field work and data collection an'd summarization.
REFERENCES Black, A.L. and Power, J.F., 1965. Effect of chemical and mechanical fallow methods on moisture storage, wheat yields, and soil erodibility. Soil Sci. Soc. Am. Proc., 29: 465-468. Black, A.L., Siddoway, F.H. and Brown, P.L., 1974. Summer fallow in the northern Great Plains (winter wheat). In: Summer Fallow in the Western United States. U.SID.A.' Conserv. Res. Rep. No. 17, U.S. Government Printing Office, Washington, DC, pp. 36-50. Bond, J.J. and Willis, W.O., 1969. Soil water evaporation: Surface residue rate and placement effects. Soil Sci. Soc. Am. Proc., 33:445 448. Deibert, E.J., French, E. and Hoag, B., 1986. Water storage and use by spring wheat under conventional tillage and no-till in continuous and alternate crop-fallow systems in the northern Great Plains. J. Soil Water Conserv., 41: 53-58• Fenster, C.R. and Peterson, G.A., 1979. Effects of no-tillage fallow as compared to conventional tillage in a wheat-fallow system. Res. Bull. 289, Agric. Exp. Stn., University of Nebraska, Lincoln, pp. 1-28. Fenster, C.R. and Wicks, G.A., 1982. Fallow systems for winter wheat in western Nebraska. Agron. J., 74: 9-13. French, E.W. and Riveland, N., 1980. Chemical fallow in a spring wheat-fallow rotation. N. D. Farm Res., 38(1): 12-15. Good, L.G. and Smika, D.E., 1978. Chemical fallow for soil and water conservation in the Great Plains. J. Soil Water Conserv., 33: 89-90. Greb, B.W., 1983. Water conservation: Central Great Plains. In: H.E. Dregne and W.O. Willis (Editors), Dryland Agriculture. Agronomy Monograph 23, American Society of Agronomy, Madison, WI, pp. 57-70. Greb, B.W. and Zimdahl, R.L., 1980. Ecofallow comes of age in the central Great Plains. J. Soil Water Conserv., 35: 230-233. Haas, H.J., Willis, W.O. and Bond, J.J., 1974. Summer fallow in the northern Great Plains ( spring wheat). In: Summer Fallow in the Western United States. U.S.D.A. Conserv. Res. Report No. 27, U.S. Government Printing Office, Washington, DC, pp. 12-35. Hammel, J.E., Papendick, R.I. and Campbell, G.S., 1981. Fallow tillage effects on evaporation and seed zone water content in a dry summer climate. Soil Sci. Soc. Am. J., 45: 1016-1022. Smika, D.E., Black, A.L. and Greb, B.W., 1969. Soil nitrate, soil water, and grain yield in a wheat-fallow rotation in the Great Plains as influenced by straw mulch. Agron. J., 61: 785-787. Steel, R.G.D. and Torrie, J.H., 1960. Principles and Procedures of Statistics. McGraw-Hill, New York, pp. 113-120. Tanaka, D.L., 1986. Wheat residue loss for chemical and stubble-mulch fallow. Soil Sci. Soc. Am. J., 50: 434-440. Whitfield, C.J., Bond, J.J., Greb, B.W., McCalla, T.M. and Siddoway, F.H., 1962. A Standardized Procedure for residue sampling. ARS 41-68, U.S.D.A., U.S. Government Printing Office, Washington, DC, pp. 3-9.
307
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
F a l l o w M e t h o d I n f l u e n c e s on Soil W a t e r a n d Precipitation Storage Efficiency I D.L. TANAKA and J.K. AASE
United States Department of Agriculture, Agriculture Research Service, P.O. Box 1109, Sidney, MT59270 (U.S.A.) (Accepted for publication 14 November 1986)
ABSTRACT Tanaka, D.L. and Aase, J.K., 1987. Fallow method influences on soil water and precipitation storage efficiency. Soil Tillage Res., 9: 307-316. Summer fallowing is practiced in the Great Plains of the U.S.A. in order to store soil water, control weeds, make nutrients available and stabilize crop yields. Soil water storage and precipitation storage efficiencies on chemical and stubble-mulch fallow plots were compared for three 14-month fallow periods for a winter-wheat-fallow rotation ( Triticum aestivum L. ) and three 21month fallow periods for spring-wheat-fallow rotation in the northern Great Plains to determine during which seasonal segment the fallow method might influence soil water storage. The experiment was conducted on a glacial till Williams loam (fine-loamy mixed, Typic Argiboroll) from July 1981 to April 1985. Soil water contents to a depth of 1.70 m were measured, using the neutron scatter technique, for seasonal segments during the fallow period. Soil water storage was similar from harvest to spring on chemical and stubble-mulch fallow plots. The over-winter to spring segment resulted in the most consistent precipitation storage efficiencies (33.3-71.1% ). Soil water storage differences as a result of a fallow method are most likely to occur during summer fallow for 14-month winter-wheat-fallow rotations and during the second overwinter for 21-month springwheat-fallow rotations. Soil water storage for the entire fallow period was greater on chemical fallow than on stubble-mulch fallow in two out of three 14-month winter-wheat fallow periods and in one out of three 21-month spring-wheat fallow periods.
INTRODUCTION S u m m e r f a l l o w i n g is p r a c t i c e d i n t h e s e m i - a r i d G r e a t P l a i n s o f t h e U . S . A . t o s t o r e soil w a t e r , c o n t r o l w e e d s , m a k e n u t r i e n t s a v a i l a b l e a n d s t a b i l i z e c r o p y i e l d s . A m o u n t o f soil w a t e r s t o r e d d e p e n d s o n c l i m a t e , soils, t i l l a g e , p r e v i o u s c r o p a n d q u a n t i t y o f s u r f a c e r e s i d u e m a i n t a i n e d d u r i n g fallow. Soil w a t e r e v a p oration makes summer fallowing very inefficient in storing summer precipi~Contribution from the United States Department of Agriculture, Agriculture Research Service, in cooperation with the Montana Agricultural Experiment Station, Journal Series No. J-1864.
0167-1987/87/$03.50
© 1987 Elsevier Science Publishers B.V.
308 tation (Black et al., 1974). Surface residue during fallow provides physical protection for soils to control wind and water erosion. Residue can reduce evaporation by decreasing air movement immediately above the soil, changing albedo and insulating the soil surface. How surface residue is managed can be a factor in minimizing evaporation and increasing soil water storage ( Bond and Willis, 1969; Smika et al., 1969; Hammel et al., 1981; Fenster and Wicks, 1982). Chemical and stubble-mulch fallow maintained 70-80% and 25-45%, respectively, of the residue present after harvest during a 14-month fallow period ( Fenster and Peterson, 1979; Tanaka, 1986 ). Greater quantities of surface residue during fallow should suppress evaporation and increase potential for chemical fallow to store more soil water than stubble-mulch fallow. Chemical fallow has stored more soil water than stubble-mulch fallow during 14 months of fallow in the central Great Plains (Good and Smika, 1978; Fenster and Peterson, 1979; Greb and Zimdahl, 1980). In the northern Great Plains, differences in soil water storage caused by increased quantities of residue during 21-month fallow periods have not been evident (Black and Power, 1965; French and Riveland, 1980; Deibert et al., 1986). In the central Great Plains, conserving crop residues by reducing or eliminating mechanical tillage has increased precipitation storage efficiency (percentage of precipitation stored in the soil profile ) from 20-24% for bare fallow to 40-55% for no-till or chemical fallow. Increased efficiencies for chemical fallow result from greater quantities of surface residue, minimum residue disturbance and better weed control ( Greb, 1983 ). In the northern Great Plains, precipitation storage efficiencies for 14- and 21-month fallow periods range from 14 to 40% (Black and Power, 1965; Haas et al., 1974). Precipitation storage efficiencies are usually the highest after harvest and through the first winter, with a range of 35-154% and averaging 71%. Summer fallow precipitation storage efficiencies are low due to evaporation, and range from a loss of 11 to a gain of 25%, with an average gain of 5.9% (Black et al., 1974). Small gains, and in most cases loss of soil water, occurred over the second winter during a 21-month fallow period. Precipitation storage efficiency data for seasonal fallow segments have been studied for stubble-mulch tillage with little or no data for chemical fallow. The purpose of our study was to compare soil water storage and precipitation storage efficiencies on chemical and stubblemulch fallow plots for 14- and 21-month fallow periods in the northern Great Plains of the U.S.A., and to determine during which seasonal segment fallow methods might influence soil water storage. MATERIALSAND METHODS The study began in April 1980 and was conducted on a glacial till Williams loam (fine-loamy mixed, Typic Argiboroll) in the northern Great Plains 11 km northwest of Sidney, Montana. The Williams loam in our study contained
309
28.8, 39.7 and 31.5%, sand, silt and clay, respectively. Four replications each of chemical and stubble-mulch fallow plots, with a plot size of 7 × 40 m were arranged in a randomized complete-block design for winter- ( T r i t i c u m aestiv u m L. ) and spring-wheat-fallow rotations. Chemical fallow plots were sprayed 3-5 times with glyphosate (N- (phosphon-methyl) glycine), a non-selective translocating herbicide, for weed control during fallow. Residue and soil were undisturbed throughout fallow. Herbicide application rates ranged from 0.4 to 0.8 kg ha 1depending on weed species and climatic conditions. Stubble-mulch fallow plots were tilled with 0.50-m sweeps to a depth of about 0.10 m in late May, followed by 3-5 tillage operations with a rod weeder, as needed, to control weeds during fallow. The quantity of surface residue on chemical and stubble-mulch fallow plots was measured after harvesting winter and spring wheat, and again before seeding winter and spring wheat, by sampling two 1-m 2 samples per plot according to methods described by Whitfield et al. (1962). The neutron scatter technique was used to determine soil water storage to a depth of 1.70 m in 0.20-m increments. The first measurement was at the 0.20m depth and accounted for the surface 0.30 m of soil water. Soil water storage was determined for the 1981-1982, 1982-1983 and 1983-1984 14-month winter-wheat fallow periods and the 1981-1983, 1982-1984 and 1983-1985 21month spring-wheat fallow periods. The 14-month winter-wheat fallow period began in late July after winter-wheat harvest, and continued through the winter and summer months until winter-wheat seeding time in mid-September; the 21-month spring-wheat fallow period began in mid-August after springwheat harvest and continued through two winters until spring-wheat seeding time in mid-April. Calculations of soil water storage and precipitation storage efficiencies began at harvest and ended with the seeding of the next crop. Each fallow period was divided into seasonal segments which consisted of harvest to soil freeze-up (harvest to October), over-winter to spring ( November to April ), summer fallow ( May to October, or until seeding winter wheat in mid-September) and second overwinter (from November until seeding spring wheat in mid-April). Precipitation storage efficiency was calculated by dividing the increase in soil water content to a depth of 1.70 m for each seasonal segment or fallow period by the precipitation received during the seasonal segment or fallow period. Precipitation adjacent to the study was measured using a standard 203-mm weighing rain gauge. Average long-term precipitation since 1949 was from 11 km southeast of the study area. Seasonal segment precipitation probabilities were calculated from long-term averages. Single degree of freedom (1 df) comparisons were used to determine residue and soil water content differences ( Steel and Torrie, 1960).
310 TABLE I Average long-term monthly precipitation (mm) and precipitation adjacent to the study (mm) from January 1981 to April 1985 Month
Average 1949-1984
1981
1982
1983
1984
1985
January February March April May June July August September October November December
l0 9 13 29 50 73 44 41 33 21 11 11
1 5 5 27 25 91 41 40 15 20 10 15
27 12 39 25 55 61 24 20 22 67 3 17
8 3 12 1 19 40 47 14 15 15 8 13
12 1 12 17 5 75 5 15 26 4 13 4
2 2 14 33
RESULTS AND DISCUSSION
Precipitation varied considerably from one fallow period to the next and for each seasonal segment. Total precipitation for the 1981-1982, 1982-1983 and 1983-1984 14-month winter-wheat fallow periods was 95, 67 and 54%, respectively, of the average 414 mm (Table I). Total precipitation for the 1981-1983, 1982-1984 and 1983-1985 21-month spring-wheat fallow periods was 90, 70 and 56%, respectively, of the average 523 mm. Seasonal segment precipitation varied from 42% of the average 95 mm from harvest to soil freeze-up in 1983-1984 and 1983-1985 fallow periods to 154% of the average 83 mm for overwinter to spring in 1981-1982 and 1981-1983 fallow periods. The quantity of surface residue after harvest and before seeding are shown in Table II. The quantity of surface residue did not differ between chemical and stubble-mulch fallow plots after harvest, but about twice as much surface residue was present on chemical fallow plots as compared with stubble-mulch fallow plots at the end of the fallow period. Soil water content changes to a depth of 1.70 m for the harvest to soil freezeup segment ranged from a loss of 14 mm to a gain of 100 mm. Precipitation storage efficiencies ranged from - 3 2 . 5 to 91.7% (Tables III-VIII). Moist soil conditions at harvest, high evaporative demand and small amounts of precipitation during the first fallow segment resulted in the loss of soil water and the negative precipitation storage efficiencies for the 1981-1983 21-month springwheat fallow period (Table IV). The chance of receiving average or aboveaverage precipitation from long-term data was about 60% for winter-wheat fallow, compared to about 40% for spring-wheat fallow during the first segment. This difference was due to a later spring-wheat harvest.
311 TABLE II The quantities of winter and spring wheat residue (kg ha 1) after harvest and prior to seeding a crop on chemical (CF) and stubble-mulch (SM) fallow plots CF
SM
1 df comparisons (CF vs. SM) ~
Winter wheat (14-month) 1981-1982 After harvest Before seeding
3700 2800
3700 1000
NS *
Spring wheat (21-month) 1981-1983 After harvest Before seeding
2400 1100
2400 300
NS *
Winter wheat (14-month) 1982-1983 After harvest Before seeding
5300 3100
4500 1500
NS *
Spring wheat (21-month) 1982-1984 After harvest Before seeding
2700 1400
2400 800
NS *
Winter wheat (14-month) 1983-1984 After harvest Before seeding
4200 3400
4600 2200
NS *
Spring wheat (21-month) 1983-1985 After harvest Before seeding
3800 1300
3900 500
NS *
~*, Significantly different at 0.05 probability level; NS, not significantly different. D u r i n g t h e s e c o n d fallow s e g m e n t , soil w a t e r gain r a n g e d f r o m 16 to 91 m m . T h i s s e g m e n t h a d t h e m o s t c o n s i s t e n t p r e c i p i t a t i o n s t o r a g e efficiencies (33.3 to 71.1% ) of a n y fallow s e g m e n t a n d were similar to w h a t G r e b (1983) r e p o r t e d for t h e s a m e p e r i o d in t h e c e n t r a l G r e a t Plains. T h e c h a n c e for receiving average or a b o v e - a v e r a g e p r e c i p i t a t i o n for this s e g m e n t was a b o u t 60% for w i n t e r a n d s p r i n g - w h e a t fallow periods. In general, p r e c i p i t a t i o n s t o r a g e efficiencies a n d soil w a t e r s t o r a g e i n c r e a s e d as s n o w a c c u m u l a t i o n increased. Soil w a t e r s t o r e d d u r i n g t h e first two fallow s e g m e n t s a v e r a g e d 70 a n d 60% of t h e t o t a l soil w a t e r s t o r e d in t h r e e s e g m e n t s of a 1 4 - m o n t h a n d four s e g m e n t s of a 21m o n t h fallow period, respectively. S p r i n g - w h e a t fallow (21 m o n t h s ) s t o r e d less soil w a t e r t h a n w i n t e r - w h e a t fallow (14 m o n t h s ) b e c a u s e of l a t e r h a r v e s t date, less s u r f a c e residue a n d longer fallow period. D u r i n g y e a r s of b e l o w - a v e r a g e p r e c i p i t a t i o n for t h e first two fallow segm e n t s , w h i c h f r o m l o n g - t e r m d a t a o c c u r r e d a b o u t 40% of t h e t i m e for w i n t e r w h e a t fallow a n d 50% of t h e t i m e for s p r i n g - w h e a t fallow, soil w a t e r s t o r a g e
312 T A B L E III The 1981-1982 14-month w i n t e r - w h e a t fallow soil water gain or loss to a depth of 1.70 m and p r e c i p i t a t i o n storage efficiency based on stored soil water and p r e c i p i t a t i o n on chemical (CF) and stubble-mulch ( S M ) fallow plots Seasonal segments
Soil water gain ( + ) or loss( ) ( r a m ) CF
Harvest to soil freeze-up Over-winter to spring S u m m e r fallow Total
SM
1 df comparisons ( CF vs. SM) ~
P r e c i p i t a t i o n storage efficiency (To) CF
SM
Precipitation (mm)
+ 16
+ 17
NS
15.2
16.2
105
+ 69 + 33
+ 74 + 19
NS NS
53.9 20.5
57.8 11.8
128 161
+ 118
+ 110
NS
29.9
27.9
394
~NS, not significantly different.
during summer fallow (third segment) can be important (Tables III, IV, VII and VIII). Soil water gain depends on previous soil water content, amount, frequency and distribution of precipitation, and quantity of surface residue to suppress evaporation. Evaporative demand during the summer fallow segment is high, limiting soil water storage. Stored soil water for the first two fallow segments during 1981-1982 (Table III) and 1982-1983 14-month winter-wheat fallow ( Table V) and during 1982-1984 21-month spring-wheat fallow (Table VI) was relatively high, and resulted in small soil water gains and low precipitation storage efficiencies during the summer fallow segment of 1982 and 1983. In contrast, stored soil water was low for the first two fallow segments during 1983-1984 (Table VII), resulting in high precipitation storage efficiencies T A B L E IV The 1981-1983 21-month spring-wheat fallow soil water gain or loss to a depth of 1.70 m and p r e c i p i t a t i o n storage efficiency based on stored soil water and p r e c i p i t a t i o n on chemical (CF) and stuble-mulch ( S M ) fallow plots Seasonal segments
Soil water gain ( + ) or loss( - ) ( m m ) CF
Harvest to soil freeze-up Over-winter to spring S u m m e r fallow Second overwinter Total
1 df comparisons (CF vs. S M ) '
SM
P r e c i p i t a t i o n storage efficiency (%) CF
SM
Precipitation (mm)
- 14
- 13
NS
-35.0
-32.5
40
+ 89 + 70 0
+91 + 53 +3
NS NS NS
69.5 28.1 0.0
71.1 21.3 7.0
128 249 43
+ 145
+ 134
NS
31.5
29.1
460
~NS, not significantly different.
313
TABLE V
The 1982-1983 14-month w i n t e r - w h e a t fallow soil water gain or loss to a d e p t h of 1.70 m and precipitation storage efficiency based on stored soil w a t e r and p r e c i p i t a t i o n on chemical (CF) and stubble-mulch ( S M ) fallow plots Seasonal segments
Soil water gain ( + ) or
loss( - ) ( m m ) CF Harvest to soil freeze-up Over-winter to spring S u m m e r fallow Total
1 df comparisons (CF vs. S M ) 1
SM
P r e c i p i t a t i o n storage efficiency (%) CF
SM
Precipitation (mm)
+ 98
÷ 88
NS
89.9
80.7
109
+ 23 + 18
+ 16 + 16
NS NS
52.3 14.3
36.4 12.7
44 126
+ 139
+ 120
*
49.8
43.0
279
L., Significantly different at 0.05 probability; NS, not significantly different.
during the 1984 summer fallow segment. Chemical fallow stored significantly more soil water than stubble-mulch fallow during the 1982-1983 (Table V) and 1983-1984 (Table VII) 14-month winter-wheat fallow periods. This was because the greater quantities of winter-wheat residue on chemical than on stubble-mulch fallow plots (Table II) more effectively suppressed evaporation. The quantities of spring-wheat residue on chemical fallow plots were too small to cause significantly more soil water storage on them than on stubblemulch fallow plots during the summer fallow segment. The second overwinter segment for 21-month spring-wheat fallow resulted in small gains and, in some instances, losses of soil water because (1) soils were near field capacity, (2) there were small quantities of surface residue, T A B L E VI
The 1982-1984 2 1 - m o n t h s p r i n g - w h e a t fallow soil water gain or loss to a d e p t h of 1.70 m and precipitation storage efficiency based on stored soil water and p r e c i p i t a t i o n on chemical ( C F ) and stubble-mulch ( S M ) fallow plots Seasonal segments
Soil water gain ( ÷ ) or loss( - ) ( m m ) CF
H a r v e s t to soil freeze-up Over-winter to spring S u m m e r fallow Second o v e r w i n t e r Total
1 df comparison (CF vs. S M ),
SM
P r e c i p i t a t i o n storage efficiency (%) CF
Precipitation (mm)
SM
+ 100
+ 88
NS
91.7
80.7
109
+ 25 + 15 - 5
+ 23 + 19 - 7
NS NS NS
56.8 10.0 - 10.6
52.3 12.7 - 14.9
44 150 47
+ 135
+ 123
NS
37.5
34.3
358
'*, Significantly different at 0.05 probability; NS, not significantly different.
314 T A B L E VII The 1983-1984 14-month w i n t e r - w h e a t fallow soil water gain or loss to a depth of 1.70 m and p r e c i p i t a t i o n storage efficiency based on stored soil water and p r e c i p i t a t i o n on chemical (CF) and stubble-mulch ( S M ) fallow plots Seasonal segments
Soil water gain ( + ) or loss(- ) (mm) CF
1 df comparisons (CF vs. S M ) I
SM
P r e c i p i t a t i o n storage efficiency (%) CF
SM
Precipitation (mm)
Harvest to soil freeze-up Over-winter to spring S u m m e r fallow
+7
+5
NS
15.9
11.4
44
+ 21 +56
+ 33 +28
NS *
33.3 48.3
52.4 24.1
63 116
Total
+ 84
+ 66
*
37.7
29.6
223
~*, Significantly different at 0.05 probability; NS, not significantly different.
and (3) frozen wet soils reduced infiltration (Tables IV and VI). If dry climatic conditions existed during previous fallow segments, soil water gains over the second winter segment can be significant, since from long-term data a 60% chance occurred for average or above-average precipitation. For example, during the 1983-1985 21-month spring-wheat fallow period, chemical fallow plots stored twice as much soil water than stubble-mulch fallow plots for the second overwinter segment (Table VIII). Twice as much surface residue, with 25% positioned vertically, held snow from late-winter storms to double precipitation storage efficiencies. As a consequence, chemical fallow plots gained significantly more soil water than stubble-mulch fallow plots at 0.20- and 0.40-m soil depths (Fig. 1 ). Only once during the three 21-month spring-wheat fallow T A B L E VIII The 1983 1985 21-month spring w h e a t fallow soil water gain or loss to a depth of 1.70 m and precipitation storage efficiency based on stored soil water and p r e c i p i t a t i o n on chemical (CF) and stubble-mulch ( S M ) fallow plots Seasonal segments
Soil water gain ( + ) or loss{ - ) ( m m ) CF
Harvest to soil freeze-up Over-winter to spring S u m m e r fallow Second overwinter Total
1 df comparisons (CF vs. SM)
SM
P r e c i p i t a t i o n storage efficiency (%) CF
SM
Precipitation (mm)
+7
+6
NS
15.9
13.6
44
+ 32 +38 +33
+ 22 +30 + 15
NS NS *
50.8 29.7 67.3
34.9 23.4 30.6
63 130 49
+ 110
+ 73
*
38.5
25.5
286
'*, Significantly different at 0.05 probability; NS, not significantly different.
315 SOIL
0
0
WATER
CONTENT
O. 10 O. 2 0 I
O. 3 0
i
0
(m 3 /m ~ )
0. i0
i
LSO X
=
CF
0
=
SM
O. 3 0
0.20
i
i
I
(0.
10) H
t
AO. 4 E v ]Z
~0.8 [D /
ml.2
1.6 23
OCT
984
12
APR
1985
Fig. 1. Profile soil water contents during the second overwinter segment on chemical (CF) and stubble-mulch (SM) fallow plots for the 1983-1985 21-month spring-wheat fallow period.
periods was there a difference in total profile soil water storage between the two fallow treatments. The extra soil water on chemical fallow plots near the soil surface can be important in seed germination and seedling establishment. Soil water storage on chemical and stubble-mulch fallow plots was similar for the first two fallow segments. Differences between chemical and stubblemulch fallow plots would only be likely to occur if plot residue orientation were altered by mechanical tillage after harvest or if weed growth caused soil water losses. One would expect soil water storage differences between chemical and stubble-mulch fallow plots to begin to appear after the first tillage operation in late May on stubble-mulch fallow plots and for them to continue throughout the remainder of fallow. The data suggest that during dry years, chemical fallow plots can store more soil water than stubble-mulch plots during 14-month winter-wheat fallow periods, but spring wheat may not produce sufficient quantities of residue for differences to occur. Soil water differences because of the fallow method are most likely to occur during the summer fallow segment for 14-month winter-wheat fallow periods. During the second overwinter of a 21-month spring-wheat fallow period, vertical and prone residue on chemical fallow plots held snow, and resulted in significantly greater soil water gains than on stubble-mulch fallow in one out of three 21-month fallow periods. Chemical fallow maintained greater quantities of surface residue, with a higher proportion of residue remaining vertical, than stubble-mulch fallow. Chemical fallow increased the chance of storing additional soil water during years with erratic precipitation patterns. Low quantities of residue and/or flattened stubble due to tillage on stubble mulch plots decreased the chance of storing additional soil water.
316 ACKNOWLEDGEMENTS
We thank Mr" R:L. Kolberg for his assistance with field work and data collection an'd summarization.
REFERENCES Black, A.L. and Power, J.F., 1965. Effect of chemical and mechanical fallow methods on moisture storage, wheat yields, and soil erodibility. Soil Sci. Soc. Am. Proc., 29: 465-468. Black, A.L., Siddoway, F.H. and Brown, P.L., 1974. Summer fallow in the northern Great Plains (winter wheat). In: Summer Fallow in the Western United States. U.SID.A.' Conserv. Res. Rep. No. 17, U.S. Government Printing Office, Washington, DC, pp. 36-50. Bond, J.J. and Willis, W.O., 1969. Soil water evaporation: Surface residue rate and placement effects. Soil Sci. Soc. Am. Proc., 33:445 448. Deibert, E.J., French, E. and Hoag, B., 1986. Water storage and use by spring wheat under conventional tillage and no-till in continuous and alternate crop-fallow systems in the northern Great Plains. J. Soil Water Conserv., 41: 53-58• Fenster, C.R. and Peterson, G.A., 1979. Effects of no-tillage fallow as compared to conventional tillage in a wheat-fallow system. Res. Bull. 289, Agric. Exp. Stn., University of Nebraska, Lincoln, pp. 1-28. Fenster, C.R. and Wicks, G.A., 1982. Fallow systems for winter wheat in western Nebraska. Agron. J., 74: 9-13. French, E.W. and Riveland, N., 1980. Chemical fallow in a spring wheat-fallow rotation. N. D. Farm Res., 38(1): 12-15. Good, L.G. and Smika, D.E., 1978. Chemical fallow for soil and water conservation in the Great Plains. J. Soil Water Conserv., 33: 89-90. Greb, B.W., 1983. Water conservation: Central Great Plains. In: H.E. Dregne and W.O. Willis (Editors), Dryland Agriculture. Agronomy Monograph 23, American Society of Agronomy, Madison, WI, pp. 57-70. Greb, B.W. and Zimdahl, R.L., 1980. Ecofallow comes of age in the central Great Plains. J. Soil Water Conserv., 35: 230-233. Haas, H.J., Willis, W.O. and Bond, J.J., 1974. Summer fallow in the northern Great Plains ( spring wheat). In: Summer Fallow in the Western United States. U.S.D.A. Conserv. Res. Report No. 27, U.S. Government Printing Office, Washington, DC, pp. 12-35. Hammel, J.E., Papendick, R.I. and Campbell, G.S., 1981. Fallow tillage effects on evaporation and seed zone water content in a dry summer climate. Soil Sci. Soc. Am. J., 45: 1016-1022. Smika, D.E., Black, A.L. and Greb, B.W., 1969. Soil nitrate, soil water, and grain yield in a wheat-fallow rotation in the Great Plains as influenced by straw mulch. Agron. J., 61: 785-787. Steel, R.G.D. and Torrie, J.H., 1960. Principles and Procedures of Statistics. McGraw-Hill, New York, pp. 113-120. Tanaka, D.L., 1986. Wheat residue loss for chemical and stubble-mulch fallow. Soil Sci. Soc. Am. J., 50: 434-440. Whitfield, C.J., Bond, J.J., Greb, B.W., McCalla, T.M. and Siddoway, F.H., 1962. A Standardized Procedure for residue sampling. ARS 41-68, U.S.D.A., U.S. Government Printing Office, Washington, DC, pp. 3-9.