Directory UMM :Data Elmu:jurnal:A:Agricultural Water Management:Vol45.Issue1.Jun2000:
Agricultural Water Management 45 (2000) 55±71
Impacts of gypsum and winter cover crops on soil
physical properties and crop productivity when
irrigated with saline water
J.P. Mitchella,*, C. Shennana,1, M.J. Singerb, D.W. Petersb,
R.O. Millera,2, T. Prichardb, S.R. Grattanb, J.D. Rhoadesc,
D.M. Mayd, D.S. Munkd
a
Department of Vegetable Crops, University of California, Davis, CA 95616, USA
Department of Land, Air and Water Resources, University of California, Davis, CA 95616, USA
c
USDA Salinity Lab, 450 West Big Springs Road, Riverside, CA 92507, USA
d
Cooperative Extension, University of California, 1720 S. Maple Avenue, Fresno, CA 93702, USA
b
Accepted 19 July 1999
Abstract
Reuse of saline subsurface drainage water for irrigation has been identi®ed as a potential option
for managing drainage volumes and sustaining crop productivity in California's San Joaquin Valley.
Soil surface structural instability, crusting and poor stand establishment may, however, be
constraints in drainage reuse systems. The objective of this ®eld study was to evaluate the
effectiveness of winter cover crop incorporation and gypsum applications relative to conventional
fallows for improving soil physical properties, stand establishment and crop productivity in a
cropping system relying on the cyclic reuse of saline drainage for irrigation. Barley (Hordeum
vulgare), Lana woolypod vetch (Vicia dasycarpa) and barley/vetch winter cover crop treatments
were compared with gypsum amended and unamended winter fallows in a rotation of tomato±
tomato±cotton as summer crops. Tomato seedling emergence was improved by 34% following
incorporation of vetch in year 1 prior to saline irrigation application, but was unaffected by
amendment treatment in year 2. Following two summer seasons in which saline drainage water was
used for about 70% of the irrigation requirements, surface-applied gypsum signi®cantly reduced
soil crust strength an average of 14%, increased soil aggregate stability an average of 46%, and
*
Corresponding author. Tel.: 1-559-646-6565; fax: 1-559-646-6593.
E-mail address: mitchell@uckac.edu (J.P. Mitchell)
1
Present address: Department of Environmental Studies, University of California, Santa Cruz, CA 95064,
USA.
2
Present address: Soil and Crop Science Department, Colorado State University, Fort Collins, CA 80523,
USA.
0378-3774/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 3 7 8 - 3 7 7 4 ( 9 9 ) 0 0 0 7 0 - 0
56
J.P. Mitchell et al. / Agricultural Water Management 45 (2000) 55±71
maintained cotton stand establishment relative to nonsaline irrigation. Emergence rates and ®nal
stand densities of cotton seedlings following incorporation of each of the cover crops, however,
were signi®cantly lower. The mechanism most likely responsible for reducing stand establishment
was the formation of stubble-reinforced surface crusts that resulted in interconnected slabs that
impeded timely emergence of seedlings. This type of physical impedance can create secondary
effects such as increased disease which could also have played a role in reducing emergence. Yields
of tomatoes irrigated with saline water were maintained relative to nonsaline irrigation in year 1, but
were decreased by 33% in year 2. No reductions in cotton lint yield occurred as a result of saline
irrigation in year 3. Soil electrical conductivity (ECe) increased from about 2±6 dS mÿ1 during the
course of this 3-year study despite leaching by winter rains. Cyclic reuse of saline subsurface
drainage water may conserve good quality water and provide a means of sustaining crop
productivity over short terms. Soil surface salt and boron accumulation, however, may be major
constraints to this cropping strategy and will limit productivity if appropriate irrigation and crop
management practices are not followed. # 2000 Elsevier Science B.V. All rights reserved.
Keywords: Salinity; Drainage water reuse; Soil degradation
1. Introduction
Intensive irrigation practices developed in this century have dramatically increased
agricultural productivity in many areas of the western US, notably in the San Joaquin
Valley (SJV) of California (Hess, 1984; Rubatzky, 1985). These practices, however, have
also resulted in increased drainage water production and chemical contamination of
groundwater (Lamb, 1986; Halberg, 1987). It is generally acknowledged that long-term
agricultural production in irrigated areas is dependent on an adequate drainage outflow
system (van Schilfgaarde and Hoffman, 1977), and a variety of management strategies are
currently being considered to reduce the volume of drainage ultimately requiring disposal
or treatment (SJV Drainage Program Management Plan, 1990). Reuse of saline drainage
water for irrigation is one option proposed. Cyclic reuse, as proposed by Rhoades (1984),
involves the use of nonsaline water in rotation with saline drainage water for the irrigation
of selected crops differing in their salinity tolerance. The potential for cyclic reuse of
saline drainage water for irrigation has been documented for several lower value crops
(Rhoades et al., 1988). Higher value crops such as processing tomato, melons and cotton
have also been produced without sustaining significant yield reductions in short-term
studies using saline drainage water (Grattan et al., 1987; Shennan et al., 1988). Similarly,
we have shown by a combination of field and greenhouse studies that processing tomato
quality is generally improved by exposure to salinity following plant establishment
(Mitchell et al., 1991a,b). Longer-term experiments in the SJV indicate that accumulation
of soil B over time is the most likely microelement limitation to cyclic drainage reuse,
whereas Se accumulation is unlikely to be a problem, because the major form of Se in the
drainage water of this region is selenate which is relatively mobile in the soil profile and
readily leached below the rootzone by periodic use of good quality water (Shennan et al.,
1995). In addition, although Se concentrations increase in crops irrigated with saline
drainage waters containing up to 300 ppb Se, it is not considered a human health hazard
(Fan and Jackson, 1988). Sulfate also plays an important role in reducing Se in plant
J.P. Mitchell et al. / Agricultural Water Management 45 (2000) 55±71
57
tissue due to sulfate±selenate competition (Wan et al., 1991; Wu and Huang, 1991). There
is a considerable body of literature documenting complex interactions between soil sodicity
and salinity and the impacts of these two factors on soil physical properties (Shainberg and
Letey, 1984; Oster et al., 1996; Oster and Jayawardane, 1998). The specific boundary of
salinity and sodicity conditions at which a soil becomes unstable varies from one soil to
the next (Pratt and Suarez, 1990). Soil slaking may, however, occur and greatly reduce
water infiltration and create a strong surface crust, resulting in poor stand establishment
and decreased crop yields when nonsaline (low EC) water is used to irrigate a field
previously irrigated with drainage water in the SJV (Biggar et al., 1988; Shennan et al.,
1988). To prevent these problems, management practices that ameliorate the degradation
of soil physical properties must be employed to allow sustainable reuse of drainage water.
The most common method for maintaining water infiltration is tillage and gypsum
incorporation into the soil, often at rates from 2±4 tons haÿ1. Another alternative is the
inclusion of a green manure crop. Several studies have shown benefits of green manure
incorporation on soil physical characteristics including water infiltration (Cassman and
Rains, 1986), aggregation (Macrae and Mehuys, 1985), and porosity (Disparte, 1987).
Cover cropping is slow to increase organic matter in semi-arid or arid irrigated regions
(Wyland et al., 1996), possibly because intermittently moist soils and high temperature
conditions may favor rapid decomposition of soil organic matter. A cover crop may also
protect the soil surface from rain or sprinkler drop impact, thereby preventing the
formation of soil crusts which often limit infiltration (Stolzy et al., 1960; Disparte, 1987).
Organic N input from a leguminous cover crop may also reduce the seasonal fertilizer-N
requirement (Shennan, 1992), reduce soil NOÿ
3 , and minimize leaching losses from
winter rains and spring preirrigations (Stivers et al., 1990). Improved seedling emergence
of crops that follow incorporated cover crops may be achieved due to reductions in soil
crust formation in cover-cropped systems (Groody, 1990; Stivers et al., 1990). Roberson
et al. (1991) found significant improvements in macroaggregate slaking resistance in
cover-cropped vs clear cultivated treatments in a California orchard. Although reference
has been made to the potential use of crop residue management in managing salinity
(Cassman and Rains, 1986), to date, no research has directly addressed this objective.
Many present row-cropping systems in the SJV could benefit from the use of cover crops
provided innovative management practices are developed.
The objectives of this study were (1) to compare the relative effectiveness of winter
cover crop incorporation and gypsum applications in cropping rotations employing cyclic
reuse of saline drainage or shallow groundwater for improving/maintaining good soil
physical properties, (2) to determine annual water budgets for the proposed cropping
system, and (3) to assess potential long-term improvements in water use efficiency of this
cropping system.
2. Materials and methods
2.1. Experimental design and cultural practices
A 3-year experiment was conducted during the 1991±1994 period on 93 m 7 m plots
of Panoche soil (fine-loamy, mixed, calcareous, thermic Typic Torriorthent) at the
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J.P. Mitchell et al. / Agricultural Water Management 45 (2000) 55±71
Table 1
Water quality for winter soil cover and summer irrigation treatments
Winter treatment
Barley cover crop
Barley/vetch cover crop
Vetch cover crop
Fallow
Fallow
Fallow
Summer irrigation treatment
Crop establishment
Post-establishment
Nonsaline
Nonsaline
Nonsaline
Nonsaline and gypsum
Nonsaline
Nonsaline
Saline
Saline
Saline
Saline
Saline
Nonsaline
University of California West Side Research and Extension Center (WSREC) located
65 km southwest of Fresno, in Five Points, CA. Thirty-year average rainfall for this
region is 154 mm. Annual rainfall during the course of this study was 166, 266 and
101 mm for 1992, 1993 and 1994, respectively. Experimental treatments are summarized
in Table 1. Summer crops consisted of two years of processing tomato (1992, 1993)
followed by a year of cotton (1994). The summer crops in each of the first five treatments
in Table 1 were irrigated with low EC water (0.53 dS mÿ1) for germination and
establishment, and with saline water (EC 6.9 dS mÿ1) following establishment
(Table 2). An additional winter fallow treatment irrigated with nonsaline water during
the summers was included to compare changes in key indices of soil quality and crop
performance that resulted from the saline water irrigations applied to each of the other
five treatments. The experiment was organized as a randomized complete block with four
replications.
Barley (Hordeum vulgare), ``Lana'' woolypod vetch (Vicia dasycarpa) and barley/vetch
mixture cover crops were seeded in mid-October of 1991, 1992 and 1993, and incorporated to a depth of 15±20 cm with a rotary tiller in mid-March of each following year.
Processing tomato `Alta' was direct-seeded in single rows on beds (1.7 m 93 m)
flanked on both sides by seeded border rows of equal dimensions on April 1 in 1992 and
April 5 in 1993. Preplant broadcast monoammonium phosphate fertilizer was applied to
all plots at 12 kg N/ha and an additional 168 kg N/ha was sidedressed at thinning except
those in which vetch cover crops had been incorporated. Cotton (cv Maxxa) was planted
on four beds each measuring 1.7 m 93 m in each plot April 18, 1994. Preplant
broadcast ammonium sulfate supplied 150 kg N/ha to all plots except those following
vetch incorporations. Weed control was accomplished using EPA-approved preplant
herbicides, mechanical cultivation and hand weeding as necessary in each year. During
the summer growing seasons, water was applied weekly to tomato and every two weeks
to cotton by furrow irrigation in quantities sufficient to replace evapotranspirative losses
based on nonstressed conditions. A 0.1 leaching fraction was calculated and included in
each irrigation application. Summaries of the quality and amount of irrigation water used
in each experimental treatment is presented in Tables 2 and 3, respectively. Crop
evapotranspiration (ET) was estimated using reference values provided by the California
Irrigation Management Information System (CIMIS) weather station located 50 m from
the experimental field and crop coefficient values developed previously at the site (Phene,
Water source
EC
(dS mÿ1)
pH
SAR
(mmol lÿ1)
Na
(mmol lÿ1)
Ca
(mmol lÿ1)
Mg
(mmol lÿ1)
SOÿ2
4
(mmol lÿ1)
Cl
(mmol lÿ1)
NOÿ
3
(mmol lÿ1)
B
(mmol lÿ1)
Saline
High quality
6.9
0.53
7.9
7.9
9.4
2.4
41.4
2.0
9.6
0.7
10.0
0.6
27.6
0.3
14.1
1.1
0.7
0.01
0.55
0.015
J.P. Mitchell et al. / Agricultural Water Management 45 (2000) 55±71
Table 2
Chemical properties of irrigation waters
59
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J.P. Mitchell et al. / Agricultural Water Management 45 (2000) 55±71
Table 3
Irrigation water applications and rainfall totals (mm) from 1991±1994
Crop
Year
Treatment
Water applied (mm)
Rainfall/cover
crop irrigations
Nonsaline
water
Saline
water
Total
Nonsaline fallow
Saline gypsum fallow
Saline fallow
Saline barley
Saline barley/vetch
Saline vetch
Nonsaline fallow
Saline gypsum fallow
Saline fallow
Saline barley
Saline barley/vetch
Saline vetch
232
232
232
232
232
232
266
266
266
304
304
304
638
219
219
219
219
219
640
117
117
117
117
117
0
419
419
419
419
419
0
523
523
523
523
523
870
870
870
870
870
870
906
906
906
944
944
944
Nonsaline fallow
Saline gypsum fallow
Saline fallow
Saline barley
Saline barley/vetch
Saline vetch
101
101
101
157
157
157
729
221
221
221
221
221
0
508
508
508
508
508
830
830
830
886
886
886
Tomato
1992
1993
Cotton
1994
1985). In-flow meters were used to measure the quantity of applied water. Prior to
planting in the spring of 1992, 11 t haÿ1 of gypsum were applied and incorporated to a
depth of 15±20 cm to the gypsum-amended plots. An additional 2.4 t haÿ1 of gypsum
were applied in spring 1994, of which about two-thirds was incorporated and one-third
was surface-applied.
2.2. Soil quality determinations
Soil samples from 0±15 cm were collected at 312 sites in a 5.7 m 7 m grid
throughout the experimental field in the spring and fall of each year for determinations by
the University of California, Division of Agriculture and Natural Resources Analytical
Laboratory of (1) total soil organic matter using a modified Walkley±Black procedure as
described by Janitzky (1986), (2) total soil nitrogen using the total Kjeldahl nitrogen
method of Carlson (1986), (3) electrical conductivity of the soil saturation extract (ECe)
using a YSI Model 32 Conductance Meter (YSI Instruments, Yellow Springs, OH), and
calculation of the sodium adsorption ratio (SARe) of the soil extract for each sample
using the equation
SAR
Na
f Ca Mg=2g1=2
;
J.P. Mitchell et al. / Agricultural Water Management 45 (2000) 55±71
61
where Na, Ca and Mg are the total concentrations of the soluble cations in mmol lÿ1.
Additional sets of 312 samples of the surface 15 cm of soil were collected in the spring of
each year for measurement of soil aggregate stability using a modi®cation of the water
stable aggregate method of Kemper and Rosenau (1986). Soil cores were taken at 1±15,
15±30, 30±60 and 60±90 cm increments four times during the experiment for ECe
determinations. Soil surface crust strength was characterized by the force required to
penetrate the top 10 mm of soil using a micropenetrometer described by Rolston et al.
(1991). A ¯at-tipped, 1.59 mm diameter probe programmed to penetrate the soil at a rate
of 8 mm minÿ1 was used. Measurements of penetration resistance were recorded at depth
intervals of 0.1 mm. A total of 200 ``pokes'' were made of air-dry soil at horizontal
intervals of 20 mm along two 1 m transects in all plots in the spring of each year of the
experiment. Based on the strong relationship between the maximum force and the total
work over the depth of penetration (Rolston et al., 1991), maximum force was used as an
index of soil surface strength. Crust samples were taken for gravimetric water content
determination along each transect at the time of measurement (Gardner, 1986).
Rhizoctonia solani colonies in the surface 15 cm of soil were quanti®ed in the spring
of 1994 using the assay procedure of Weinhold (1977).
2.3. Biological and economic crop yield and quality determination
Tomato and cotton seedling emergence rates were determined by counting the number
of seedlings that emerged along a given length of single-row-planted treatment plot
during the four weeks following planting. Seedling counts were made at 13 sites along a
total of 40 m in each plot in 1992, 52 m in 1993 and 104 m in 1994. Tomato plant
aboveground biomass was determined six times during the 1992 season and nine times in
1993 by harvesting, drying and weighing the total plant mass from 2.4 m2 of treatment
plot. Tomato leaf blades and petioles were collected 10 times during the 1993 season for
determination of total N by the Kjeldahl method.
2.4. Statistical analysis
Statistical computations of effects due to winter soil cover and summer irrigation
treatments were made using the PROC GLM procedure of SAS Version 6.1 software.
Response variables considered were soil water stable aggregates, crust strength, crop
yields and quality. Significance was determined using an F-test and in the event of
significant effects due to treatments, means were separated at the 0.05 probability level
and contrasts between groups of treatments were calculated.
3. Results
3.1. Water inputs
Variations in total water applied among treatments in 1993 and 1994 are due to the
irrigation water that was applied to cover crop plots for germination and establishment
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J.P. Mitchell et al. / Agricultural Water Management 45 (2000) 55±71
Fig. 1. Changes in soil ECe, SAR, organic matter (%) and nitrogen (5) over time at 0±15 cm depth. Standard
error of the mean bars are shown when they exceed the height of the symbol.
during the fall months preceding each of these seasons. Saline drainage water reuse
accounted for 70% of the total amount of irrigation water applied to summer crops during
the three-year cropping cycle (Table 3).
3.2. Soil quality determinations
Reuse of drainage water led to significant fluctuations in salinity in the soil surface
(Fig. 1) and throughout the soil profile (Figs. 2 and 3). Leaching by winter rains had little
effect on salinity in lower parts of the soil profile (Fig. 2). SARe values for the surface
15 cm of soil also increased relative to initial soil values after irrigations with saline
drainage water (Fig. 1). By spring 1994 these soils became sodic (i.e., SARe > 11). Soil
organic matter (SOM) content increased in cover crop amended plots in the spring of
each year relative to winter fallowed plots (Fig. 1). Increases in SOM were highest in the
barley and barley/vetch mix plots and were almost twice as high as initial values. Total
soil N was also increased in the spring of each year in cover crop plots, but declined to
levels of the fallowed plots by the fall of each year (Fig. 1).
J.P. Mitchell et al. / Agricultural Water Management 45 (2000) 55±71
63
Fig. 2. Soil saturation extract electrical conductivity (ECe) pro®les over time. Standard error of mean bars are
shown when they exceed the height of the symbol.
Fig. 3. Cotton seedling emergence 1994. Each value is a mean of four replicates for the total number of
seedlings emerged along 104 m of plot. Standard error of the mean bars are shown when they exceed the height
of the symbol.
64
J.P. Mitchell et al. / Agricultural Water Management 45 (2000) 55±71
Table 4
Water stable aggregates (%) in 1992 and 1994
1992
1994
Treatment
Fresh fallow
Saline/gypsum fallow
Saline fallow
Saline/barley
Saline/barley-vetch
Saline/vetch
35.5
35.9
34.0
39.4
38.9
36.5
33.5
18.7
17.5
28.8
22.9
25.0
Contrasts
cover crop vs saline
Fresh vs saline
Gypsum vs saline
Cover vs gypsum
Barley vs barley/vetch and vetch
Barley vs vetch
p-values
0.02
0.36
0.37
0.16
0.33
0.16
Changes in means 1992±1994
Fresh vs saline
Cover crop vs saline
Gypsum vs saline
Cover crop vs gypsum
Barley vs barley plus vetch and vetch
Brley vs vetch
0.0001
0.0001
0.44
0.0001
0.001
0.018
0.0001
0.12
0.81
0.07
0.23
0.76
Water stable aggregation was not affected by amendment treatment in Spring 1992
(Table 4). Aggregation was significantly reduced in soils irrigated with saline drainage
water for two years (Table 4). Cover cropping resulted in higher aggregation percentages
than the saline fallow in 1994. The soil aggregate stability of the treatment irrigated with
nonsaline water was maintained at pre-treatment levels.
Average soil crust strength in Spring 1992 was lowest following incorporation of
barley (Table 5). Crust strength did not differ among treatments in 1993. In 1994,
Table 5
Average maximum penetration force for the 0±10 mm depth in 1992, 1993 and 1994 and each value is a mean of
about 200 ``pokes''a
Treatment
Nonsaline fallow
Saline gypsum fallow
Saline fallow
Saline barley
Saline barley plus vetch
Saline vetch
a
Penetration force (N)
1992
1993
1994
6.79ab
5.34abc
7.31a
3.78c
4.73bc
5.66abc
6.05a
6.23a
6.82a
4.34a
5.75a
5.64a
5.75bc
4.88c
8.16a
6.47bc
6.33bc
7.43ab
Means in any column followed by different letter (a, b) differ signi®cantly at the 0.05 probability level.
65
J.P. Mitchell et al. / Agricultural Water Management 45 (2000) 55±71
Fig. 4. Changes in cotton plant height in 1994. Standard error of the mean bars are shown when they exceed the
height of the symbol.
following two seasons of saline irrigations, penetration resistance was highest in the
saline fallow plot and lowest following surface application of gypsum.
3.3. Crop productivity and quality
Tomato stand establishment (number of emerged seedlings per meter of row) was
increased by 46% in 1992 (prior to drainage water application) in plots following vetch
incorporation, but was unaffected by either cover crop incorporation or gypsum
application in 1993 (data not shown). Cotton seedling emergence following incorporation
of each of the cover crops, however, was significantly lower (Fig. 4). Total tomato plant
biomass production was similar among treatments early in the season, and slightly lower
in plots following cover crops as the crop matured (data not shown). Cotton plants
Table 6
Tomato fruit and cotton lint yieldsa
Tomato (t haÿ1)
Fresh/fallow
Saline/gypsum fallow
Saline/fallow
Saline/barley
Saline/barley-vetch
Saline/vetch
a
Cotton (t haÿ1)
1992
1993
1994
100.6a
102.6a
97.2a
84.0b
75.7b
78.8b
69.9a
47.7b
48.2b
43.9b
50.0b
49.7b
1.89a
2.17a
2.19a
2.13a
2.03a
2.00a
Means in any column followed by different letter (a, b) differ signi®cantly at the 0.05 probability level.
66
J.P. Mitchell et al. / Agricultural Water Management 45 (2000) 55±71
Fig. 5. Crusted soil surface
Table 7
Number of Rhizoctonia solani colony forming units per 100 g dry soil (0±15 cm) standard deviation of the
mean
Fresh/fallow
Saline/gypsum fallow
Saline/fallow
Saline/barley
Saline/barley plus vetch
Saline/vetch
1.8 0.52
0.9 0.88
1.3 0.89
0.3 0.52
0.5 0.59
1.7 1.20
irrigated with saline water were significantly shorter through much of the season than
plants irrigated with nonsaline water (Fig. 5).
Two patterns of tomato red fruit yields are seen in the two tomato seasons (Table 6). In
1992, yields were lower in plots following each of the three cover crops, while in 1993,
yields were reduced by a third in all plots that had been irrigated with drainage water
relative to the control plot that had been irrigated with nonsaline water. Cotton lint yields
were not affected by saline irrigation (Table 6). No significant difference in the number of
Rhizoctonia colony forming units was found in surface soil samples collected during the
period of cotton emergence in 1994 (Table 7).
4. Discussion
Reuse of saline subsurface drainage water for irrigation has been identified as an
important component of regional management strategies for drainage water management
J.P. Mitchell et al. / Agricultural Water Management 45 (2000) 55±71
67
and sustained agricultural production in the SJV (San Joaquin Valley Drainage Program,
1990). The cyclic method of reuse, as proposed by Rhoades (1984), and used in this
study, theoretically maximizes the usability of saline drainage water by confining
irrigations with saline water to certain crops in a rotation at suitably salt-tolerant growth
stages (Rhoades et al., 1988). Several recent field studies have shown the viability of the
cyclic irrigation approach practiced over short terms (Rhoades and Dinar, 1991; Mitchell
et al., 1991a; Shennan et al., 1995). In the first year of this study, yields of tomatoes
irrigated with saline water following winter fallow were maintained relative to plants
irrigated with nonsaline water (Table 6). Yields in this first year of saline drainage water
reuse were significantly reduced following incorporation of each of the cover crops.
Nitrogen availability to the summer tomato crop in this year may have been suboptimal
due to N-immobilization that may have occurred following incorporation of cover crop
residues. Work by Miller (1991) supports the finding that nitrogen recovery from
incorporated cover crops added to systems without recent organic nitrogen addition tends
to be lower than from systems with a longer history of cover crop addition.
The feasibility of a drainage water reuse strategy over longer-terms depends on its
ability to sustain crop yields as well as soil quality. The substantial tomato yield reduction
that occurred in the second year of our study in plots irrigated with saline drainage water
(Table 6), as well as the significant salt buildup in lower portions of the crop root zone
(Fig. 2) following drainage water irrigations demonstrate definitive limitations of the
reuse approach. They also confirm the findings from a recent six-year cyclic drainage
water reuse study (Shennan et al., 1995). An important difference between this study and
the study of Shennan et al. (1995) is the fact that saline water was applied to two
consecutive tomato crops in our rotation. Fig. 2 indicates that average rootzone salinities
were about 2.8 and 3.8 dS mÿ1 in 1992, the first tomato season, and in 1993, the second
tomato season, respectively. Using these values and production functions of Maas (1987),
expected tomato yield declines due to salinity would be about 3% in 1992 and 13% in
1993. In the absence of stand reductions in either tomato year, soil salinity likely resulted
in the yield decline observed in 1993. The discrepancy between the 13% yield reduction
predicted by the Maas function and the 33% decrease we determined, may in part have
resulted from a higher percentage of green fruit in saline plots (Shennan et al., 1995) at
the time of harvest. The percentage of green fruit, however, was not determined in our
study and, therefore, is not included in our fruit yield data.
These results demonstrate that gypsum application and cover crop incorporation may
improve soil physical properties in cyclic saline drainage water reuse systems, but that
these improvements do not necessarily correlate with higher crop stands or yields.
Following two summer seasons in which saline drainage water supplied about 70% of the
irrigation water requirement, surface-applied gypsum significantly reduced soil crust
strength and improved cotton stand establishment. Cover cropping decreased crust
strength and increased soil aggregate stability, but significantly depressed emergence
rates and final stand densities of cotton seedlings in the third year of the study when soils
had become salinized.
Two mechanisms may account for the lack of observed relationship between the soil
physical properties and seedling emergence patterns following cover crop incorporation.
First, when soil surface crusts form in cover-cropped soils, undecomposed barley stubble
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J.P. Mitchell et al. / Agricultural Water Management 45 (2000) 55±71
and vetch stems horizontally and vertically reinforced dispersed soil crusts as ``rebar''
does in reinforced concrete. The result is interconnected surface slabs. This stubblereinforced crust presents a formidable barrier to seedlings and impedes timely seedling
emergence. The frequency and width of cracks were not measured here, but these surface
attributes did not seem to vary with treatment. Surface plates that occurred in covercropped plots, however, appeared to be considerably more interconnected (Figure. 7). The
lifting force of cotton seedlings was in many cases inadequate to overcome the
gravitational force of these adjacent, interconnected plates that effectively ``capped'' the
seedlings and inhibited their emergence (Arndt, 1965). We do not know if such stubblereinforced crusts would similarly impede emergence under nonsaline conditions because
practical limitations on the number of treatments we could impose in this study did not
permit evaluation of cover-cropped nonsaline soil conditions. Results of a single year
study by Groody (1990) indicated slightly earlier emergence of tomato seedlings and
lower soil penetration resistance following vetch incorporation relative to fallowed plots
that were fertilized with inorganic N under nonsaline conditions. We also detected
increased emergence following vetch incorporation in the first tomato crop before
drainage water irrigations were applied. In our study, however, higher numbers of
emerged tomato seedlings following vetch incorporation were not correlated with
reduced crust strength as measured by micropenetrometer. This finding suggests that
other, as yet unspecified attributes of vetch such as certain decomposition products may
somehow promote tomato seedling growth and emergence under nonsaline conditions.
Second, the longer that seedlings are under such slabs, the more susceptible they are to
pathogen infection, the weaker they become, and if they emerge at all, they often do not
survive. Colyer and Vernon (1993) reported higher incidences of various Rhizoctonia and
Fusarium fungi in cotton seedlings as well as reduced plant populations in conservation
tillage systems relative to a conventional system. Our soil assays for Rhizoctonia revealed
no significant increase in fungal colonies in cover crop incorporated soils relative to
fallowed soils. Delays in seedling emergence due to surface slabs, however, resulted in
longer exposure times to existing soil pathogens and likely contributed to the increase in
diseased seedlings and stand reductions that we observed, and that may have been
exacerbated by soil salinity (Snapp et al., 1991). Finally, it is likely that soil aggregation
and soil surface crust strength are inadequate measures of the phenomena that
fundamentally contribute to seedling emergence performance in cover-cropped soils.
Problems identifying universal measures of emergence impedance for all crops and soil
conditions were described over thirty years ago by Arndt (1965). The micropenetrometer
used in this study is capable of detecting and quantifying differences in soil surface
characteristics associated with soil physical properties and irrigation management that
may influence seedling emergence. No strong relationship between seedling emergence
performance and soil crust strength was found in our study, however. R2 values for linear
regression analyses of emergence vs penetration resistance for the three crops in this
study were only 0.003, 0.053 and 0.01. Furthermore, the penetrometer inadequately
detected the influence of undecomposed cover crop residues that linked surface plates in
interconnected slabs.
The fact that these interconnected surface crusts severely impeded emergence in 1994
but not in either of the two earlier years may be due to several factors. First, sprinkler
J.P. Mitchell et al. / Agricultural Water Management 45 (2000) 55±71
69
irrigations for germination and emergence were done more frequently for each of the two
tomato crops than for the cotton crop. These irrigations were deemed necessary to
establish the tomato crops, but they may have diminished, to some extent, influences of
amendment treatment on tomato emergence. Second, tomato and cotton seedlings differ
in growth and emergence patterns. Tomato seedlings, though very thin, may have greater
ability to wind and detour around impedances, and emerge through cracks than thicker
cotton seedlings that have greater lifting force but less lateral mobility.
Shortcomings of soil aggregation determinations are well known (Allison, 1968). Our
study did not fully test possible relationships between aggregation and seedling
emergence for the gypsum amendment treatment because samples for aggregation
determination in the spring of 1994 were collected before the application of gypsum to
the soil surface as were samples for soil chemical analyses. We cannot, therefore, relate
our finding of distinct reductions in soil crust strength following surface application to
our data for water stable aggregation which was based on sample material collected prior
to surface gypsum application.
In summary, the use of gypsum and cover crops as amendments may be beneficial in
improving soil physical properties in saline drainage water reuse systems. Cover cropping
will require innovative management, however, before long-term benefits to soil quality
can be optimally expressed in crop productivity in these systems. Detailed economic
evaluations of integrated reuse systems are needed.
Acknowledgements
The research reported in this paper was supported in part by a grant from the University
of California Water Resources Center, as part of Project UCAL-WRC-W-783. Gratitude
is also expressed for a research fellowship to the senior author from the University of
California, Davis. We also graciously acknowledge the contributions of the anonymous
reviewers of this manuscript.
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Agric. Ecosystems Environ. 59, 1±17.
Impacts of gypsum and winter cover crops on soil
physical properties and crop productivity when
irrigated with saline water
J.P. Mitchella,*, C. Shennana,1, M.J. Singerb, D.W. Petersb,
R.O. Millera,2, T. Prichardb, S.R. Grattanb, J.D. Rhoadesc,
D.M. Mayd, D.S. Munkd
a
Department of Vegetable Crops, University of California, Davis, CA 95616, USA
Department of Land, Air and Water Resources, University of California, Davis, CA 95616, USA
c
USDA Salinity Lab, 450 West Big Springs Road, Riverside, CA 92507, USA
d
Cooperative Extension, University of California, 1720 S. Maple Avenue, Fresno, CA 93702, USA
b
Accepted 19 July 1999
Abstract
Reuse of saline subsurface drainage water for irrigation has been identi®ed as a potential option
for managing drainage volumes and sustaining crop productivity in California's San Joaquin Valley.
Soil surface structural instability, crusting and poor stand establishment may, however, be
constraints in drainage reuse systems. The objective of this ®eld study was to evaluate the
effectiveness of winter cover crop incorporation and gypsum applications relative to conventional
fallows for improving soil physical properties, stand establishment and crop productivity in a
cropping system relying on the cyclic reuse of saline drainage for irrigation. Barley (Hordeum
vulgare), Lana woolypod vetch (Vicia dasycarpa) and barley/vetch winter cover crop treatments
were compared with gypsum amended and unamended winter fallows in a rotation of tomato±
tomato±cotton as summer crops. Tomato seedling emergence was improved by 34% following
incorporation of vetch in year 1 prior to saline irrigation application, but was unaffected by
amendment treatment in year 2. Following two summer seasons in which saline drainage water was
used for about 70% of the irrigation requirements, surface-applied gypsum signi®cantly reduced
soil crust strength an average of 14%, increased soil aggregate stability an average of 46%, and
*
Corresponding author. Tel.: 1-559-646-6565; fax: 1-559-646-6593.
E-mail address: mitchell@uckac.edu (J.P. Mitchell)
1
Present address: Department of Environmental Studies, University of California, Santa Cruz, CA 95064,
USA.
2
Present address: Soil and Crop Science Department, Colorado State University, Fort Collins, CA 80523,
USA.
0378-3774/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 3 7 8 - 3 7 7 4 ( 9 9 ) 0 0 0 7 0 - 0
56
J.P. Mitchell et al. / Agricultural Water Management 45 (2000) 55±71
maintained cotton stand establishment relative to nonsaline irrigation. Emergence rates and ®nal
stand densities of cotton seedlings following incorporation of each of the cover crops, however,
were signi®cantly lower. The mechanism most likely responsible for reducing stand establishment
was the formation of stubble-reinforced surface crusts that resulted in interconnected slabs that
impeded timely emergence of seedlings. This type of physical impedance can create secondary
effects such as increased disease which could also have played a role in reducing emergence. Yields
of tomatoes irrigated with saline water were maintained relative to nonsaline irrigation in year 1, but
were decreased by 33% in year 2. No reductions in cotton lint yield occurred as a result of saline
irrigation in year 3. Soil electrical conductivity (ECe) increased from about 2±6 dS mÿ1 during the
course of this 3-year study despite leaching by winter rains. Cyclic reuse of saline subsurface
drainage water may conserve good quality water and provide a means of sustaining crop
productivity over short terms. Soil surface salt and boron accumulation, however, may be major
constraints to this cropping strategy and will limit productivity if appropriate irrigation and crop
management practices are not followed. # 2000 Elsevier Science B.V. All rights reserved.
Keywords: Salinity; Drainage water reuse; Soil degradation
1. Introduction
Intensive irrigation practices developed in this century have dramatically increased
agricultural productivity in many areas of the western US, notably in the San Joaquin
Valley (SJV) of California (Hess, 1984; Rubatzky, 1985). These practices, however, have
also resulted in increased drainage water production and chemical contamination of
groundwater (Lamb, 1986; Halberg, 1987). It is generally acknowledged that long-term
agricultural production in irrigated areas is dependent on an adequate drainage outflow
system (van Schilfgaarde and Hoffman, 1977), and a variety of management strategies are
currently being considered to reduce the volume of drainage ultimately requiring disposal
or treatment (SJV Drainage Program Management Plan, 1990). Reuse of saline drainage
water for irrigation is one option proposed. Cyclic reuse, as proposed by Rhoades (1984),
involves the use of nonsaline water in rotation with saline drainage water for the irrigation
of selected crops differing in their salinity tolerance. The potential for cyclic reuse of
saline drainage water for irrigation has been documented for several lower value crops
(Rhoades et al., 1988). Higher value crops such as processing tomato, melons and cotton
have also been produced without sustaining significant yield reductions in short-term
studies using saline drainage water (Grattan et al., 1987; Shennan et al., 1988). Similarly,
we have shown by a combination of field and greenhouse studies that processing tomato
quality is generally improved by exposure to salinity following plant establishment
(Mitchell et al., 1991a,b). Longer-term experiments in the SJV indicate that accumulation
of soil B over time is the most likely microelement limitation to cyclic drainage reuse,
whereas Se accumulation is unlikely to be a problem, because the major form of Se in the
drainage water of this region is selenate which is relatively mobile in the soil profile and
readily leached below the rootzone by periodic use of good quality water (Shennan et al.,
1995). In addition, although Se concentrations increase in crops irrigated with saline
drainage waters containing up to 300 ppb Se, it is not considered a human health hazard
(Fan and Jackson, 1988). Sulfate also plays an important role in reducing Se in plant
J.P. Mitchell et al. / Agricultural Water Management 45 (2000) 55±71
57
tissue due to sulfate±selenate competition (Wan et al., 1991; Wu and Huang, 1991). There
is a considerable body of literature documenting complex interactions between soil sodicity
and salinity and the impacts of these two factors on soil physical properties (Shainberg and
Letey, 1984; Oster et al., 1996; Oster and Jayawardane, 1998). The specific boundary of
salinity and sodicity conditions at which a soil becomes unstable varies from one soil to
the next (Pratt and Suarez, 1990). Soil slaking may, however, occur and greatly reduce
water infiltration and create a strong surface crust, resulting in poor stand establishment
and decreased crop yields when nonsaline (low EC) water is used to irrigate a field
previously irrigated with drainage water in the SJV (Biggar et al., 1988; Shennan et al.,
1988). To prevent these problems, management practices that ameliorate the degradation
of soil physical properties must be employed to allow sustainable reuse of drainage water.
The most common method for maintaining water infiltration is tillage and gypsum
incorporation into the soil, often at rates from 2±4 tons haÿ1. Another alternative is the
inclusion of a green manure crop. Several studies have shown benefits of green manure
incorporation on soil physical characteristics including water infiltration (Cassman and
Rains, 1986), aggregation (Macrae and Mehuys, 1985), and porosity (Disparte, 1987).
Cover cropping is slow to increase organic matter in semi-arid or arid irrigated regions
(Wyland et al., 1996), possibly because intermittently moist soils and high temperature
conditions may favor rapid decomposition of soil organic matter. A cover crop may also
protect the soil surface from rain or sprinkler drop impact, thereby preventing the
formation of soil crusts which often limit infiltration (Stolzy et al., 1960; Disparte, 1987).
Organic N input from a leguminous cover crop may also reduce the seasonal fertilizer-N
requirement (Shennan, 1992), reduce soil NOÿ
3 , and minimize leaching losses from
winter rains and spring preirrigations (Stivers et al., 1990). Improved seedling emergence
of crops that follow incorporated cover crops may be achieved due to reductions in soil
crust formation in cover-cropped systems (Groody, 1990; Stivers et al., 1990). Roberson
et al. (1991) found significant improvements in macroaggregate slaking resistance in
cover-cropped vs clear cultivated treatments in a California orchard. Although reference
has been made to the potential use of crop residue management in managing salinity
(Cassman and Rains, 1986), to date, no research has directly addressed this objective.
Many present row-cropping systems in the SJV could benefit from the use of cover crops
provided innovative management practices are developed.
The objectives of this study were (1) to compare the relative effectiveness of winter
cover crop incorporation and gypsum applications in cropping rotations employing cyclic
reuse of saline drainage or shallow groundwater for improving/maintaining good soil
physical properties, (2) to determine annual water budgets for the proposed cropping
system, and (3) to assess potential long-term improvements in water use efficiency of this
cropping system.
2. Materials and methods
2.1. Experimental design and cultural practices
A 3-year experiment was conducted during the 1991±1994 period on 93 m 7 m plots
of Panoche soil (fine-loamy, mixed, calcareous, thermic Typic Torriorthent) at the
58
J.P. Mitchell et al. / Agricultural Water Management 45 (2000) 55±71
Table 1
Water quality for winter soil cover and summer irrigation treatments
Winter treatment
Barley cover crop
Barley/vetch cover crop
Vetch cover crop
Fallow
Fallow
Fallow
Summer irrigation treatment
Crop establishment
Post-establishment
Nonsaline
Nonsaline
Nonsaline
Nonsaline and gypsum
Nonsaline
Nonsaline
Saline
Saline
Saline
Saline
Saline
Nonsaline
University of California West Side Research and Extension Center (WSREC) located
65 km southwest of Fresno, in Five Points, CA. Thirty-year average rainfall for this
region is 154 mm. Annual rainfall during the course of this study was 166, 266 and
101 mm for 1992, 1993 and 1994, respectively. Experimental treatments are summarized
in Table 1. Summer crops consisted of two years of processing tomato (1992, 1993)
followed by a year of cotton (1994). The summer crops in each of the first five treatments
in Table 1 were irrigated with low EC water (0.53 dS mÿ1) for germination and
establishment, and with saline water (EC 6.9 dS mÿ1) following establishment
(Table 2). An additional winter fallow treatment irrigated with nonsaline water during
the summers was included to compare changes in key indices of soil quality and crop
performance that resulted from the saline water irrigations applied to each of the other
five treatments. The experiment was organized as a randomized complete block with four
replications.
Barley (Hordeum vulgare), ``Lana'' woolypod vetch (Vicia dasycarpa) and barley/vetch
mixture cover crops were seeded in mid-October of 1991, 1992 and 1993, and incorporated to a depth of 15±20 cm with a rotary tiller in mid-March of each following year.
Processing tomato `Alta' was direct-seeded in single rows on beds (1.7 m 93 m)
flanked on both sides by seeded border rows of equal dimensions on April 1 in 1992 and
April 5 in 1993. Preplant broadcast monoammonium phosphate fertilizer was applied to
all plots at 12 kg N/ha and an additional 168 kg N/ha was sidedressed at thinning except
those in which vetch cover crops had been incorporated. Cotton (cv Maxxa) was planted
on four beds each measuring 1.7 m 93 m in each plot April 18, 1994. Preplant
broadcast ammonium sulfate supplied 150 kg N/ha to all plots except those following
vetch incorporations. Weed control was accomplished using EPA-approved preplant
herbicides, mechanical cultivation and hand weeding as necessary in each year. During
the summer growing seasons, water was applied weekly to tomato and every two weeks
to cotton by furrow irrigation in quantities sufficient to replace evapotranspirative losses
based on nonstressed conditions. A 0.1 leaching fraction was calculated and included in
each irrigation application. Summaries of the quality and amount of irrigation water used
in each experimental treatment is presented in Tables 2 and 3, respectively. Crop
evapotranspiration (ET) was estimated using reference values provided by the California
Irrigation Management Information System (CIMIS) weather station located 50 m from
the experimental field and crop coefficient values developed previously at the site (Phene,
Water source
EC
(dS mÿ1)
pH
SAR
(mmol lÿ1)
Na
(mmol lÿ1)
Ca
(mmol lÿ1)
Mg
(mmol lÿ1)
SOÿ2
4
(mmol lÿ1)
Cl
(mmol lÿ1)
NOÿ
3
(mmol lÿ1)
B
(mmol lÿ1)
Saline
High quality
6.9
0.53
7.9
7.9
9.4
2.4
41.4
2.0
9.6
0.7
10.0
0.6
27.6
0.3
14.1
1.1
0.7
0.01
0.55
0.015
J.P. Mitchell et al. / Agricultural Water Management 45 (2000) 55±71
Table 2
Chemical properties of irrigation waters
59
60
J.P. Mitchell et al. / Agricultural Water Management 45 (2000) 55±71
Table 3
Irrigation water applications and rainfall totals (mm) from 1991±1994
Crop
Year
Treatment
Water applied (mm)
Rainfall/cover
crop irrigations
Nonsaline
water
Saline
water
Total
Nonsaline fallow
Saline gypsum fallow
Saline fallow
Saline barley
Saline barley/vetch
Saline vetch
Nonsaline fallow
Saline gypsum fallow
Saline fallow
Saline barley
Saline barley/vetch
Saline vetch
232
232
232
232
232
232
266
266
266
304
304
304
638
219
219
219
219
219
640
117
117
117
117
117
0
419
419
419
419
419
0
523
523
523
523
523
870
870
870
870
870
870
906
906
906
944
944
944
Nonsaline fallow
Saline gypsum fallow
Saline fallow
Saline barley
Saline barley/vetch
Saline vetch
101
101
101
157
157
157
729
221
221
221
221
221
0
508
508
508
508
508
830
830
830
886
886
886
Tomato
1992
1993
Cotton
1994
1985). In-flow meters were used to measure the quantity of applied water. Prior to
planting in the spring of 1992, 11 t haÿ1 of gypsum were applied and incorporated to a
depth of 15±20 cm to the gypsum-amended plots. An additional 2.4 t haÿ1 of gypsum
were applied in spring 1994, of which about two-thirds was incorporated and one-third
was surface-applied.
2.2. Soil quality determinations
Soil samples from 0±15 cm were collected at 312 sites in a 5.7 m 7 m grid
throughout the experimental field in the spring and fall of each year for determinations by
the University of California, Division of Agriculture and Natural Resources Analytical
Laboratory of (1) total soil organic matter using a modified Walkley±Black procedure as
described by Janitzky (1986), (2) total soil nitrogen using the total Kjeldahl nitrogen
method of Carlson (1986), (3) electrical conductivity of the soil saturation extract (ECe)
using a YSI Model 32 Conductance Meter (YSI Instruments, Yellow Springs, OH), and
calculation of the sodium adsorption ratio (SARe) of the soil extract for each sample
using the equation
SAR
Na
f Ca Mg=2g1=2
;
J.P. Mitchell et al. / Agricultural Water Management 45 (2000) 55±71
61
where Na, Ca and Mg are the total concentrations of the soluble cations in mmol lÿ1.
Additional sets of 312 samples of the surface 15 cm of soil were collected in the spring of
each year for measurement of soil aggregate stability using a modi®cation of the water
stable aggregate method of Kemper and Rosenau (1986). Soil cores were taken at 1±15,
15±30, 30±60 and 60±90 cm increments four times during the experiment for ECe
determinations. Soil surface crust strength was characterized by the force required to
penetrate the top 10 mm of soil using a micropenetrometer described by Rolston et al.
(1991). A ¯at-tipped, 1.59 mm diameter probe programmed to penetrate the soil at a rate
of 8 mm minÿ1 was used. Measurements of penetration resistance were recorded at depth
intervals of 0.1 mm. A total of 200 ``pokes'' were made of air-dry soil at horizontal
intervals of 20 mm along two 1 m transects in all plots in the spring of each year of the
experiment. Based on the strong relationship between the maximum force and the total
work over the depth of penetration (Rolston et al., 1991), maximum force was used as an
index of soil surface strength. Crust samples were taken for gravimetric water content
determination along each transect at the time of measurement (Gardner, 1986).
Rhizoctonia solani colonies in the surface 15 cm of soil were quanti®ed in the spring
of 1994 using the assay procedure of Weinhold (1977).
2.3. Biological and economic crop yield and quality determination
Tomato and cotton seedling emergence rates were determined by counting the number
of seedlings that emerged along a given length of single-row-planted treatment plot
during the four weeks following planting. Seedling counts were made at 13 sites along a
total of 40 m in each plot in 1992, 52 m in 1993 and 104 m in 1994. Tomato plant
aboveground biomass was determined six times during the 1992 season and nine times in
1993 by harvesting, drying and weighing the total plant mass from 2.4 m2 of treatment
plot. Tomato leaf blades and petioles were collected 10 times during the 1993 season for
determination of total N by the Kjeldahl method.
2.4. Statistical analysis
Statistical computations of effects due to winter soil cover and summer irrigation
treatments were made using the PROC GLM procedure of SAS Version 6.1 software.
Response variables considered were soil water stable aggregates, crust strength, crop
yields and quality. Significance was determined using an F-test and in the event of
significant effects due to treatments, means were separated at the 0.05 probability level
and contrasts between groups of treatments were calculated.
3. Results
3.1. Water inputs
Variations in total water applied among treatments in 1993 and 1994 are due to the
irrigation water that was applied to cover crop plots for germination and establishment
62
J.P. Mitchell et al. / Agricultural Water Management 45 (2000) 55±71
Fig. 1. Changes in soil ECe, SAR, organic matter (%) and nitrogen (5) over time at 0±15 cm depth. Standard
error of the mean bars are shown when they exceed the height of the symbol.
during the fall months preceding each of these seasons. Saline drainage water reuse
accounted for 70% of the total amount of irrigation water applied to summer crops during
the three-year cropping cycle (Table 3).
3.2. Soil quality determinations
Reuse of drainage water led to significant fluctuations in salinity in the soil surface
(Fig. 1) and throughout the soil profile (Figs. 2 and 3). Leaching by winter rains had little
effect on salinity in lower parts of the soil profile (Fig. 2). SARe values for the surface
15 cm of soil also increased relative to initial soil values after irrigations with saline
drainage water (Fig. 1). By spring 1994 these soils became sodic (i.e., SARe > 11). Soil
organic matter (SOM) content increased in cover crop amended plots in the spring of
each year relative to winter fallowed plots (Fig. 1). Increases in SOM were highest in the
barley and barley/vetch mix plots and were almost twice as high as initial values. Total
soil N was also increased in the spring of each year in cover crop plots, but declined to
levels of the fallowed plots by the fall of each year (Fig. 1).
J.P. Mitchell et al. / Agricultural Water Management 45 (2000) 55±71
63
Fig. 2. Soil saturation extract electrical conductivity (ECe) pro®les over time. Standard error of mean bars are
shown when they exceed the height of the symbol.
Fig. 3. Cotton seedling emergence 1994. Each value is a mean of four replicates for the total number of
seedlings emerged along 104 m of plot. Standard error of the mean bars are shown when they exceed the height
of the symbol.
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J.P. Mitchell et al. / Agricultural Water Management 45 (2000) 55±71
Table 4
Water stable aggregates (%) in 1992 and 1994
1992
1994
Treatment
Fresh fallow
Saline/gypsum fallow
Saline fallow
Saline/barley
Saline/barley-vetch
Saline/vetch
35.5
35.9
34.0
39.4
38.9
36.5
33.5
18.7
17.5
28.8
22.9
25.0
Contrasts
cover crop vs saline
Fresh vs saline
Gypsum vs saline
Cover vs gypsum
Barley vs barley/vetch and vetch
Barley vs vetch
p-values
0.02
0.36
0.37
0.16
0.33
0.16
Changes in means 1992±1994
Fresh vs saline
Cover crop vs saline
Gypsum vs saline
Cover crop vs gypsum
Barley vs barley plus vetch and vetch
Brley vs vetch
0.0001
0.0001
0.44
0.0001
0.001
0.018
0.0001
0.12
0.81
0.07
0.23
0.76
Water stable aggregation was not affected by amendment treatment in Spring 1992
(Table 4). Aggregation was significantly reduced in soils irrigated with saline drainage
water for two years (Table 4). Cover cropping resulted in higher aggregation percentages
than the saline fallow in 1994. The soil aggregate stability of the treatment irrigated with
nonsaline water was maintained at pre-treatment levels.
Average soil crust strength in Spring 1992 was lowest following incorporation of
barley (Table 5). Crust strength did not differ among treatments in 1993. In 1994,
Table 5
Average maximum penetration force for the 0±10 mm depth in 1992, 1993 and 1994 and each value is a mean of
about 200 ``pokes''a
Treatment
Nonsaline fallow
Saline gypsum fallow
Saline fallow
Saline barley
Saline barley plus vetch
Saline vetch
a
Penetration force (N)
1992
1993
1994
6.79ab
5.34abc
7.31a
3.78c
4.73bc
5.66abc
6.05a
6.23a
6.82a
4.34a
5.75a
5.64a
5.75bc
4.88c
8.16a
6.47bc
6.33bc
7.43ab
Means in any column followed by different letter (a, b) differ signi®cantly at the 0.05 probability level.
65
J.P. Mitchell et al. / Agricultural Water Management 45 (2000) 55±71
Fig. 4. Changes in cotton plant height in 1994. Standard error of the mean bars are shown when they exceed the
height of the symbol.
following two seasons of saline irrigations, penetration resistance was highest in the
saline fallow plot and lowest following surface application of gypsum.
3.3. Crop productivity and quality
Tomato stand establishment (number of emerged seedlings per meter of row) was
increased by 46% in 1992 (prior to drainage water application) in plots following vetch
incorporation, but was unaffected by either cover crop incorporation or gypsum
application in 1993 (data not shown). Cotton seedling emergence following incorporation
of each of the cover crops, however, was significantly lower (Fig. 4). Total tomato plant
biomass production was similar among treatments early in the season, and slightly lower
in plots following cover crops as the crop matured (data not shown). Cotton plants
Table 6
Tomato fruit and cotton lint yieldsa
Tomato (t haÿ1)
Fresh/fallow
Saline/gypsum fallow
Saline/fallow
Saline/barley
Saline/barley-vetch
Saline/vetch
a
Cotton (t haÿ1)
1992
1993
1994
100.6a
102.6a
97.2a
84.0b
75.7b
78.8b
69.9a
47.7b
48.2b
43.9b
50.0b
49.7b
1.89a
2.17a
2.19a
2.13a
2.03a
2.00a
Means in any column followed by different letter (a, b) differ signi®cantly at the 0.05 probability level.
66
J.P. Mitchell et al. / Agricultural Water Management 45 (2000) 55±71
Fig. 5. Crusted soil surface
Table 7
Number of Rhizoctonia solani colony forming units per 100 g dry soil (0±15 cm) standard deviation of the
mean
Fresh/fallow
Saline/gypsum fallow
Saline/fallow
Saline/barley
Saline/barley plus vetch
Saline/vetch
1.8 0.52
0.9 0.88
1.3 0.89
0.3 0.52
0.5 0.59
1.7 1.20
irrigated with saline water were significantly shorter through much of the season than
plants irrigated with nonsaline water (Fig. 5).
Two patterns of tomato red fruit yields are seen in the two tomato seasons (Table 6). In
1992, yields were lower in plots following each of the three cover crops, while in 1993,
yields were reduced by a third in all plots that had been irrigated with drainage water
relative to the control plot that had been irrigated with nonsaline water. Cotton lint yields
were not affected by saline irrigation (Table 6). No significant difference in the number of
Rhizoctonia colony forming units was found in surface soil samples collected during the
period of cotton emergence in 1994 (Table 7).
4. Discussion
Reuse of saline subsurface drainage water for irrigation has been identified as an
important component of regional management strategies for drainage water management
J.P. Mitchell et al. / Agricultural Water Management 45 (2000) 55±71
67
and sustained agricultural production in the SJV (San Joaquin Valley Drainage Program,
1990). The cyclic method of reuse, as proposed by Rhoades (1984), and used in this
study, theoretically maximizes the usability of saline drainage water by confining
irrigations with saline water to certain crops in a rotation at suitably salt-tolerant growth
stages (Rhoades et al., 1988). Several recent field studies have shown the viability of the
cyclic irrigation approach practiced over short terms (Rhoades and Dinar, 1991; Mitchell
et al., 1991a; Shennan et al., 1995). In the first year of this study, yields of tomatoes
irrigated with saline water following winter fallow were maintained relative to plants
irrigated with nonsaline water (Table 6). Yields in this first year of saline drainage water
reuse were significantly reduced following incorporation of each of the cover crops.
Nitrogen availability to the summer tomato crop in this year may have been suboptimal
due to N-immobilization that may have occurred following incorporation of cover crop
residues. Work by Miller (1991) supports the finding that nitrogen recovery from
incorporated cover crops added to systems without recent organic nitrogen addition tends
to be lower than from systems with a longer history of cover crop addition.
The feasibility of a drainage water reuse strategy over longer-terms depends on its
ability to sustain crop yields as well as soil quality. The substantial tomato yield reduction
that occurred in the second year of our study in plots irrigated with saline drainage water
(Table 6), as well as the significant salt buildup in lower portions of the crop root zone
(Fig. 2) following drainage water irrigations demonstrate definitive limitations of the
reuse approach. They also confirm the findings from a recent six-year cyclic drainage
water reuse study (Shennan et al., 1995). An important difference between this study and
the study of Shennan et al. (1995) is the fact that saline water was applied to two
consecutive tomato crops in our rotation. Fig. 2 indicates that average rootzone salinities
were about 2.8 and 3.8 dS mÿ1 in 1992, the first tomato season, and in 1993, the second
tomato season, respectively. Using these values and production functions of Maas (1987),
expected tomato yield declines due to salinity would be about 3% in 1992 and 13% in
1993. In the absence of stand reductions in either tomato year, soil salinity likely resulted
in the yield decline observed in 1993. The discrepancy between the 13% yield reduction
predicted by the Maas function and the 33% decrease we determined, may in part have
resulted from a higher percentage of green fruit in saline plots (Shennan et al., 1995) at
the time of harvest. The percentage of green fruit, however, was not determined in our
study and, therefore, is not included in our fruit yield data.
These results demonstrate that gypsum application and cover crop incorporation may
improve soil physical properties in cyclic saline drainage water reuse systems, but that
these improvements do not necessarily correlate with higher crop stands or yields.
Following two summer seasons in which saline drainage water supplied about 70% of the
irrigation water requirement, surface-applied gypsum significantly reduced soil crust
strength and improved cotton stand establishment. Cover cropping decreased crust
strength and increased soil aggregate stability, but significantly depressed emergence
rates and final stand densities of cotton seedlings in the third year of the study when soils
had become salinized.
Two mechanisms may account for the lack of observed relationship between the soil
physical properties and seedling emergence patterns following cover crop incorporation.
First, when soil surface crusts form in cover-cropped soils, undecomposed barley stubble
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J.P. Mitchell et al. / Agricultural Water Management 45 (2000) 55±71
and vetch stems horizontally and vertically reinforced dispersed soil crusts as ``rebar''
does in reinforced concrete. The result is interconnected surface slabs. This stubblereinforced crust presents a formidable barrier to seedlings and impedes timely seedling
emergence. The frequency and width of cracks were not measured here, but these surface
attributes did not seem to vary with treatment. Surface plates that occurred in covercropped plots, however, appeared to be considerably more interconnected (Figure. 7). The
lifting force of cotton seedlings was in many cases inadequate to overcome the
gravitational force of these adjacent, interconnected plates that effectively ``capped'' the
seedlings and inhibited their emergence (Arndt, 1965). We do not know if such stubblereinforced crusts would similarly impede emergence under nonsaline conditions because
practical limitations on the number of treatments we could impose in this study did not
permit evaluation of cover-cropped nonsaline soil conditions. Results of a single year
study by Groody (1990) indicated slightly earlier emergence of tomato seedlings and
lower soil penetration resistance following vetch incorporation relative to fallowed plots
that were fertilized with inorganic N under nonsaline conditions. We also detected
increased emergence following vetch incorporation in the first tomato crop before
drainage water irrigations were applied. In our study, however, higher numbers of
emerged tomato seedlings following vetch incorporation were not correlated with
reduced crust strength as measured by micropenetrometer. This finding suggests that
other, as yet unspecified attributes of vetch such as certain decomposition products may
somehow promote tomato seedling growth and emergence under nonsaline conditions.
Second, the longer that seedlings are under such slabs, the more susceptible they are to
pathogen infection, the weaker they become, and if they emerge at all, they often do not
survive. Colyer and Vernon (1993) reported higher incidences of various Rhizoctonia and
Fusarium fungi in cotton seedlings as well as reduced plant populations in conservation
tillage systems relative to a conventional system. Our soil assays for Rhizoctonia revealed
no significant increase in fungal colonies in cover crop incorporated soils relative to
fallowed soils. Delays in seedling emergence due to surface slabs, however, resulted in
longer exposure times to existing soil pathogens and likely contributed to the increase in
diseased seedlings and stand reductions that we observed, and that may have been
exacerbated by soil salinity (Snapp et al., 1991). Finally, it is likely that soil aggregation
and soil surface crust strength are inadequate measures of the phenomena that
fundamentally contribute to seedling emergence performance in cover-cropped soils.
Problems identifying universal measures of emergence impedance for all crops and soil
conditions were described over thirty years ago by Arndt (1965). The micropenetrometer
used in this study is capable of detecting and quantifying differences in soil surface
characteristics associated with soil physical properties and irrigation management that
may influence seedling emergence. No strong relationship between seedling emergence
performance and soil crust strength was found in our study, however. R2 values for linear
regression analyses of emergence vs penetration resistance for the three crops in this
study were only 0.003, 0.053 and 0.01. Furthermore, the penetrometer inadequately
detected the influence of undecomposed cover crop residues that linked surface plates in
interconnected slabs.
The fact that these interconnected surface crusts severely impeded emergence in 1994
but not in either of the two earlier years may be due to several factors. First, sprinkler
J.P. Mitchell et al. / Agricultural Water Management 45 (2000) 55±71
69
irrigations for germination and emergence were done more frequently for each of the two
tomato crops than for the cotton crop. These irrigations were deemed necessary to
establish the tomato crops, but they may have diminished, to some extent, influences of
amendment treatment on tomato emergence. Second, tomato and cotton seedlings differ
in growth and emergence patterns. Tomato seedlings, though very thin, may have greater
ability to wind and detour around impedances, and emerge through cracks than thicker
cotton seedlings that have greater lifting force but less lateral mobility.
Shortcomings of soil aggregation determinations are well known (Allison, 1968). Our
study did not fully test possible relationships between aggregation and seedling
emergence for the gypsum amendment treatment because samples for aggregation
determination in the spring of 1994 were collected before the application of gypsum to
the soil surface as were samples for soil chemical analyses. We cannot, therefore, relate
our finding of distinct reductions in soil crust strength following surface application to
our data for water stable aggregation which was based on sample material collected prior
to surface gypsum application.
In summary, the use of gypsum and cover crops as amendments may be beneficial in
improving soil physical properties in saline drainage water reuse systems. Cover cropping
will require innovative management, however, before long-term benefits to soil quality
can be optimally expressed in crop productivity in these systems. Detailed economic
evaluations of integrated reuse systems are needed.
Acknowledgements
The research reported in this paper was supported in part by a grant from the University
of California Water Resources Center, as part of Project UCAL-WRC-W-783. Gratitude
is also expressed for a research fellowship to the senior author from the University of
California, Davis. We also graciously acknowledge the contributions of the anonymous
reviewers of this manuscript.
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