Impacts of gypsum and winter cover crops on soil
physical properties and crop productivity when
irrigated with saline water
J.P. Mitchell
a,*, C. Shennan
a,1, M.J. Singer
b, D.W. Peters
b,
R.O. Miller
a,2, T. Prichard
b, S.R. Grattan
b, J.D. Rhoades
c,
D.M. May
d, D.S. Munk
daDepartment of Vegetable Crops, University of California, Davis, CA 95616, USA bDepartment of Land, Air and Water Resources, University of California, Davis, CA 95616, USA
cUSDA Salinity Lab, 450 West Big Springs Road, Riverside, CA 92507, USA
dCooperative Extension, University of California, 1720 S. Maple Avenue, Fresno, CA 93702, USA
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
Agricultural Water Management 45 (2000) 55±71
*Corresponding author. Tel.:1-559-646-6565; fax:1-559-646-6593.
E-mail address: mitchell@uckac.edu (J.P. Mitchell)
1Present address: Department of Environmental Studies, University of California, Santa Cruz, CA 95064,
USA.
2Present address: Soil and Crop Science Department, Colorado State University, Fort Collins, CA 80523,
USA.
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 mÿ1during 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
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 NOÿ3, 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 m7 m plots of Panoche soil (fine-loamy, mixed, calcareous, thermic Typic Torriorthent) at the
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 (EC6.9 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 incor-porated 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 m93 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 (cvMaxxa) was planted on four beds each measuring 1.7 m93 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,
Table 1
Water quality for winter soil cover and summer irrigation treatments
Winter treatment Summer irrigation treatment
Table 2
Chemical properties of irrigation waters
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 6.9 7.9 9.4 41.4 9.6 10.0 27.6 14.1 0.7 0.55
High quality 0.53 7.9 2.4 2.0 0.7 0.6 0.3 1.1 0.01 0.015
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1985). In-flow meters were used to measure the quantity of applied water. Prior to planting in the spring of 1992, 11 t haÿ1of 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 m7 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;
Table 3
Irrigation water applications and rainfall totals (mm) from 1991±1994
Crop Year Treatment Water applied (mm)
Rainfall/cover
1992 Nonsaline fallow 232 638 0 870
Saline gypsum fallow 232 219 419 870
Saline fallow 232 219 419 870
Saline barley 232 219 419 870
Saline barley/vetch 232 219 419 870
Saline vetch 232 219 419 870
1993 Nonsaline fallow 266 640 0 906
Saline gypsum fallow 266 117 523 906
Saline fallow 266 117 523 906
Saline barley 304 117 523 944
Saline barley/vetch 304 117 523 944
Saline vetch 304 117 523 944
Cotton
1994 Nonsaline fallow 101 729 0 830
Saline gypsum fallow 101 221 508 830
Saline fallow 101 221 508 830
Saline barley 157 221 508 886
Saline barley/vetch 157 221 508 886
Saline vetch 157 221 508 886
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 minÿ1was 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 m2of 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
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).
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.
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.
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 4
Water stable aggregates (%) in 1992 and 1994
1992 1994
cover crop vs saline 0.02 0.0001
Fresh vs saline 0.36 0.0001
Gypsum vs saline 0.37 0.44
Cover vs gypsum 0.16 0.0001
Barley vs barley/vetch and vetch 0.33 0.001
Barley vs vetch 0.16 0.018
Barley vs barley plus vetch and vetch 0.23
Brley vs vetch 0.76
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 Penetration force (N)
1992 1993 1994
Nonsaline fallow 6.79ab 6.05a 5.75bc
Saline gypsum fallow 5.34abc 6.23a 4.88c
Saline fallow 7.31a 6.82a 8.16a
Saline barley 3.78c 4.34a 6.47bc
Saline barley plus vetch 4.73bc 5.75a 6.33bc
Saline vetch 5.66abc 5.64a 7.43ab
aMeans in any column followed by different letter (a, b) differ signi®cantly at the 0.05 probability level.
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
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.
Table 6
Tomato fruit and cotton lint yieldsa
Tomato (t haÿ1) Cotton (t haÿ1)
1992 1993 1994
Fresh/fallow 100.6a 69.9a 1.89a
Saline/gypsum fallow 102.6a 47.7b 2.17a
Saline/fallow 97.2a 48.2b 2.19a
Saline/barley 84.0b 43.9b 2.13a
Saline/barley-vetch 75.7b 50.0b 2.03a
Saline/vetch 78.8b 49.7b 2.00a
aMeans in any column followed by different letter (a, b) differ signi®cantly at the 0.05 probability level.
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
Rhizoctoniacolony 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
Fig. 5. Crusted soil surface
Table 7
Number ofRhizoctonia solanicolony forming units per 100 g dry soil (0±15 cm)standard deviation of the mean
Fresh/fallow 1.80.52
Saline/gypsum fallow 0.90.88
Saline/fallow 1.30.89
Saline/barley 0.30.52
Saline/barley plus vetch 0.50.59
Saline/vetch 1.71.20
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 mÿ1in 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
and vetch stems horizontally and vertically reinforced dispersed soil crusts as ``rebar'' does in reinforced concrete. The result is interconnected surface slabs. This stubble-reinforced 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 cover-cropped 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 stubble-reinforced 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 variousRhizoctoniaand
Fusariumfungi in cotton seedlings as well as reduced plant populations in conservation
tillage systems relative to a conventional system. Our soil assays forRhizoctoniarevealed 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.R2values 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
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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