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Agronomy Journal 92:924-932 (2000)
© 2000 American Society of Agronomy

SOIL MANAGEMENT

Soil Physical Properties and Tomato Yield and Quality in Alternative Cropping Systems

Giuseppe Collaa, Jeffrey P. Mitchellb, Brian A. Joyceb, Leisa M. Huyckb, Wesley W. Wallenderb, Steve R. Templeb, Theodore C. Hsiaob and Durga D. Poudelb

a Dep. of Crop Production, Univ. of Tuscia, VT 01100, Tuscia, Italy
b Dep. of Agronomy and Range Sci., Univ. of California, Davis, CA 95616 USA

mitchell{at}uckac.edu


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusions
 REFERENCES
 
The Sustainable Agriculture Farming Systems (SAFS) Project has studied the transition to low-input and organic alternatives in California's Sacramento Valley. This project compares a 4-yr rotation of tomato (Lycopersicon esculentum Mill.), safflower (Carthamus tinctorius L.), corn (Zea mays L.), and wheat (Triticum aestivum L.) followed by double-cropped bean (Phaseolus vulgaris L.) in the conventional system and oat (Avena sativa L.)–purple vetch (Vicia benghalensis L.) in the low-input and organic systems. A conventional 2-yr rotation (tomato–wheat) is also studied. In 1997 and 1998, we evaluated the transition to alternative systems on soil bulk density, water holding capacity, infiltration and storage, water use efficiency, and `Brigade' tomato yield and quality. No differences in laboratory determinations of soil bulk density and water holding capacity were found; however, in situ water holding capacity was highest in the organic system, lowest in the conventional 4-yr rotation and intermediate in the low-input and conventional 2-yr rotations. In 1998, infiltration during 3-h irrigations was 0.028 m3 m-1 for the conventional, and 0.062 m3 m-1 and 0.065 m3 m-1 for the low-input and organic systems, respectively. Similar results were observed in 1997. The alternative systems required more water per irrigation for uniform application, resulting in higher soil water content in the organic systems throughout 1998. Evapotranspiration was higher in the conventional systems in both years relative to other systems. Tomato yields did not differ among systems in either year. Fruit quality was highest in the conventional 4-yr system.

Abbreviations: CCOF, California Certified Organic Farmers • CONV2, conventionally managed 2-yr rotation • CONV4, conventionally managed 4-yr rotation • E, evaporation • ET, evapotranspiration • LOW, low-input 4-yr rotation • ORG, organically managed 4-yr rotation • SAFS, Sustainable Agriculture Farming Systems • WUE, water use efficiency


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
Materials and methods
 Results and discussion
 Conclusions
 REFERENCES
 
IN RECENT YEARS, there has been growing interest among farmers, researchers, governmental agencies, and environmental conservation groups in investigating and adopting alternative crop production practices that are less chemical-intensive, less dependent on nonrenewable fossil fuels, and that function to conserve soil and water resources (Gliessman, 1998). This interest has resulted in part from studies documenting negative impacts of conventional agriculture on long-term profitability and resource stewardship, including declines in soil organic matter levels due to intensive tillage, surface water quality degradation due to reduced water infiltration rates, and reduced soil tilth (National Research Council, 1989). Over the last decade, alternative farming strategies have been increasingly investigated for opportunities to sustain and improve the soil resource base while meeting the needs and concerns of farmers (Jackson et al., 1993; Temple et al., 1994; Drinkwater et al., 1995; Mitchell et al., 1997, 1998).

Winter cover cropping is an alternative agriculture practice that has received much attention as a means of ameliorating soil physical properties. It has been shown to increase soil water retention (Keisling et al., 1994) and infiltration (Williams, 1966; Gulick et al., 1994) and to decrease soil surface strength (Folorunso et al., 1992; Bauer and Busscher, 1996). Keisling et al. (1994) also showed that hydraulic conductivity and bulk density were significantly improved as a result of winter cover cropping. The use of winter cover crops also has been shown to be effective in stabilizing soil aggregates (Jordahl and Karlen, 1993; Hermawan and Bomke, 1997).

However, the benefits of improved soil physical properties must be evaluated in light of the economic costs of additional water demands and potential impacts on crop quality and yield (Sainju and Singh, 1997). Higher infiltration rates can increase irrigation needs, and the cover crops can deplete soil moisture as they mature, competing with the cash crop for water unless killed early enough in the spring (Prichard et al., 1989; Zhu et al., 1991). Changes in soil–water relations can also affect the quality and yield of crop harvests depending upon irrigation frequency and availability of soil water to the plants (Mitchell et al., 1991).

The Sustainable Agriculture Farming Systems Project was established in 1988 to study the transition from conventional to low-input and organic cover crop-based farm management in California's Sacramento Valley (Temple et al., 1994). Multidisciplinary investigations in this project over the past 10 yr have shown many differences in soil biology (Gunapala and Scow, 1998), physical and chemical properties (Clark et al., 1998), water relations (Mitchell, 1996, 1997), abundance and diversity of weed, pathogen, arthropod and nematode populations (Lanini et al., 1994; Van Bruggen, 1995; Ferris et al., 1996), crop performance (Clark et al., 1999), and economic efficiency (Klonsky and Livingston, 1994) of the farming systems. Our objectives were to evaluate the effects of alternative farming systems on soil compaction, water holding capacity, infiltration and water storage in relation to tomato evapotranspiration, yield, and fruit quality within the SAFS cropping systems comparison 10 yr after it had been established.


   Materials and methods
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
Results and discussion
 Conclusions
 REFERENCES
 
Experimental Site and Farming System
The research site was located on 11.3 ha at the University of California, Davis, at the Agronomy Farm (38°32' N, 121°47' W, 18-m elevation). Basic soil characterization study of the SAFS research plots showed, on average, 36% sand, 46% silt, and 18% clay at 0- to 30-cm depth. The soil is classified as Reiff loam (coarse-loamy, mixed, nonacid, thermic Mollic Xerofluvents) and Yolo silt loam (fine-silty, mixed, nonacid, thermic Typic Xerorthents). The study consisted of four treatment systems, which differed in crop rotation and use of external inputs. The farming systems included 4-yr rotations under conventional (CONV4), low-input (LOW), and organic (ORG) management and a conventionally managed 2-yr rotation (CONV2) (Fig. 1) .



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  		Fig. 1 Rotation and cover crop sequences in the four farming systems evaluated in the Sustainable Agriculture Farming Systems Project

 
The 2-yr rotations included processing tomato, safflower, corn, and bean with winter wheat. In the LOW and ORG treatments, wheat was replaced by a mixture of oat and purple vetch, which was incorporated as green manure. In these two systems, the cover crops were grown during the fall and winter preceding tomato, safflower, and corn. The CONV2 system was a tomato and wheat rotation. The CONV4 and CONV2 treatments were managed using practices typical of the surrounding area, which included the application of synthetic pesticides and fertilizers. In the LOW systems, the fertilizers and pesticides were reduced by using cover crops to improve soil fertility and mechanical cultivation and cover cropping for winter weed control. Organic plots were managed according to California Certified Organic Farmers (CCOF) requirements, through use of cover crops, composted animal manure, and mechanical cultivation and limited use of other CCOF-approved products. The experimental design was a randomized complete block split plot with four replicates. The four farming systems were the main plots and the crops in rotation were the subplots. The 56 subplots measured 68 by 18 m (0.12 ha) each.

Cover Crop and Tomato Management
This study was conducted in 1997 and 1998 tomato plots. The ORG and LOW tomato plots were preceded in both years by common vetch (Vicia sativa L.) as a winter cover crop. Before the vetch was planted, the soil was disked and rolled and beds were prepared. Two hundred millimeters of water was applied by furrow irrigation to the ORG plots on 2 Oct. 1996, and 170 mm was applied to the LOW plots on 16 Oct. 1996. On 25 Sept. 1997, both the ORG and LOW plots were sprinkle-irrigated with 50 mm of water. The vetch was seeded at 60 kg ha-1 on 8 Oct. 1996 in the organic plots, on 24 Oct. 1996 in the low-input plots and on 1 Oct. 1997 in both systems. The cover crop was mowed and incorporated to a 0.15-m soil depth on 25 Mar. 1997 and on 9 Apr. 1998. In the ORG system, after mowing the cover crop, composted turkey manure (2.8% N, 72% dry weight) at 12 t ha-1 in 1997 and 7 t ha-1 in 1998 was spread and disked. Total N provided by the manure was 242 kg ha-1 in 1997 and 141 kg ha-1 in 1998. The beds were reformed with furrows to a 0.15-m soil depth. Total precipitation from cover crop planting to incorporation was 421 mm in 1996 to 1997 and 642 mm in 1997 to 1998. The fallow CONV4 plots were tilled in the same manner as the ORG and LOW plots. Herbicide (glyphosate at 1.1 kg ha-1) was applied to CONV2 and CONV4 plots in February of both years. Preplant trifluralin (5 kg ha-1) and EPTC (s-ethyl dipropylcarbamothioate) (3 kg ha-1) were applied in 1997. In 1998, preplant napropramide (1.1 kg ha-1) was applied and incorporated into the soil and trifluralin (5 kg ha-1) was applied as a postplant herbicide. In the LOW plots, trifluralin (2 kg ha-1) was incorporated prior to planting in 1997 and about one month after planting in 1998.

Brigade tomato was direct-seeded on 10 Mar. 1997 and on 10 Apr. 1998 in the conventional systems (2- and 4-yr rotations) and transplanted on 16 Apr. 1997 and on 20 Apr. 1998 in the LOW and ORG systems. Transplants were used in these two systems to increase the vetch biomass incorporated as green manure and to lessen competition from weeds relative to the direct-seeded plots. In all farming systems, spacing between rows was 1.52 m, with a density of 2.2 plants m-2. The tomato plants were sprinkle-irrigated when necessary for about 3 wk after planting and furrow-irrigated about every 7 to 14 d throughout the rest of the season. Crop water needs were determined by a farm manager visually monitoring the plants and soil. Overall irrigation frequency and weed management practices are summarized in Table 1 . The total amounts of water applied in the different systems, measured using in-line flow meters (Water Specialties, Porterville, CA) are shown in Table 2 .


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  		Table 1 Irrigation and weed management practices and their frequency in organic, low-input, and conventional (2- and 4-yr rotations) tomato systems in 1997 and 1998

 

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  		Table 2 Water applied in organic, low-input, and conventional (2- and 4-yr rotations) tomato systems

 
In both years, starter fertilizer 8 N–9.6 P–4.8 K–0.5 Zn at 166 kg ha-1 was placed in the plant row at planting in all systems except in the ORG system, where fish powder (12 N–0.10 P–0.83 K) at 5 kg ha-1 and seaweed (3 N–0.10 P–0.12 K) at 11 kg ha-1 were applied as foliar fertilizers. CONV2 and CONV4 systems were sidedressed with 340 kg ha-1 of urea (46%) on 29 Apr. 1997 and on 20 May 1998. The LOW plots received 182 kg ha-1 of urea on 5 May 1997 and 195 kg ha-1 of urea on 18 May 1998.

Soil–Plant Measurements
Three soil samples per plot for bulk density determinations were taken in the surface 0.60 m at 0.15-m increments using a 0.076-m-diam. hydraulic auger in the center of tomato beds in June 1998. To avoid differences in surface roughness, the top 2.5 cm of soil was removed from the sample area before the sample was taken. The undisturbed soil cores were dried in an oven for 24 h at 105°C, then weighed to calculate bulk density (Blake and Hartge, 1986).

In situ soil water holding capacity was measured using a neutron hydroprobe (Campbell Scientific, Martinez, CA) in July 1998. Six polyethylene vinyl chloride tubes were installed in each plot to a depth of 1.15 m. Metal rings (66-cm-diam.) were then inserted into the soil to a depth of 0.15 m around each tube, and 0.23 m3 of water was applied to the soil in each ring. After ponding, the surface was covered with polyethylene sheets to minimize evaporation. Soil water content was determined with a hydroprobe at 0- to 0.15-, 0.15- to 0.30-, 0.30- to 0.45-, and 0.45- to 0.60-m depths at 24, 48, and 72 h after the application of water.

Six undisturbed soil cores were also collected from each plot at 0.15 and 0.45 m using a hand sampler in July 1998 to provide an additional characterization of water holding capacity. The soil cores were placed on a ceramic plate for 24 h in a shallow pan of water to permit uniform wetting by capillarity. The soil cores were then placed on a pressure plate at 33 kPa, and after reaching equilibrium, which was determined when drainage stopped, the cores were removed, weighed, and oven-dried for 24 h at 105°C. The dry soil was weighed to estimate the water holding capacity (Cassel and Nielsen, 1986).

Volumetric soil-water content was monitored about every 2 wk at 0.15, 0.30, 0.60, 0.90, 1.20, 1.50, and 1.80 m using a hydroprobe in access tubes located in the center of the middle row at 15 m from the beginning and the end of each plot from October 1997 to March 1998. Each access tube was installed snugly in the soil so as to avoid water channeling down between the tube and soil. A snug fit was ensured by using an auger that matched the outside diameter of the access tube and by carefully placing a small amount of soil around the tube at the soil surface. During the 1998 tomato season, measurements were taken just prior to and 24 h after each irrigation to carefully monitor pre- and postirrigation soil water profiles. The hydroprobe was calibrated for soils of each cropping system.

During the 1998 tomato season, infiltration rates were determined using the two-point method as described by Walker and Skogerboe (1987). The advance and recession phases of application were monitored at 27 and 54 m from the head of each furrow. Kostiakov infiltration functions were developed for each furrow:

where Z is the cumulative depth of infiltrated water, t is the intake opportunity time, and k and a are empirical constants. This equation has been confirmed to be appropriate for furrow irrigation conditions in California by Hanson et al. (1990). To investigate heterogeneities between furrows within the field, three to six furrows were monitored in each plot.

Infiltration rate determinations were made during four irrigation events throughout the 1998 tomato season for all plots in the ORG, LOW, and CONV4 replications. CONV2 plots were included in the third irrigation. For each furrow, discharge from the gate, cross-sectional area at the upstream end, and elapsed advance and recession times at the 27- and 54-m points were recorded. Cross-sectional furrow area was estimated by measuring the distance from a horizontal reference board that was placed across the furrow between two tomato beds down to the bottom of the furrow at 2.54-cm increments along the horizontal reference board. These parameters were used to calculate the empirical constants in the above equation. Furrow characteristics were the same at the beginning of each irrigation except the third, in which the CONV4 and CONV2 plots had been cultivated prior to irrigation. A total of 162 furrows were characterized. Cumulative infiltration, reported as cubic meters of water infiltrated per meter of furrow length, was plotted against time for each test.

Daily tomato evapotranspiration (ET) for 1997 and 1998 was simulated using a computer model developed for irrigation scheduling by Hsiao et al. (1985). This model requires reference evapotranspiration, the extent of canopy cover as related to the canopy growth coefficient of the crop, and the frequency of soil surface wetting. Canopy growth coefficient is defined as the rate of increase in the percentage of canopy cover relative to the existing percentage of cover. When canopy cover is incomplete and at the early vegetative stage, its growth is essentially exponential and its growth coefficient tends to be a constant. The model uses this coefficient to calculate canopy cover as a function of time. After wetting, evaporation from the exposed soil surface is divided into Stage 1 and Stage 2 phenomena (Philips, 1957).

Tomato canopy growth was monitored with a digital infrared camera (Dycam, Chatsworth, CA) every 2 wk during the growing season. Canopy coverage was calculated from the images using the software program BRIV32 (S. Heinold, personal communication, 1998).

Tomato fruits were harvested on 25 July 1997 and on 25 Aug. 1998 in the CONV2 and CONV4 systems and on 26 July 1997 and on 10 Aug. 1998 in the LOW and ORG plots. Fruits were sorted into red, unripe, rotten, cracked, and insect-damaged and weighed to determine yield in 1998. A subsample of 30 red fruit from each plot from the CONV4, LOW, and ORG systems was used for quality determinations of soluble solids, pH, titrable acidity, viscosity, and color. Each sample was blended and sieved using the procedure developed by the Food Science Department, University of California, Davis (Barret and Osuga, 1997). The serum was used for soluble solids measurements by a digital refractometer and for pH determination. Titratable acidity was determined by titration of the serum with 0.1 N NaOH to 8.1. The viscosity was measured with a Bostwick consistometer by measuring the distance in centimeters that the product flowed when both product and consistometer had equilibrated to 25°C. Color readings were made on deaerated homogenate using a colorimeter (Model 25A-PC2, HunterLab, Reston, VA) to measure spectral reflectance. For each sample, all quality determinations were replicated three times and averaged.

Statistical Analysis
Analysis of variance of the treatment effects on measured traits was performed using the GLM procedure of SAS (SAS Inst., 1985). Yield and quality parameters were analyzed by one-way analyses of variance within each year. Bulk density data were analyzed as a split-plot design, with farming systems as the main plots and soil depths as subplots. The effects of farming systems, soil depth, and time on water content at field capacity and the influences of farming systems on soil water storage were analyzed by a randomized complete block design with repeated measures (24, 48, and 72 h after water application and days of year, respectively) at the subplot level. In these analyses, all subplot effects (block effects and block interactions) were treated as random effects. The analysis relative to the soil water storage was done separately for the first period (26 Sept.–9 May 1998) and the second period (9 May–7 Aug. 1998) because the CONV2 system was only monitored during the second period. Because of missing values, analyses were run as mixed models. When F-tests showed statistical significance, the Duncan's Multiple Range Test was used to separate means for particular comparisons.


   Results and discussion
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
Conclusions
 REFERENCES
 
Soil Physical Properties
Soil bulk density was not significantly different among the farming systems, and no interaction was found between farming system and depth. Bulk density was affected by depth (P < 0.001). Average bulk density of the three core samples in each plot increased with depth from 1.36 ± 0.03 Mg m-3 for the 0.025- to 0.15-m depth interval to 1.43 ± 0.008 Mg m-3 for the 0.15- to 0.30-m interval and decreased thereafter, from 1.35 ±0.02 Mg m-3 (0.30- to 0.45-m interval) to 1.32 ± 0.03 Mg m-3 (0.45- to 0.60-m interval). These results agree with previous determinations made at the site earlier in the rotation cycle (Clark et al., 1998). Many studies have reported that organic matter decreases the bulk density of the soil (Zhang et al., 1997). In our study, however, bulk density measures were not changed despite very different amounts of organic matter having been incorporated into the soils over the previous 10 yr. Because of cover crop growth and mechanical weed control, LOW and ORG systems were tilled more than CONV2 and CONV4 systems, so extra tillage may have negated the effect of organic matter incorporation from cover crops. Frequent tillage during all the rotations may account for the similarities in bulk density that we observed in this study (Martens and Frankenberger, 1992; Clark et al., 1998).

In situ water holding capacity measured with the hydroprobe was significantly affected by farming system (P < 0.05), soil depth (P < 0.001), and time (P < 0.001) (Table 3) . There was no interaction among farming system, soil depth, and time, so each was considered independently. The ORG system had significantly higher soil water content at field capacity than the CONV4 system (27.3 vs. 25%) when averaged over all depths and measurement times, while intermediate values were found in the LOW (25.6%) and CONV2 (25.9%) systems (Fig. 2) .


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  		Table 3 Analysis of variance for farming system, soil depth, and measurement time effects on soil water content

 


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  		Fig. 2 Mean gravimetric soil water holding capacity as affected by farming system (average of four soil depths and three measurement times). CONV 2, conventionally managed 2-yr rotation; CONV 4, conventionally managed 4-yr rotation; LOW, low-input 4-yr rotation; ORG, organically managed 4-yr rotation

 
Soil water content, averaged across all farming systems and measurement times, was highest at the 0- to 0.15-m (27.3%) and 0.15- to 0.30-m (27.5%) soil depth intervals and significantly decreased at the 0.30- to 0.45-m (25.7%) and 0.45- to 0.60-m (23.4%) depths (Fig. 3) . Lower clay and silt content in deep soil layers (Cavero et al., 1998) may explain the lower soil water content at field capacity at the 0.30- to 0.45-m and 0.45- to 0.60-m layers than at the soil surface. The water content significantly decreased with the increase of time after water application when averaged over all farming systems and soil depths (data not shown).



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  		Fig. 3 Mean gravimetric soil water holding capacity as affected by soil depth (average of four farming systems and three measurement times)

 
Water holding capacity determinations of undisturbed soil cores did not show differences among systems (data not shown). Larger soil volumes and less disturbance in the in situ determinations of water holding capacity than in cores used for the laboratory determination may explain the disparity of these results.

Werner (1997) reported increased water holding capacity as a result of organic management in a California orchard. Higher water retention may be directly related to the increased soil organic matter content of both the LOW and the ORG systems, previously documented for the SAFS Project soils by Drinkwater et al. (1995) and Clark et al. (1998). Additionally, differences in the constituents of the humic acid fractions of the soils compared here may account for these differences in water holding capacity (Devevre and Horwath, 1999). Jamison and Kroth (1959) found this type of effect to be limited to soil of medium-low clay content (13–20%). Given the low clay content of the soils in the SAFS Project (Cavero et al., 1998), even the modest increase in soil organic matter content may help to explain the observed differences in water holding capacity.

Cumulative water infiltration in 5 h was highest for all irrigation events in the ORG and LOW systems and lowest in the CONV2 and CONV4 systems (Fig. 4) . Infiltration in the conventional systems (2- and 4-yr rotations) was lower in the late season even though these soils were drier than those in LOW and ORG systems. L.J. Schwankl (unpublished data, 1993) and Mitchell et al. (1997) also observed higher infiltration rates in the LOW and ORG systems in the SAFS Project. After 3 h, cumulative infiltration in the ORG and LOW plots was more than twice that of the conventional system except in the second irrigation event. Infiltration rates after 3 h in the ORG system, averaging 0.065 m3 m-1, were sustained throughout the season. In the CONV4 system, the highest infiltration rates were observed in the two mid-season irrigations (0.037 and 0.035 m3 m-1 vs. 0.019 and 0.023 m3 m-1) as a result of prior cultivation. Infiltration rates in the LOW system were lower in the second irrigation event than in the first, third, and fourth irrigations (0.037 m3 m-1 vs. 0.062, 0.072, and 0.064 m3 m-1, respectively).



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  		Fig. 4 Cumulative water infiltration for conventional, low-input, and organic systems by irrigation events on (A) 17 June, (B) 29 June, (C) 17 July, and (D) 27 July 1998. Error bars represent standard error. CONV 2, conventionally managed 2-yr rotation; CONV 4, conventionally managed 4-yr rotation; LOW, low-input 4-yr rotation; ORG, organically managed 4-yr rotation

 
Improvement of water infiltration resulting from applications of organic materials have been reported in several studies (Pillsbury and Huberty, 1941a and 1941b; Johnson, 1957; Williams and Doneen, 1960; Williams, 1966; Roberson et al., 1991; Gulick et al., 1994). Organic residues improve macroaggregate stability, macroporosity, and saturated hydraulic conductivity and reduce the strength of surface crusts (Folorunso et al., 1992; Mitchell et al., 1999). Martens and Frankenberger (1992) reported that bulk density and aggregate stability are the major factors affecting water infiltration rates. In our study, the lack of observed differences in bulk density among the systems may suggest that surface sealing was the major limiting factor in determining water infiltration. Visual observations of increased surface macroporosity in the LOW and ORG systems may also explain the differences in water infiltration rates seen in our study. Surface cover crop residues and greater aggregate stability found under cover crops (McVay et al., 1989) may also have reduced crusting of the surface soil, thereby allowing greater infiltration. Folorunso et al. (1992) similarly found, in field studies similarly conducted in Davis, CA, that cover crops reduced soil surface strength and that this increased water permeability. The similarity between infiltration rates in the LOW and ORG systems may indicate that cover crop residues are more effective than animal manure for enhancing water infiltration.

The more rapid infiltration in the LOW and ORG systems resulted in increased total irrigation applications. The cropping systems evaluated in this study were furrow-irrigated, as is typical for row crops in the Sacramento Valley. In these systems, irrigation water is turned off when it reaches the end of the furrow. If infiltration is higher, it takes longer for the water to reach the end of the furrow, which results in more water applied (Table 2) and conceivably higher production costs.

Soil water storage from fall 1997 to tomato planting in spring 1998 (first period) was not significantly affected by farming system, and no significant interaction between farming system and time was found. Unger and Vigil (1998) reported that the water use of the cover crops could reduce soil water available for the next crop. In our study, however, no significant differences in soil water storage were found among the systems. This could be due to the extended wet conditions experienced in the spring of 1998.

During the tomato growing season, both farming system and time had significant effects on water storage (P < 0.05 and P < 0.0001, respectively). Overall, the ORG system had a higher mean water content (422 mm) than CONV4 system (365 mm). Water storage in LOW (403 mm) and CONV2 systems (382 mm) was intermediate and not significantly different from either the ORG or the CONV4 systems. There was a significant (P < 0.0001) interaction between farming system and time (Fig. 5) . After the first week of June, soil water content continually decreased due to the high evapotranspiration of the tomato crop. The decrease in water content was greater in the CONV2 and CONV4 systems than in the LOW and ORG systems as a result of the different water infiltration rates and water applied, so that significant differences among the systems were found on 9 May 1998 (day of year 129) and from 25 May 1998 (day of year 145) until harvest time. More water was applied per irrigation to the LOW and ORG systems because infiltration rates were higher, but the CONV2 and CONV4 systems needed to be irrigated more frequently because their infiltration rates were lower.



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  		Fig. 5 Soil water storage and precipitation in the four tomato systems in 1998. CONV 2, conventionally managed 2-yr rotation; CONV 4, conventionally managed 4-yr rotation; LOW, low-input 4-yr rotation; ORG, organically managed 4-yr rotation

 
Tomato Evapotranspiration, Yield, and Fruit Quality
Estimates of ET for 1997 and 1998 are summarized in Table 4 . No significant differences were found in canopy growth coefficients (0.077) or in maximum canopy cover developed (60%) among the systems. Simulated ET for the CONV2 and CONV4 systems was essentially the same, because planting and harvest dates and irrigation management were similar for these treatments. Evapotranspiration was higher in these systems than in LOW and ORG systems mainly as a result of 10 extra days of evaporation associated with the fact they were seeded 10 d ahead of the transplanting time for LOW and ORG systems. In both years, evaporation (E) as a percent of ET was highest in the conventional systems as a result of direct seeding. Crop establishment took longer with direct seeding than with transplanting, thereby increasing the soil evaporation in the early part of the crop's growth. In 1998, because of the frequent rainfall in the early part of the season, the difference in the percentage ET as E between CONV2 and CONV4 systems and LOW and ORG systems was higher than in 1997. The consumptive water use efficiency (marketable tomato plants per water consumed in ET) was higher for the LOW-ORG systems than for the CONV2-CONV4 systems due to their use of transplants, and the resulting shorter growing season and reduced soil evaporation. Consumptive water use efficiency (WUE) for the LOW-ORG systems was 14.7 and 15.2 kg m-3, respectively, for 1997 and 1998. For the CONV2-CONV4 systems, consumptive WUE was 12.4 and 12.6 kg m-3, respectively, for 1997 and 1998. This was remarkably similar to the consumptive WUE of 12.3 kg m-3 reported by Cuenca (1978) for tomato also grown in Davis, suggesting that the simulated values of ET were reasonably realistic.


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  		Table 4 Cumulative values (mm) of simulated transpiration (T), soil evaporation (E), evapotranspiration (ET), soil evaporation as a percentage of ET, and seasonal water use efficiency (WUE) for tomato in 1997 and 1998 in conventional (2- and 4-yr rotations), low-input, and organic systems

 
Comparisons of applied water (Table 2) with simulated ET in 1997 indicate that the CONV2 system was moderately overirrigated, and the LOW and ORG systems were severely overirrigated. In contrast, in 1998 irrigation quantity was well matched with ET for the LOW and ORG systems, while the CONV2 and CONV4 plots were underirrigated. This underirrigation can be clearly seen in the soil water data (Fig. 5). The extent of underirrigation was not fully accounted for by the measured soil water storage (Fig. 5), probably because there was significant deep percolation of soil water during the rainy part of the season for the LOW and ORG systems. The differences in the amount of water applied regardless of ET reflect the difference in infiltration rates of the systems.

Yields were not significantly different in CONV2, CONV4, LOW, and ORG systems in either year (Table 5) . During both years, there were fewer cracked fruits in CONV2 and CONV4 systems than in LOW and ORG systems. These results may be related to the greater fluctuations in the soil water content in LOW and ORG systems during the ripening stage of the growing season (Fig. 5), as reported by Grierson and Kader (1986). These fluctuations in soil water content may have resulted from higher lateral water infiltration and water applied in the LOW and ORG systems as measured from neutron hydroprobe access tubes, which were in the middle of the tomato bed. A reduction in watering during ripening stage (Peet and Willits, 1995) should be considered in LOW and ORG systems to reduce fruit cracking.


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  		Table 5 Marketable and unmarketable tomato yield in the four farming systems in 1997 and 1998

 
Relative amounts of green, rotten, and insect-damaged fruits did not differ consistently among the cropping systems in either year. Fruit soluble solids, titrable acidity, and color determinations differed significantly among CONV4, LOW, and ORG systems (Table 6) . The highest levels of soluble solids and titrable acidity and the best fruit color were achieved in CONV4. The ORG fruit had lowest overall quality, while intermediate values were found in the LOW system. No significant differences were found in pH and viscosity among the systems. Soluble solids content was inversely correlated with the amount of water applied . These data suggest that low water tensions during the ripening stage were a major factor improving soluble solids, titratable acidity, and fruit color in the CONV4 system. In the LOW and ORG systems, higher infiltration increased water availability, resulting in lower overall fruit quality. Growers typically attempt to develop water management practices that maintain yield but impose a moderate, controlled level of stress on their crops to improve water quality. Controlled periods of soil water deficit have been imposed by increasing the interval between irrigations (Aljibury and May, 1970) or by withholding irrigations before harvest (Martin et al., 1996; Rudich et al., 1977; Sanders et al., 1989; Mitchell et al., 1991) or reducing the water applied in the ripening stage (Colla et al., 1999). These methods have improved fruit quality by increasing fruit soluble solids concentration, titratable acidity, and viscosity and improving color. These data confirm the importance of water management as a determinant of fruit quality.


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  		Table 6 Quality parameters of tomato fruits for conventional 4-yr rotation, low-input, and organic farming systems in 1998

 

   Conclusions
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusions
REFERENCES
 
This field study demonstrates that alternative farming systems that include cover crops can increase water holding capacity and soil permeability to irrigation water in California's Sacramento Valley. Enhanced water infiltration in the organic and low-input systems of this study was not related to a lower bulk density of the soils in these systems, but appeared to be related to greater surface macroporosity in the soil. The alternative production systems thus required more water per irrigation to achieve uniform water application, resulting in higher soil water content in the root zone throughout much of the growing season. Differences in soil water storage among the systems did not affect tomato productivity in either year, but had major effects on fruit quality. The conventional system produced superior fruit quality to both the organic and the low-input systems. Because of the higher soil water storage of the organic and low-input systems, irrigation management and the timing of the last irrigation in these systems need to be optimized to sufficiently stress the crop and thereby ensure a higher quality fruit.California Certified Organic Farmers 1995; SAS Institute 1985

Received for publication May 10, 1999.
   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusions
 REFERENCES
 





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