Plastic mulch is commonly used to produce many vegetable crops because of its potential to decrease days to harvest, control weeds, and improve soil moisture conservation. However, use of plastic mulch is relatively new for sweet corn (Zea mays L.) in North America. We compared five plastic soil-biodegradable mulches [BDMs; Bio360, Organix AG, Clear Organix AG, Naturecycle, and Experimental polylactic acid/polyhydroxyalkanoates (Metabolix, Inc., Cambridge, MA)] and a paper mulch (WeedGuardPlus) against standard black polyethylene (PE; nonbiodegradable) mulch and bare ground cultivation for growth, yield, and quality of sweet corn cultivar Xtra Tender 2171. This field experiment was carried out in Mount Vernon, WA, which has a Mediterranean-type climate with an average air temperature of 16.1 °C during the 2017 and 2018 growing seasons. The experiment was drip irrigated; and in both years, preemergence herbicides were applied to the entire experimental area 1 to 2 days after seeding, and post-emergence herbicides were applied to alleys. While most mulches remained intact until the end of the growing season, Clear Organix AG started to split shortly after laying, resulting in significant weed pressure by midseason in both 2017 and 2018. Plant height toward the end of the season was lowest for plants grown on bare ground, intermediate for Clear Organix AG and WeedGuardPlus, and highest for the black plastic BDM and PE mulch treatments both years, except for Bio360 in 2018 where plant height was intermediate. Days to 50% tasseling and 50% silking were delayed 9 and 13 days, respectively, for bare ground and WeedGuardPlus compared with all other treatments in both years. Marketable ear yield was highest with the black plastic BDMs and PE mulch and lowest with bare ground, WeedGuardPlus, and Clear Organix AG treatments in both years. Total soluble solid content of kernels, and length and diameter of ears grown on the plastic BDM and PE mulch treatments were equal to or greater than, but never lower than, bare ground and WeedGuardPlus. These results indicate that growth, yield, and quality of sweet corn grown with black plastic BDMs are comparable to PE mulch, making black plastic BDMs an effective alternative to black PE mulch for sweet corn production in a Mediterranean-type climate.
Sweet corn (Zea mays L.) is a warm-season crop adapted to temperate climates (Aguyoh et al., 1999). Optimum growth of sweet corn occurs when soil temperatures range from 20 to 30 °C, and soil moisture is at 75% to 100% field capacity (Kebede et al., 2014; Schneider and Gupta, 1985; Stall et al., 2019). Sweet corn production in northwest Washington is limited due to cool spring and early summer air (13 °C average) and soil temperature (16 °C average at 5 cm depth). Emergence is slowed and subsequently reduced due to diseases that are prevalent under cold soil conditions (Jackson, 2008). In this region, the summer is also relatively cool (16 and 18 °C average air and soil temperature, respectively) and plant growth is often slow, resulting in a late-season crop harvest that has a lower price in the market.
Polyethylene (PE) mulch is widely used for numerous crops to increase soil temperature, control weeds, reduce evaporation, and conserve water in the soil (Liu et al., 2010; Wang et al., 2009). These benefits can lead to earlier harvest and increased crop yield (Liu et al., 2009; Steinmetz et al., 2016). Zhang et al. (2017) evaluated the effects of PE mulch (8 μm, Gansu Tianbao Plastic Plant, China) on soil temperature, soil moisture, water use efficiency (WUE; yield per unit water), yield, and revenue of grain corn. Soil temperatures (5 to 25 cm deep) and soil moisture (0 to 200 cm deep) were greater in field plots with PE mulch than in bare ground, leading to increased WUE and yield in the PE mulch treatment. Other studies have shown that clear PE mulch can raise soil temperature, accelerate germination, promote growth, decrease the days to harvest, and increase the yield of marketable sweet corn ears (Aguyoh et al., 1999; Dinkel, 1966; Hopen, 1965; Kwabiah, 2004). Qin et al. (2015) conducted a meta-analysis of the effects of PE and straw mulching on corn (type not specified) and wheat (Triticum aestivum L.) using 1310 yield observations from 74 studies conducted in 19 countries. Mulching significantly increased yields, WUE, and nitrogen use efficiency (NUE; yield per unit nitrogen) by up to 60%, compared with no mulch. PE mulch increased WUE and NUE to a greater extent than straw mulch. These effects were larger for corn than wheat, and effects tended to decrease with increasing water inputs. Plastic (primarily PE) mulch is used to optimize crop water usage in regions such as northeast China (Xu et al., 2015; Zhang et al., 2017), while its potential to enhance soil warming is more important in other regions such as Canada (Waterer, 2010; Zandstra et al., 2007).
In recent years, farmers in northwest Washington have been using black PE mulch for sweet corn production, with the objective of achieving earlier harvests with price premiums. While clear PE mulch is extensively used for sweet corn production in northeast China, France, and Germany, its use is not common in the United States (Xu et al., 2015; Zhang et al., 2017). Currently for sweet corn production, nonbiodegradable PE mulch is most widely used. However, PE mulch needs to be removed and disposed at the end of the season, which can cost up to $584 per ha (Galinato and Walters, 2012; Galinato et al., 2012). As PE mulch is contaminated with soil and debris, only about 10% is recycled (Kasirajan and Ngouajio, 2012). Soil-biodegradable mulches (BDMs) are made from biodegradable polymers such as poly(butylene adipate-co-terephthalate), polylactic acid, polyhydroxyalkanoate, and thermoplastic starch. Unlike PE mulch, BDM can be tilled into the soil after use, thereby eliminating the need to dispose of mulch off-farm. Plastic BDM has been reported to perform similarly to PE mulch regarding soil temperature and crop yield for eggplant (Solanum melongena L.), muskmelon (Cucumis melo L. var. reticulatus), pepper (Capsicum annuum L.), pumpkin (Cucurbita pepo L.), sweet corn, tomato (Solanum lycopersicum L.), and zucchini (Cucurbita pepo L.) (Anzalone et al., 2010; Cowan et al., 2014; Ghimire et al., 2018; Martin-Closas et al., 2008; Waterer, 2010). For sweet corn, Waterer (2010) compared black, clear, and wavelength selective BDMs (15 μm; Biotelo, DuBois Agrinovation, St. Remi, QC, Canada) and PE mulches over three growing seasons in Saskatchewan, Canada. No significant differences in soil temperature, crop growth, or yield responses were found for the BDMs as compared with the same color of PE mulch. Zandstra et al. (2007) compared five types of clear degradable mulches (PE-based mulches containing an additive to initiate degradation) to clear PE mulch and bare ground in sweet corn production at two locations of Ontario, Canada: Ridgetown, with 14 °C average air temperature in May (sweet corn was seeded on 29 May); and Simcoe, with 25 °C average air temperature in June (sweet corn was seeded on 13–20 June). Plant height, weight per ear, and ear length did not differ due to mulch treatment at either location. However, marketable ear yield was higher for all mulch treatments compared with bare ground at both locations, and ear diameter was higher for all plastic mulches compared with bare ground at the Ridgetown location. This showed that BDMs can perform as well as PE mulch.
There are a limited number of studies that have tested BDMs for sweet corn production, and no studies have been conducted in the northwest United States. Waterer (2010) is the only study that evaluated both clear and black BDMs for sweet corn production, and there are no studies that also included paper BDMs, which are biobased and completely soil-biodegradable. As growing sweet corn on PE mulch is gaining acceptance in the United States, the objective of the current study was to compare sweet corn production with PE mulch and five plastic BDMs (including one clear mulch) and one paper BDM in Mount Vernon, WA.
Materials and Methods
This experiment was carried out at the Washington State University (WSU) Northwestern Washington Research and Extension Center at Mount Vernon (lat. 48°43′24″ N, long. 122°39′09″ W, elevation 6 m) in 2017 and 2018. The experimental site has a Mediterranean climate and rainfall in the spring delays soil preparation, while rainfall in the fall prevents heavy equipment from entering the fields. In the 2017 crop growing season (mid-May through Sept.), the average daily temperature was 16.3 °C, average relative humidity (RH) was 75%, and total rainfall was 56 mm (AgWeatherNet, 2018). In the 2018 crop growing season, the average daily temperature was 15.9 °C, average RH was 75%, and total rainfall was 75 mm. The experimental site consisted of a Skagit silt loam soil characterized as a fine-silty, mesic Fluvaquentic Endoaquepts with a pH of 6.5 and an organic matter content of 2.7% by weight. The field site was planted with a winter wheat cover crop each fall from 2015 through 2018. The cover crop was incorporated in the spring, and pie pumpkins were grown during the summers of 2015 and 2016.
Experimental design and planting.
The experiment used a randomized complete block design with four replicates of eight mulch treatments (Table 1). Each replicate plot had five rows, each 9 m long, and rows were spaced 2.4 m center-to-center. Center-to-center bed spacing was greater than farmers’ practice for sweet corn in this area because plot layout and treatment assignments were maintained following a 2-year BDM experiment with pie pumpkin (Ghimire et al., 2018). To avoid cross-treatment mulch and soil contamination, the same BDMs were incorporated into the soil after crop harvest every fall. One exception to the consistency in mulch treatments was that thickness of Clear Organix AG, which was raised by 5 µm from 2017 to 2018 because it deteriorated too quickly in 2017 and was thinner than other plastic BDMs (Table 1). All mulches were 1.2 m wide, and were black except Clear Organix AG and brown WeedGuardPlus. Raised beds 15 to 20 cm high and 0.8 m wide were formed in all plots by machine (Model 2600 Bed Shaper; Rain-Flo Irrigation, East Pearl, PA), and mulch was laid simultaneously in respective plots. Using a wheel dibble, planting holes were punched at 15 cm spacing in two off-set rows spaced 30 cm apart. One seed of sweet corn ‘Xtra Tender 2171’ (Johnny’s Selected Seed, Winslow, ME) was seeded in each planting hole on 23 May 2017 and 22 May 2018. In each plot, there were 120 plants per double-row bed and 600 plants total.
Treatments tested at Mount Vernon, WA in 2017 and 2018, and mulch information from manufacturers.
Fertilizer and irrigation.
Each year, following recommendations for sweet corn production in this region, fertilizer (Wilbur-Ellis; 22N–15P–11K) was broadcasted and incorporated before planting at the rate of 171 kg·ha−1 N, 51 kg·ha−1 P, 71 kg·ha−1 K, 12 kg·ha−1 Mg, 23 kg·ha−1 S, 3 kg·ha−1 Zn, and 53 kg·ha−1 Cl. Drip irrigation tape (T-Tape, Model 508-08-340, 0.20 mm, 20-cm emitter spacing, 4.23 L per min per 100-m flowrate; Rivulis, San Diego, CA) was laid on the center of the bed under the mulch as beds were formed and mulch was laid. Moisture sensors (5TM; Decagon Devices, Inc., Pullman, WA) were installed at 10 and 20 cm depths in the third row of each plot in the first replicate. Data were recorded with data loggers (EM50G; Decagon Devices) and used to schedule irrigation. In 2017, before tasseling started, irrigation was triggered when the water content at 20 cm depth fell below 0.16 cm3·cm−3 in the PE mulch plot, but before it fell below 0.15 cm3·cm−3 at either depth in the bare ground plot. This threshold is above the permanent wilting point for silt loam soil, which is 0.10 cm3·cm−3 [Natural Resource Conservation Service (NRCS), 2008]. Water demand increased as the crop developed; so once tasseling began, irrigation was triggered at 0.18 cm3·cm−3 at 20 cm depth in the PE mulch plot, but before it fell below 0.16 cm3·cm−3 at either depth in the bare ground plot. In 2018, irrigation was always triggered before soil moisture content fell below 0.18 cm3·cm−3 at either depth in any plot. The objective was to reach field capacity (0.35 cm3·cm−3 for the silt loam soil type) in the PE plot by the end of each irrigation event.
Air temperature, solar radiation, RH, wind speed, and rainfall data during the cropping season were collected from the WSU AgWeatherNet station located ≈140 m north of the experimental site. Soil temperature and moisture data were recorded at 10 cm depth at 60 s intervals and averaged hourly using data loggers, with sensors installed in the third row of each plot in the first replicate. Daily soil growing degree days (GDDsoil) were calculated for each treatment based on the soil temperature recorded at 10 cm depth using the following formula:
Base temperature was set at 10 °C, as this is the threshold for corn growth (Stall et al., 2019). Accumulated GDDsoil was calculated for each treatment by adding GDDsoil for the entire growing season.
Pre-emergence herbicides (Outlook 1.1 L·ha−1 and Callisto 0.37 L·ha−1) were applied to the entire experimental area on 24 May in 2017 and 2018, 1 and 2 d after seeding, respectively. Post-emergence herbicides (Atrazine 2.3 L·ha−1, Callisto 0.18 L·ha−1, R-11 1.2 L·ha−1) were applied to the entire experimental area on 22 June 2017 and 2018 to control weeds in the bare ground plots, on the bed sides, and in the alleyways.
Mulch deterioration ratings and weed measurement.
The percent soil exposure (PSE) was visually assessed in the center 1 m of the third row of each plot two times per month for the entire growing season. Zero PSE denoted mulch that was completely intact, and 100 PSE denoted fully deteriorated mulch. Ratings were in 1% increments up to 20% PSE, and in 5% increments thereafter (Ghimire et al., 2018). Dry shoot biomass of weeds was recorded three times from three different 1-m–long sections in either the second or fourth row in each plot at: 1) 3 weeks after seeding, 2) midseason, and 3) 1 week before first harvest. Weeds were clipped at the soil surface then dried at 60 °C for 48 h. Total dry weight per sample was recorded, and the measurement was converted to weight per square meter for standard reporting.
Plant height from the soil surface to the top vegetative point, and Soil Plant Analysis Development (SPAD) readings (SPAD-502 Chlorophyll Meter; Minolta, Osaka, Japan) were recorded from five randomly selected representative plants in the third row of each plot two times per month. SPAD readings were taken on the topmost fully expanded leaf before silking or the first ear leaf after silking. Days to 50% emergence, 50% tasseling, and 50% silking were also recorded. Dry root and shoot biomass were measured by randomly selecting two representative plants in the second row of each plot 2 weeks before the first harvest. The ratio of dry root to shoot weight was calculated.
Ear yield and quality.
Ears were harvested in each plot up to three times as they reached fresh market maturity. In 2017, ears from all BDM and PE mulch plots were harvested on 28 Aug. and 13 Sept., and ears from bare ground and paper mulch plots were harvested on 13 and 26 Sept. In 2018, ears were harvested on 27 Aug., 12 Sept., and 26 Sept. from all plots except WeedGuardPlus, where no ears were ready for harvest on 27 Aug. The number and total weight of marketable ears were recorded at each harvest. Ear quality was measured for five randomly selected ears from each plot at the second harvest, and included length and diameter, tipfill, kernel alignment, and total soluble solids (TSS). Length of unfilled tip was measured in increments of 0.5 cm. Kernel alignment was rated where 1 denoted straight, 2 denoted somewhat skewed, and 3 denoted very skewed alignment. To assess TSS, kernels were sliced off from one side of each ear into a plastic bag, squeezed, then two to three drops of the resulting juice were placed onto a refractometer (Palm Abbe PA201; Misco, Cleveland, OH), and the TSS value was recorded.
All data were subjected to analysis of variance using generalized linear mixed model (GLIMMIX) procedure in SAS (Statistical Analysis System Version 9.2 for Windows; SAS Institute, Cary, NC). Data were analyzed as a randomized complete block design with repeated measures for PSE, plant height, and SPAD data. The Slice statement was used to subdivide means by observation dates to simplify means comparisons. The assumptions of normality and homogeneity of variances were assessed using the Shapiro-Wilk test (W > 0.80) and Levene’s test (α = 0.05), respectively. The MMAOV macro (Saxton, 2010) in SAS was used to build all PROC GLIMMIX procedures. Fisher’s least significant difference test (α = 0.05) was used to compare treatment means. No transformation satisfied the assumptions of normality and homogeneity of variance for PSE, weed data, and shoot and root weights; therefore, a nonparametric transformation was applied for these data using PROC RANK in SAS, but the reported means are based on the raw data.
All main effects (mulch treatment, year, and sampling time) and their interactions were assessed (Table 2). Treatment by year interaction was significant for most parameters; so, to simplify the presentation of results, all data are presented by year. However, insignificant treatment by year interactions are discussed where applicable.
Results (P values) from analysis of variance of the main factors “Mulch treatment” (Trt), “Year,” and “sampling time” (Time), and their interactions, for the parameters measured for sweet corn in 2017 and 2018.
During the sweet corn growing season (mid-May through Sept.) in 2017 and 2018, the total solar radiation was 2586 and 2376 MJ·m−2 and total rainfall was 56 and 75 mm, respectively (Table 3). During this time period, the overall average soil temperature at 10 cm depth was 19 °C both years. The average soil temperature at 10 cm depth for all black plastic BDMs and PE mulch was 19.3 °C in 2017 and 19.5 °C in 2018; for WeedGuardPlus it was 18.0 °C in 2017 and 18.1 °C in 2018; and for bare ground it was 18.7 °C in 2017 and 18.1 °C in 2018 (Table 3). Accumulated GDDsoil for all treatments was similar in both years, and it was 1162 in 2017 and 1158 in 2018 (Table 3). Accumulated GDDsoil for bare ground and WeedGuardPlus was 1067 on average for the two years, which was 126 GDDsoil lower than all plastic mulch treatments both years (1194 GDDsoil on average). The total amount of irrigation applied was 163 mm in 2017 and 320 mm in 2018. This difference was attributed to irrigation rate being based on moisture content under PE mulch alone in 2017 but on average moisture content across all treatments in 2018. In 2017, average soil moisture for all treatments was 0.20 cm3·cm−3 at 10 cm depth, and there was no consistent difference among the various mulch treatments (Table 3). In 2018, overall average soil moisture at 10 cm depth was 0.19 cm3·cm−3; and it was 0.20 cm3·cm−3 for plastic BDMs and PE mulch treatments; 0.14 cm3·cm−3 for WeedGuardPlus mulch; and 0.18 cm3·cm−3 for bare ground.
Environmental and soil temperature data during the sweet corn growing season at Mount Vernon, WA in 2017 and 2018.
Percent soil exposure.
PSE differed among the mulch treatments and increased over time as the season progressed for both years (P < 0.05 for all). PSE was highest for Clear Organix AG mulch and reached 51% and 39% by the end of the season in 2017 and 2018, respectively. Thickness of Clear Organix AG mulch in 2018 was increased by 5 µm from 2017 because of how quickly it deteriorated in 2017. PSE was 0% to 5% through to the end of the growing season for all other mulch treatments both years (Fig. 1).
Predominant weeds were pigweed (Amaranthus sp. L.), chickweed (Stellaria media L.), and common lambsquarter (Chenopodium sp. L.). Weed dry biomass was negligible (<1 g·m−2) for all mulch treatments at all three sampling times in both years, except for Clear Organix AG mulch at midseason (12 g·m−2) in 2017, and midseason (39 g·m−2) and late-season (134 g·m−2) in 2018.
The treatment differences for days to 50% emergence and 50% tasseling were consistent over both years (P > 0.05). Days to 50% emergence differed among the mulch treatments (P < 0.0001) for both years. It was lowest for Clear Organix AG, Organix AG, and Bio360 treatments (8 d for all), and greatest for bare ground (12 d) in 2017 (Table 4). In 2018, days to 50% emergence was lowest for Clear Organix AG and PE treatments (average 7.8 d), and greatest for bare ground (13.8 d). Days to 50% tasseling also differed due to treatment (P < 0.0001) in both years. It was lowest for Clear Organix AG, Organix AG, and PE treatments (average 58 d for all), and highest for bare ground (68 d) and WeedGuardPlus mulch (67 d) in 2017 (Table 4). In 2018, days to 50% tasseling was lowest for Clear Organix AG and PE treatments (average 52.7 d), and again highest for bare ground (66 d) and WeedGuardPlus mulch (63.5 d). In both years, days to 50% silking was lowest for all plastic BDMs and PE mulch treatments (average 74.8 d in 2017 and 71.3 d in 2018), and highest for WeedGuardPlus and bare ground treatments (average 87.7 d in 2017 and 87.3 d in 2018).
Days to sweet corn emergence, tasseling, and silking at Mount Vernon, WA in 2017 and 2018.
Plant height differed among the mulch treatments at each sampling time (P < 0.0001 for all) in both years. In 2017, plant height at 90 d after seeding was highest for Bio360, Organix AG, and PE treatments (average 141 cm), intermediate for Exp. PLA/PHA and Naturecycle treatments (average 135 cm), and lowest for bare ground, WeedGuardPlus and Clear Organix AG treatments (average 113 cm) (Fig. 2). In 2018, plant height at 92 d after seeding was highest for PE, Exp. PLA/PHA, Naturecycle, and Organix AG treatments (average 172 cm), intermediate for Bio360 and Clear Organix AG treatments (average 166 cm), followed by WeedGuardPlus mulch (160 cm), and lowest for bare ground (137 cm).
SPAD readings also differed among the mulch treatments at each sampling time in both years (P < 0.01 all). In 2017, SPAD readings were greatest throughout the season for plants grown with Exp. PLA/PHA, Naturecycle and PE treatments, and lowest for plants grown with Clear Organix AG and bare ground treatments. However, plants grown on Clear Organix AG mulch had the highest SPAD readings early in the season before mulch deterioration started (Fig. 3). This trend was the same in the 2018 growing season.
The treatment effects on dry root and shoot biomass and root-to-shoot ratio were consistent over both years (P > 0.05). Dry root biomass differed among the mulch treatments (P < 0.01) in both years. In 2017, dry root biomass was greatest for plants grown with PE mulch (23.4 g/plant) and lowest for plants grown on bare ground (3.4 g/plant). In 2018, dry root biomass was greatest for plants grown with PE and Naturecycle treatments (average 199 g/plant) and was lowest again for plants grown with bare ground (85 g/plant). Dry shoot biomass also differed among the treatments (P < 0.01) in both years. In 2017, dry shoot biomass was greatest for plants grown with Exp. PLA/PHA, PE, Organix AG, Naturecycle, and Bio360 treatments (average 114.4 g/plant), and lowest for plants grown with Clear Organix AG, bare ground, and WeedGuardPlus treatments (average 55 g/plant) (Fig. 4). In 2018, dry shoot biomass was highest for all black plastic BDM and PE mulch treatments (average 320 g/plant), intermediate for Clear Organix AG and WeedGuardPlus treatments (average 259 g/plant), and lowest for bare ground (196 g/plant). The root-to-shoot ratio did not differ among the mulch treatments (P = 0.15) in 2017, but differed (P = 0.007) in 2018 when it was greatest for Naturecyle, PE, Bio360, and WeedGuardPlus treatments (average 0.57), and lowest for Organix AG mulch (0.36).
Ear yield and quality.
Number of marketable sweet corn ears per acre differed among the mulch treatments (P = 0.0002) in 2017 but not in 2018 (P = 0.10). In 2017, marketable ear number was greater for all the plastic BDM and PE treatments (average 35,616 per ha) than for bare ground and WeedGuardPlus treatments (average 19,426), whereas average number of marketable ears in all treatments in 2018 was 48,875 (Fig. 5).
The treatment differences for weight of ears, ear width, and kernel alignment were consistent over both years (P > 0.05). Weight of ears differed due to mulch treatments (P < 0.01) in both years. In 2017, weight of marketable ears was greatest for PE mulch (12.1 t·ha−1) followed by Organix AG and Exp. PLA/PHA treatments (average 10.1 t·ha−1), and it was lowest for bare ground, WeedGuardPlus, and Clear Organix AG treatments (average 6.6 t·ha−1) (Fig. 6). In 2018, marketable ear weight was greatest for PE, Exp. PLA/PHA, Naturecycle and Organix AG treatments (average 19.5 t·ha−1), intermediate for Clear Organix AG and Bio360 treatments (average 17.3 t·ha−1), and lowest for bare ground and WeedGuardPlus treatments (average 13.7 t·ha−1).
Ear length and diameter differed among the mulch treatments (P = 0.0006 and P = 0.01, respectively) in 2017 but not in 2018 (P = 0.79 and P = 0.53, respectively). In 2017, both ear length and diameter were greater for all black plastic BDM and PE mulch treatments than for Clear Organix AG, bare ground, and WeedGuardPlus treatments (Table 5). TSS and kernel alignment did not differ among the treatments (P > 0.05) in either year. Tip fill differed due to treatment (P = 0.02) in 2017, and ears harvested from the bare ground treatment had the longest section of unfilled tip (2.95 cm) followed by Organix AG mulch (1.85 cm); whereas in 2018, unfilled tip length was 0 for all treatments (Table 5).
Quality parameters [length, diameter, total soluble solids (TSS), kernel alignment, and unfilled tip length] of sweet corn grown at Mount Vernon, WA in 2017 and 2018.
Discussion and Conclusions
Soil temperature, weed control, sweet corn plant growth, and ear yield and quality were comparable among the black plastic BDM and PE mulch treatments. BDMs were incorporated into the respective plots for 3 consecutive years, indicating that incorporation of BDMs did not negatively impact crop yield and quality when compared with PE mulch that was removed at the end of the growing season per common grower practices. During the growing season, PE mulch and plastic BDMs raised soil temperature, which resulted in 10% greater accumulated GDDsoil for PE mulch and plastic BDMs compared with bare ground and WeedGuardPlus mulch. Similar increase in soil temperature using plastic mulches is reported by Ghimire et al. (2018), Cowan et al. (2014), Miles et al. (2012), and Moreno and Moreno (2008). Though there was not a consistent trend in soil moisture across the mulch treatments over years, increased moisture conservation under the plastic mulches may have contributed to greater plant vigor and greater soil moisture uptake. For example, in 2018, plant vigor (as demonstrated by plant height and biomass) was lower in bare ground plots than in all other treatments.
Soil temperature under Clear Organix AG mulch was high at the beginning of the growing season, but it declined over time as this mulch deteriorated and soil on the bed surface was exposed to ambient air temperature. Deterioration of Clear Organix AG mulch started shortly after laying both years, despite increasing the mulch thickness by 5 µm in 2018 compared with 2017. In contrast, all other mulches had minimal (<5%) deterioration throughout the growing season in both years. As light penetrated the Clear Organix AG mulch, weeds germinated and grew, pushing the mulch up from the soil surface, leading to increased deterioration, possibly because of stretching of the mulch. At midseason, weed pressure was high in the Clear Organix AG treatment, which corresponded with the high PSE of this mulch. Though post-emergence herbicide was applied to the whole field, as Clear Organix AG mulch deteriorated, new unsprayed areas of the bed were exposed, enabling weeds to germinate and grow. No other mulch treatments had much weed development because the mulches remained intact throughout the growing season. These findings are confirmed by other studies that also found that black plastic BDMs controlled weeds as effectively as black PE mulch (Anzalone et al., 2010; Cirujeda et al., 2012; Cowan et al., 2014; Ghimire et al., 2018; Miles et al., 2012), but clear PE mulch was not effective in controlling weeds (Johnson and Fennimore, 2005; Karlsson, 2017). Thus in the current study, the negative impact on crop productivity due to weed pressure throughout the growing season outweighed the positive impact of increased soil temperature at the beginning of the growing season with Clear Organix AG mulch.
Plant emergence, tasseling, silking, and ear harvest occurred earlier with all plastic BDM and PE mulch treatments compared with bare ground. Additionally, plant growth and yield were greater in plastic BDM and PE mulch treatments compared with bare ground. These differences correspond to greater accumulated GDDsoil in plastic BDM and PE mulch treatments than in bare ground in this study. While this difference in GDDsoil was only 10%, the increase in soil temperature in plastic BDM and PE mulch treatments likely provided the sweet corn crop with significantly more favorable growing conditions. Other studies have also reported that clear and black plastic BDM and PE mulches decreased days to germination, tasseling, silking, and harvest of sweet corn compared with bare ground (Aguyoh et al., 1999; Waterer, 2010). In the current study, plant height, shoot and root biomass, SPAD readings, and ear weight and number were greater for all black plastic BDM and PE mulch treatments compared with bare ground, WeedGuardPlus, and Clear Organix AG treatments. Other studies have also reported increased sweet corn plant vigor and yield with the use of PE mulches compared with bare ground (Rajablariani and Sheykhmohamady, 2015; Xu et al., 2015; Zandstra et al., 2007; Zhang et al., 2017).
In the current study, the measures of plant vigor (plant height, root and shoot biomass, and SPAD readings), yield, and earliness were greater for all treatments in 2018 compared with 2017. The differences among the treatments were also smaller in 2018 than 2017. Additionally, in 2017, ears grown on bare ground had poorer tip fill compared with all other treatments; but, in 2018, no differences were observed in ear quality (ear length and width, TSS, kernel alignment, and tip fill) because of treatment. These differences between years likely were related to increased irrigation rate in 2018 (total irrigation 320 mm) compared with 2017 (163 mm). Later in the season in 2017, and for the entire 2018 growing season, we took account of both sensor readings and direct observation of plant stress to schedule irrigation. As the irrigation was adequate for all the plots in 2018, there was less difference in plant vigor across the treatments. But earlier in the season in 2017, when the irrigation rate was based on soil moisture levels in the PE mulch treatment alone, plants in other treatments were moisture stressed, as demonstrated by their reduced plant vigor, yield, and ear quality. Due to its low permeability, PE mulch likely conserved more moisture in the soil so that it was available for plant uptake, whereas plastic BDMs have higher permeability and so water was evaporated from those plots and not available for plant uptake (Saglam et al., 2017). Adequate soil moisture in the PE mulch treatment may have improved mineralization of plant available nutrients, thereby leading to increased nutrient uptake and resulting in improved crop performance. Ertek and Kara (2013) also reported inadequate irrigation reduced sweet corn ear yield, ear quality (sugar and protein contents), and WUE. Other studies have also reported deficit irrigation reduced plant growth and yield of grain corn (Payero et al., 2009; Singh et al., 2007). Xu et al. (2015) reported greater soil moisture conservation leading to higher plant height of grain corn grown with clear plastic mulch compared with bare ground. Saglam et al. (2017) reported increased soil moisture conservation under black PE mulch and BDMs compared with paper mulch and bare ground when pumpkin was drip irrigated and rainfall was sparse. Costa et al. (2014) also reported equal soil moisture conservation with the use of PE mulch and BDM with strawberry.
The current study indicates that in the Mediterranean climate of northwest Washington, emergence, growth, and harvest of sweet corn were accelerated, and marketable ear yield and quality were increased with the use of black plastic BDMs or PE mulch compared with bare ground and paper mulch. This suggests that black plastic BDMs can replace black PE mulch for sweet corn production. Although WeedGuardPlus mulch remained intact over the growing season, it had a soil cooling effect, which delayed and reduced sweet corn yield in this study. This soil cooling effect may be beneficial in regions or for crops where heat stress is a problem. While clear BDM appeared to accelerate plant growth early in the season, plants were stressed due to weed competition and low soil moisture when the mulch deteriorated later in the growing season. Thus, to maximize crop performance for clear BDM, different crop management practices than for black BDM and PE may be required, specifically regarding weed control and irrigation rate.
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