permeable soil
Résumé
Il importe d’optimiser la gestion des irrigations du fraisier à jours neutres afin de répondre adéquatement au besoin de la plante tout en diminuant la pression du secteur agricole sur l’eau. Dans les sols de l’île d’Orléans (Québec, Canada), la fraction des particules de sol supérieure à 2 mm peut varier de 15 à 30%. L’eau s’écoule principalement verticalement sous le tube de goutte-à-goutte et le mouvement vertical rapide de l’eau entraine un assèchement du sol en bordure de l’andain et une perte de nutriments. Diverses techniques combinées à l’utilisation de tensiomètres peuvent être envisagées afin d’améliorer l’efficacité d’utilisation de l’eau d’irrigation (EUEI) dans ce type de sol. L’irrigation fractionnée, l’ajustement du seuil de déclenchement (SD) de l’irrigation selon l’ETc prévisionnel, l’installation de matelas capillaires sous la zone racinaire et un système de production hors-sol sur butte profilée ont été testés durant les saisons de production 2013 et 2014 dans un contexte de production commerciale. L’effet des traitements durant la saison 2014 a été limité des maladies et les précipitations. Les traitements ont permis d’améliorer EUEI par rapport au témoin mais n’ont pas permis une augmentation significative du rendement total de la saison. Le système hors-sol a toutefois permis d’augmenter significativement de 86% le rendement vendable durant le premier mois de production. En raison de sa simplicité et de sa tendance à augmenter le rendement, l’irrigation fractionnée est recommandée pour le type de sol à l’étude.
Abstract
It is important to optimize the irrigation management of day-neutral strawberry plants in order to adequately meet the plant needs while reducing the pressure of agriculture on water. Some soils in the area of Île d’Orléans (Québec, Canada) present an important proportion of schist fragments (15-30%). Because of the high hydraulic conductivity, water flows mainly vertically under the drip tape with little horizontal movement, causing losses of water and nutrients. Different techniques, combined with the use of tensiometer, may be considered to improve the irrigation water use efficiency (IWUE) in this type of soil. Pulse irrigation, irrigation threshold (IT) adjusted according to ETc, installation of capillary mat under the root zone and a raised bed trough system with peat substrate were tested during the 2013 and 2014 cropping seasons in commercial fields. Treatment effects during the 2014 season were limited by diseases and rainfall. Treatments tested allowed to increase IWUE compared to the control but the total marketable yield increases were not significant. The soilless system has, however, significantly increased the marketable yield by 86% in the first months of production for both years. Because of its simplicity and its yield increase, pulse irrigation is recommended for this type of soil in order to reduce the amount of irrigation water.
Introduction
The province of Québec produces more than 50% of all strawberries in Canada with 1758 ha (ISQ and MAPAQ, 2014). Strawberry plants have a shallow root system which make them susceptible to drought (Galletta and Himelrick, 1990) and are generally considered as a high water requirement crop. Whereas insufficient water supply results in yield reduction (Krüger et al., 1999; Kumar and Dey, 2012; Liu et al., 2007; Yuan et al., 2004), excessive irrigation can contribute to disease development and nutrient leaching.
Québec has an appearance of abundant fresh water resources. However, some regions have significant problems of water supply in terms of quantity or quality especially where there is a high concentration of irrigated crops (e.g. Montérégie, Lanaudière and Île d’Orléans near Québec City) (AAC, 2003). It is therefore important to optimize the management of irrigation for strawberry to meet crop needs while reducing the pressure of agriculture on the water resource.
It has been shown that marketable yield and irrigation water use efficiency (IWUE) could benefit from soil water potential (SWP) based irrigation management for strawberry (Bergeron, 2010; Evenhuis and Alblas, 2002; Hoppula and Salo, 2006; Létourneau, n.d.). Yet, as pointed out by Morillo et al. (2015), optimizing irrigation management do not imply solely high technology irrigation systems with sensors in the field.
Hydraulic properties influence the shape and dimension of the wetting pattern (Cote et al., 2003). Letourneau et al. (n.d.) reported a limited influence of SWP based irrigation management in highly permeable soils with important schist fragments (particles over 2 mm) due to inadequate wetting pattern. In highly permeable soils, water drains easily and quickly from the root zone because of gravity (Cote et al., 2003). In these soils, wetting patterns present a downward oriented elliptical shape which makes it difficult to wet the side of the bed; making most of the applied water rapidly unavailable to plants. Improved irrigation techniques or production systems combined with SWP management could be considered to increase IWUE of strawberry in this type of soil.
Pulse irrigation could reduce drainage under the root zone (Cote et al., 2003) and could increase horizontal water movements (Cote et al., 2003; Skaggs et al., 2010). High frequency irrigation resulted in equivalent or improved yield and increased IWUE for potato (Wang et al., 2006) and bell pepper (Dukes et al., 2003). Pulse irrigation consists in dividing the volume of water to be applied in more than one event, separated by non- irrigation periods, and can be considered as a high frequency irrigation method. Létourneau (n.d.) obtained increased yield and IWUE for strawberry with pulse irrigation. However, the study was conducted for only one season.
Deficit irrigation can minimize water use and leaching of biocides into groundwater (Mpelasoka et al., 2001). Deficit irrigation studies on strawberry showed to improve fruit quality (Giné Bordonaba and Terry, 2010; Terry et al., 2008) but also generally resulted in a reduction of yield and fruit size (Blatt, 1984; Krüger et al., 1999; Liu et al., 2007; Serrano et al., 1992). However, some studies reported that a mild water deficit had no measurable effect on yield (El-Farhan and Pritts, 1997) or on flowering pattern and the number of flowers (Johnson et al., 2014). Drought stress, up to a certain point, may increase plant water use efficiency. As suggested by Letourneau et al. (n.d.), adjusting the irrigation threshold (IT) throughout the season as a function of crop evapotranspiration (ETc) could lead to an increased IWUE. Therefore, varying IT to induce a mild drought when ETc is low could improve IWUE without compromising yield.
Soil water retention barriers (SWRB) have shown to help save water in turf grass production (Demirel and Kavdir, 2012) and increase yield of cucumber, green pepper and maize grown on a sandy soil (Smucker and Basso, 2014). SWRB can provide water and nutrients to the plant root zone for a longer time period (Kavdir et al., 2014). The depth of the barrier is important to avoid the negative impact of excess water. Trials with capillary mats have generated water saving and improved plant growth in nursery (Caron et al., 2002) and reduced the amount of nutrient solution applied in greenhouse tomato (Lemay et al., 2012).
An alternative production system was developed in Europe as a cheap option for growers to convert their traditional soil grown strawberries to a soilless culture. It consists of machine made troughs in the top of a raised bed of soil. The trough is lined with a fabric barrier and is filled with substrate. It is known as the Belgian/Dutch bed system in Europe (Lieten, 2013) or raised bed troughs system (RaBeT) in California (Wang et al., 2012). It has led to increased, or equivalent, yield compared to standard field systems in Europe (Lieten, 2013) and California (Wang et al., 2012). In soilless culture, the growing medium is more homogeneous than the soil of a field, which can facilitate irrigation management. Also, crop rotation is no longer necessary which is an economic advantage.
The primary objective of this study was to determine the potential of 1) pulse irrigation, 2) IT adjustment throughout the season, 3) SWRB and 4) RaBeT, in the context of a commercial production. The hypothesis tested was that these techniques combine with SWP irrigation management could lead to an increase of yield and IWUE compared to a control treatment. The secondary objective was to determining the effect of these methods on day-neutral strawberry plant development, yield, fruit quality and water use, under Québec climate and on a highly permeable soil with an important schist fraction.
Materials and methods
Experimental setup
Five treatments were applied in a randomized complete block design with four replicates in 2013 and in a randomized incomplete block design with six replicates in 2014. The first treatment was conventional irrigation (control) with an irrigation threshold (IT) of -15 kPa. The second treatment was pulse irrigation with an IT of -15 kPa. The irrigation was divided into two events separated by more than one hour, each event lasting half the time of the control irrigation treatment. The third treatment was pulse irrigation with IT adjusted according to predicted crop evapotranspiration (ETc) and will be referred as pulse ETc. When daily predicted ETc was equal or more than 2 mm day-1, the IT of -15 kPa was applied but when it was less than 2 mm day-1, an IT of -
30 kPa was used. The idea was that inducing a moderate water stress could save water when ETc was below 2 mm day-1 without decreasing yield. Daily ET values were calculated with the Rochette and Dubé (1989)
equation, and crop coefficients derived from Bergeron (2010) and Hanson and Bendixen (2004). The fourth treatment consisted of a capillary mat buried at the lower limit of the root zone (30 cm) with an IT of -15 kPa. The capillary mat was installed directly under the drip tape and was 40 cm wide. It was designed to capture water flow and create a water reserve. It will be referred as the SWRB treatment. The fifth treatment was a raised bed trough system (RaBeT) using a peat substrate (Agro Mix™ AF G10, Fafard et Frère) with an IT of - 5 kPa. The trough was lined with a capillary mat. The IT of soil and substrate were selected based on the results of previous studies (Bergeron, 2010; Létourneau, n.d.; Watters, n.d.) and applied after an establishment period.
The treatments were drip irrigated independently. One drip line per bed was buried 5 cm under soil surface with a 1 gpm/100pi flow and 4-inch emitter spacing for the RaBeT treatment and a 0.34 gpm/100pi flow and 8- inch emitter spacing for open field treatments. The soil water potential was measured by tensiometers (Hxm 80, Hortau) and recorded online by the IRROLIS 3 system (Hortau, Lévis, Canada) thus allowing real time monitoring. For both years, three replicates were monitored with a station installed at mid-length of the bed with tension sensors at the depths of 15 and 30 cm. Irrigation was triggered simultaneously in all replicates of a treatment when the average value of soil water potential at 15 cm depth reached the IT.
Site and crop
The experiments were conducted for two years in commercial strawberry fields at Saint-Jean-de-l’Île- d’Orléans, Québec, Canada (46°54’N, 70°56’W). The growing season is short and rainy, and has a humid continental climate. Table 1 and Table 2 present soil textures and basic physical and chemical properties of the experimental sites. Conventional (250 cm³) and undisturbed (5.5 cm height and 9 cm diameter cylinder)
soil samples were collected at 15 and 30 cm depths in the growing bed in the beginning of the season at six locations each year. Conventional samples were used to measured initial EC (Rhoades, 1982), pH (CPVQ 1988), organic matter content (CEAEQ 2003) and soil texture (Gee and Or, 2002). Proportions of rock fragments in the 0-15 and 15-30 cm horizons were measured with the Grossman and Reinsch (2002) procedure. Undisturbed samples were used to measure soil bulk density (ρbulk) by weighting after oven drying.
Saturated hydraulic conductivity (Ksat) was measured on site (Banton et al., 1991).
According to the Canadian Soil Classification System, soil texture of the experimental sites varies from silty clay loam to clay loam. The fraction of schist fragments (particle > 2 mm) observed in the 0-30 cm surface layer varies between 18 to 24 % (Table 1). This important fraction greatly influences the hydraulic behavior of the soil and explains its high saturated hydraulic conductivity (Table 2).
All cultural operations (planting, harvest, pest, disease and weed management, flower and runner cutting and general maintenance) were done similarly on all plots by the grower crew. Bare root day-neutral strawberry (cv. monterey) plants were transplanted in double-row per bed with a spacing of 7.8 and 10 plants m-1 for open
field and RaBeT treatment, respectively. The 90 cm wide and 30 cm high beds were spaced 1.4 m apart and covered with black polyethylene mulch. The length of bed varied between 100 to 228 m depending on the field. Plantation was done on May 15th and 20th, harvest started on July 13th and 10th and ended on October 18th and
10th for 2013 and 2014, respectively. Fertigation was applied according to Guérineau et al. (2003) for the
RaBeT treatment and according to grower instructions for open field treatments.
Measurements
Crown diameter was measured with a slide gauge and leaf area was determined as the longest leaf-covered distance in two perpendicular directions. Measurements were done weekly from planting until the first harvest on the same six randomly selected plants per plot. Mortality plant rate was evaluated four times during the season on a full bed length per plot.
For each plot, marketable yield of the full length beds was measured by the grower harvesting crew according to his schedule (2-3 times a week). Fruit quality parameters were measured weekly or bi-weekly throughout the harvesting season on one randomly chosen package of about 1.5 L per plot. Average fruit size was measured as the net weight of package divided by the number of fruits. Two randomly fruits were then selected for further measurements. Firmness was measured by a hand penetrometer (FT-02, QA Supplies LLC) with a 2.5 mm tip. Sugar content (expressed in °Brix) was evaluated with a refractometer (PAL-1, Atago Co) by hand squeezing the fruit until juice covered the prism of the refractometer. The amount of irrigation water was measured for each treatment by water meters (Les compteurs Lecompte, Saint-Hyacinthe,
Canada). Irrigation water use efficiency (IWUE) was calculated as the marketable yield (kg) divided by the total amount of irrigation water (m3).
At the end of the harvest season, strawberry plants were collected with their roots. The plants were then cut at the collar and the leaf dry mass was determined after drying at 65 °C. The proportion of root affected by disease was evaluated visually by three persons after washing the roots. The pH and the EC were measured weekly on the soil/substrate solution extracted at 20 cm depth by suction lysimeters (Soil water sampler, Soilmoisture Equipment Corp.).
Statistical analyses
Data were analyzed by an analysis of variance (ANOVA) using the MIXED procedure of the SAS 9.4 version (SAS Institute Inc, Cary, USA). When the effect of treatment was significantly different according to the year, results were presented by year. Otherwise, means of the two years were presented. When treatments were significantly different at the P < 0.05 level, main effect contrasts were performed with LSD tests. When necessary, values were log (mortality rate and root disease) or square root (montly yield) transformed to stabilize the variances.
Results and discussion
Climatic parameters
Table 3 presents climatic parameters observed during the two experimental seasons. Rainfall contributes minimally to the water requirement of the crop because of the plastic mulch covering the bed. Rainfall distribution and frequency differed according to the year. The establishment and plant development period in 2013 were less favorable due to rain and cold temperature. Therefore, the delay between planting and harvesting was shorter in 2014. The end of the 2014 season was characterized by more rain than in 2013 and the normal. There were more days with precipitation over 25 mm in 2014 than in 2013 and the normal during the harvesting period. These flooding days can create conditions favorable to the onset of diseases. A frost occurred on September 19th 2014 witch affected plant strength and fruits development and explains partially
the earlier ending of that harvesting season. ET was 584.1 and 592.8 mm for May to October in 2013 and 2014, respectively. ETc was 257.2 mm for the 2013 cropping season and 240.1 mm for 2014. There were 52 and 49 days where ETc was equal or above 2 mm in 2013 and 2014, respectively.
Soil water potential
Table 4 presents the average seasonal soil water potential (SSWP) and the average observed irrigation threshold (OIT) for both seasons. Seasonal soil water potentials were generally higher in 2014 than in 2013.
OIT values were generally lower than the intended IT due to practical delays such as time to go to experimental site, valve adjustments and pump maintenance and availability. The irrigation pump used in 2014 was able to irrigate only one treatment at the time. The pulse and RaBeT treatments have been more frequently delayed than the other treatments since their OIT was lower in 2014. Temporary wilting symptoms were observed a few times for RaBeT treatments. It is also difficult to re-humidify a substrate that has dried. Inadequate wetting patterns, particularly in this soil, resulted in management difficulties. Irrigation was frequently triggered as one sensor shown greatly higher or lower soil matric potential than the two others. Capillary mats of SWRB treatment allowed maintaining a relatively constant SWP only in the lower limits of the root zone. Mean season SWP at 30 cm was kept close to field capacity.
Soil EC and pH evolution in the season
The EC of all soil treatments tended to decrease during both production seasons as plants withdrawed nutriments from the soil (Figure 3). Therefore, there was no salt accumulation induced by any treatment. The RaBeT treatment EC presented more variation than the other treatments. The EC is an important parameter because it can be linked to the amount of fertilizer in the soil or in the substrate. It is even more important in soilless culture because all the fertilizers are supplied by fertigation. It has been difficult to maintain EC in the substrate within the recommended values of Guérineau et al. (2003) due to frequent leaching caused by rainfall. In the beginning of the seasons, because of frequent rainfall and low water requirement of the plants, the IT value was rarely reached. The low frequency of irrigation resulted in a decrease in EC.
The general evolution of soil pH was similar between treatments (Figure 4). The lower values at the beginning of the 2013 season were not caused by irrigation since the first irrigation started at end of June. The pH tended to increase during the 2014 season and reached values above the recommendations (6.5-6.6) (Guérineau et al., 2003; OMAFRA, 2015). The pH of irrigation water increased from 7.40 to 8.88 during the 2014 season whereas it varied between 4.84 and 6.95 during the 2013 season. Despite the addition of acid in the fertilisation solution of the RaBeT treatment, its pH could not be maintained near the recommended value of 5.8 ± 0.8 (Guérineau et al., 2003). This could have restricted nutrients availability.
Treatment effect on plants
Growth rate
Treatment effect on leaf area and crown diameter growth rates were not significant (Table 5), which is consistent with other studies (Hoppula and Salo, 2006; Létourneau, n.d.; Pires et al., 2006). Crop water needs were mainly fulfilled by rainfall until the end of June which explains the absence of treatment effect during the plant development stage. However, pulse and RaBeT treatments tended to have a higher leaf area growth
rate. That trend for the RaBeT treatment can be explained by the smaller root volume, substrate temperature would then rise more rapidly due to the small leaf area and black mulch during the establishment period. In a study by Lieten (1997), high root temperatures (>15-16°) have resulted in improved yield earliness by speeding up plant growth, but also resulted in a reduction of subsequent yield and fruit size.
Above ground biomass
The RaBeT treatment presented a significantly lower plant dry biomass (Table 5) because its plant density was higher. Dion (n.d.) also noticed a decrease in the number of crowns, leaves, flower stalks and fresh biomass with increasing plant density. Furthermore, plants of the RaBeT treatments showed more declining signs than the control after the first frost and with the temperature decreasing at the end of the season due to smaller root volume, which is consistent with Lieten (1992). The temperature fluctuations are however reduced with the RaBeT system compare to the tabletop soilless system due to benefit of soil radiation. Létourneau (n.d.) also