Agua-suelo-clima

 

Irrigation scheduling of peach trees (Prunus persica L.) by continuous measurement of soil water status

 

Programación del riego en melocotoneros (Prunus persica L.) mediante medición continua del status hídrico del suelo

 

Oussama H. Mounzer¹, Juan Vera^1,2, Luis M. Tapia³, Yelitza García-Orellana⁴, Wenceslao Conejero¹, Isabel Abrisqueta¹, Ma. Carmen Ruiz-Sánchez^1,2, José Ma. Abrisqueta-García^1,2*

 

¹ Departamento de Riego. Centro de Edafología y Biología Aplicada del Segura (CEBAS-CSIC). 30100. Espinardo, Murcia, España.

² Unidad Asociada al CSIC de Horticultura Sostenible en Zonas Áridas (UPCT-CEBAS). *Author for correspondence. (jmabrisq@cebas.csic.es).

³ Instituto Nacional de Investigaciones Forestales y Agropecuarias (INIFAP). Uruapan, México.

⁴ Departamento de Ingeniería Agrícola. Universidad Centro Occidental Lisandro Alvarado (UCLA). Barquisimeto, Venezuela.

 

Received: September, 2007.
Aproved: June, 2008.

 

Abstract

Continuous real time soil moisture measurements facilitate the control of soil water availability for plants. The aim of this research was to study soil water content in two irrigation treatments and its effect on the physiological response of young peach trees (Prunus pérsica (L.) Batsch cv. Flordastar) during one growing season. The irrigation treatments were: T-1, scheduled to fulfil crop water requirements (100% ETc, according to FAO, Penman-Monteith methodology), and T-2, in which irrigation doses and frequency are adjusted continuously to maintain the water content in the main root zone (0-50 cm depth) at +10% to -5% field capacity, as indicated by the continuous measurement of the soil water content by capacitance Frequency Domain Reflectrometry (FDR) probes. After irrigation, the different phases of soil water depletion in the root zone were defined according to the evaporation and root absorption processes. The first two phases, which occurred during daylight, included a rapid decrease in the soil water content (149 to 129 mm a third phase of slower soil water depletion (129 to 127 mm 0.5 m^-1) occurred during the night, and a fourth involved a rapid decrease (127 to 122 mm 0.5 m^-1), followed by a slight recovery, also during the night. A comparative study of the soil-plant water status patterns observed in the two treatments during one growing season was made. The irrigation treatment scheduled according to capacitance probe measurements provided a saving of 18% of irrigation water with respect to the conventional scheduled treatment and had no effect on total yield or vegetative growth of young peach trees.

Key words: Prunus pérsica L. Batsch, capacitance probe, drip irrigation scheduling, soil-plant water relationship, soil water content measurement.

 

Resumen

La medición continua de la humedad del suelo en tiempo real facilita el control de la disponibilidad de agua del suelo para las plantas. El objetivo de esta investigación fue el estudio del contenido de agua del suelo en dos sistemas de riego y su efecto en la respuesta fisiológica de árboles jóvenes de durazno (Prunus pérsica (L.) Batsch cv. Flordastar) durante una temporada de cultivo. Los sistemas de riego fueron: T-1, diseñado para satisfacer los requerimientos de agua para cultivo [100% ETc (evapotranspiración del cultivo), según la FAO, metodología de Penman-Monteith], y T-2, en el que las dosis y la frecuencia de riego se ajustan continuamente para mantener el contenido de agua en la parte central de la raíz (0-50 cm de profundidad) a entre +10% y - 5% de la capacidad de campo, como lo muestran las mediciones continuas del contenido de agua del suelo mediante sondas capacitivas FDR (Frequency Domain Reflectometry). Después del riego, las distintas etapas de agotamiento de agua del suelo en la zona radicular se determinaron en función de la evaporación y de los procesos de absorción radicular. En las dos primeras etapas, que tuvieron lugar durante el día, se registró un rápido descenso en el contenido de agua del suelo (de 149 a 129 mm 0.5 m^-1); durante la noche hubo una tercera etapa de disminución de agua del suelo, que fue mas lenta (de 129 a 127 mm 0.5 m^-1), y hubo una cuarta con un descenso rápido (de 127 a 122 mm 0.5 m^-1) seguida de una ligera recuperación, también por la noche. Se realizó un estudio comparativo de los modelos de comportamiento agua-suelo-planta observados en los dos sistemas durante una temporada de cultivo. El sistema de riego programado a partir de instrumentos de medición de capacitancia generó un ahorro de 18% de agua de riego con respecto al convencional y no influyó en el rendimiento total ni en el crecimiento vegetativo de árboles jóvenes de durazno.

Palabras clave: Prunus pérsica L. Batsch, sonda capacitiva, sistema de riego por goteo, relación agua-suelo-planta, medición del contenido de agua del suelo.

 

INTRODUCTION

Peach (Prunus pérsica (L.) Batsch) is one of the most widely cultivated and important deciduous fruit trees in the world. Spain is the fourth producer of peaches after China, Italy and the United States, with 1.2 million metric tonnes per year (FAO, 2006). About 70% is consumed fresh in Spain, 20% is processed (canned, juice, jam or dried), and 10% is exported as fresh fruit.

Beginning with the first studies on irrigation in deciduous fruit trees, authors have generally agreed that irrigation provides three main benefits: improved tree growth (Frecon, 2002), increased yield (Bryla, 2004)) and increased fruit size (Day, 2002)). Nevertheless, such benefits depend on how, when and how much water is applied to the plant. Excess or deficit irrigation water both have an impact on peach crop productivity, and the best irrigation practices should reduce water use and associated on-site environmental impacts, such as nutrient and chemical leaking and a decline in soil quality. All these have even more importance in areas with scarce water resources, such as the Spanish Mediterranean region.

Water management practices have been proposed to reduce water use without decreasing yield and fruit quality in peach. Regulated deficit irrigation during phenological stages less sensitive to water deficit can save water, with no effect on fruit yield (Ruiz-Sánchez and Girona, 1995; Dichio et al., 2004; Naor, 2006). Partial root zone irrigation reduced water used by 35-40% with no effects on plant physiology or volume canopy (Gong ét al., 2005). All these practices were based on calculations of evapotranspiration demand.

In semiarid lands, the extended use of high frequency localised irrigation methods changes the root system growth patterns as well as plant water uptake because of differences in the way water is distributed through the soil profile (Bryla, 2004). Girona ét al. (2002) indicated that monitoring the available soil water content is critical for irrigation timing, due to variable tree responses, wetting patterns, soil depth and root exploration in high frequency irrigation methods.

Frequent data collection is essential for understanding the water dynamics of the soil-plant-atmosphere continuum. Much effort has been invested in the determination and characterization of the variables that control soil water flow and water absorption by the roots (Goldhamer ét al., 1999), because crop production is more closely related to the available soil water (Girona ét al., 2002) than to any meteorological variable (DeJong and Bootsma, 1996). Thus, monitoring the soil water content is essential for estimating plant water needs and for scheduling efficient irrigation. Soil water status has long been used for scheduling irrigation (Campbell and Campbell, 1982), and several different methods and techniques exist for estimating soil water content. These include the use of neutron probe, although this method is expensive and labour intensive (Letey, 2007). Other methods for indirectly determining the soil water content involve the use of tensiometers or electrical resistance sensors (Lowery et al., 1986; Hanson et al., 2000).

Newer tools allow the continuous measurement of the soil water content. For example, numerous studies have determined the soil water content using TDR (Time Domain Reflectometry) (Green and Clothier, 1999; Polak and Wallach, 2001). Recently, capacitance

FDR probe (Frequency Domain Reflectometry) has given excellent results as far as precision, facility of calibration, installation, and data interpretation and transmission are concerned (Paltineanu and Starr, 1997).

The main goal of this paper was to study soil water content variations measured continuously by capacitance FDR probes, and the physiological response of young peach trees (cv. Flordastar) to different irrigation scheduling treatments (a conventional, based on ETc, and another based on soil water content measurements).

 

MATERIAL AND METHODS

The experiments were performed during 2004, in an experimental 0.8 ha plot located in Santomera-Murcia (S.E. Spain): 38° 06' N, 1° 02' W. Soil are calcareous, rocky and shallow, with a clay-loam texture and low organic matter and cationic exchange capacity values classified as Lithic xeric haploxeroll (Soil Survey Staff, 2006). The bulk density of the soil was 1.45 g cm^{- 3} down to 50 cm, but more compact (1.67 g cm^{- 3}) at deeper levels. The soil water content at field capacity (θ_FC) and at permanent wilting point (0pwp), as determined in undisturbed soil samples by the Richards pressure plate technique (Richards, 1965), were 0.24 and 0.15 cm³ cm^{- 3}, respectively, which implied an available soil water content of 90 mm m^-1.

Agrometeorological data were recorded by an automated station located within the peach orchard. During the experimental period the average maximum and minimum air temperatures were 30.1 and 7.7 °C. The annual reference evapotranspiration (ETo) determined by the FAO, Penman-Monteith equation (Allen et al., 1998) was 1100 mm, with a maximum of 7 mm day^-1 in August. Rainfall was 440 mm, from which 245 mm occurring in spring.

The plant material consisted of three-year-old peach trees (Prunus persica (L.) Batsch cv. Flordastar, on GF-677 peach rootstock), spaced 5×5 m. The trees were irrigated by a drip irrigation system consisting of a single lateral line per tree row, with four emitters per tree, placed 1 and 1.5 m of each side of the trunk, providing 2 L h^-1.

Two irrigation scheduling treatments were considered. In the T-1 treatment, the trees were irrigated to 100% of ETc, estimated by the product of reference evapotranspiration (ETo, penman-Monteith), crop coefficients (0.40 Feb, 0.60 Mar, 0.70 Apr, 0.95 May, 0.80 Jun, 0.50 Jul, 0.40 Aug-Oct, 0.30 Nov) and the percentage of ground area shaded by the tree (Doorenbos and pruitt, 1977; Fereres and Goldhamer, 1990; Allen et al., 1998; O'Connell and Goodwin, 2003). In the T-2 treatment the irrigation doses and frequency were adjusted continuously in order to maintain the soil water content of the main root zone (0-50 cm depth) at between + 10% and —5% of field capacity. The upper limit is equivalent to the value of the soil water content registered when the wetted front of a nightly irrigation event was detected at 50 cm depth. The lower limit was chosen close to the field capacity to prevent any soil water shortage (Mounzer, 2005).

Irrigation was scheduled for the T-1 treatment, on the basis of weekly estimated ETc and automatically controlled by a head unit programmer operating on electro-hydraulic valves (Agrosolmen S.L., 25 mm, Solenoid Bacara, Latch 2-way). For the T-2 treatment, the irrigation applications were scheduled on the basis of real-time soil water content measurements controlled by a radio telemetry system. The irrigation water volumes for each treatment were measured with in-line flowmeters (Agrosolmen S. L., Aurus, 25 mm, pulse emitter 10 L).

Treatments were distributed in a completely randomized design with four repetitions, each consisting of one row of 13 trees. The central nine trees were used for experimental measurements and the others served as guard trees.

The volumetric water content through the soil profile was monitored in continuous real time, using two multisensor capacitance probes (C-probe, v.1, Agrilink Inc., Australia) per treatment, placed 10 cm from the emitter, inside a PVC access tube installed within the wetted area. The probe had sensors at 10, 20, 30 and 50 cm depth, which covered 95 % of the active root system (Abrisqueta et al., 2008).

The PVC access tube of each probe was installed with a special auger to ensure good contact between the tube and the soil. The bottom of the PVC access tube was plugged with a rubber bung to avoid water and water vapour from entering the tube. An intrusion plastic column was used to hold the four sensors at their respective depths. A rain gauge was installed just below the emitter to continuously monitor the emitter water flow. Each probe and rain gauge were connected to a radio transmission unit (RTU) with a 7-pin cable. The RTU read the value of each sensor every 5 min and stored an average value every 15 min. The stored raw data were sent by radio signal through a relay station to the laboratory, 17 km from the experimental site, downloaded into a personal computer and graphically displayed as volumetric water content after modification by a locally established calibration equation.

The water content in each soil layer, expressed in mm, was calculated by multiplying the measured volumetric moisture content by the thickness of the corresponding soil layer. The soil water store was calculated by integrating the soil water content of the 5 layers within the monitored soil profile, including the layer corresponding to 40 cm depth, which was estimated as the mean of the 30 and 50 cm readings. The resulting value was compared with the predetermined field capacity to either trigger or halt an irrigation event.

Leaf water relations were determined by measuring stem water potential (Ψ_stem) using a pressure chamber (Soil Moisture Equipment Co., mod. 3000), on mature leaves on the north face near the trunk. They were placed in a plastic bag covered with aluminium foil for at least 2 h prior to the measurements, which were carried out at midday every 10-15 days from April to October. Four leaves per treatment (one leaf per tree and one tree per replication) were cut and immediately placed in the chamber according to Hsiao (1990).

Extension shoot length was recorded at biweekly intervals in four tagged shoots per tree, one from each compass direction on four trees per treatment (one tree per replicate). Trunk diameter was determined in all the experimental trees about 30 cm above the graft union in January and December 2004. Canopy shaded area was estimated in August for all the experimental trees. A large tarpaulin (5×5 m) divided into 625 cm² squares was placed under the tree between 11:00 and 13:00 solar time, and the total number of shaded squares counted. Peaches were harvested twice between 7 and 18 May. The total weight of the fruit and the total number of fruit per tree at each picking were recorded in five experimental trees of each replicate and treatment.

Data were analysed using the SPSS software (SPSS, 2002). Analysis of variance (ANOVA) was used to discern treatment effect. Statistical comparisons were considered significant at p≤0.05.

 

RESULTS AND DISCUSSION

The soil water store (SWS) registered continuously at 0-50 cm depth between two successive irrigation events (16 July and 18 July) in the T-2 treatment is shown in Figure 1. The SWS showed a sharp increase when irrigation began on the 16th of July and then decreased rapidly as it was turned off. In the morning of the next day, the decrease in soil water content again speeded up as the evaporative demand of the atmosphere gradually increased. In the absence of any irrigation the following night, a very light increase in the soil moisture content was observed (Figure 1).

This slight increase in the soil water store can be explained by water displacement as a result of the lateral hydraulic gradient. The hypothesis is that the sensors are installed near the centre of the active root system (Mounzer, 2005). During the day, the intensive root water uptake depletes the soil water content and generates a hydraulic gradient. In the afternoon, the water uptake decreases and the water in the soil far from the roots tends to equilibrate the hydraulic gradient by moving toward the sensors, slightly raising the soil water content.

A similar behaviour was described in the soil water depletion dynamic of irrigated orange trees using TDR probes, and was explained by the free drainage and root uptake processes (Polak and Wallach, 2001).

Based on the effect of different driving forces on soil water flow, the transient soil moisture content pattern between the end of irrigation on 16 July and the start of the irrigation on 18 July was fitted for different stages, whose regression models are described in Table 1. Stages I and II corresponded to water distribution, root absorption and soil evaporation processes with different rates of soil water depletion: —13.6 and —1.9 mm h^—1.

Stage III represents a night time stage, when root water absorption and evaporation are minimal (vapour pressure deficit, VPD<0.5 kPa); consequently, the soil water depletion rate was lower (— 0.2 mm h^—1). Stage IV of the soil water depletion process was not constant (third degree polynomial model), it started during the day time when evaporation and root water absorption were substantial (VPD≥3 kPa), and depletion occurred at a similar rate to that observed in stage II. At about 12:00 h on 17 July the soil water content reached the threshold value (lower limit) at which irrigation would normally be started, and continued to decrease because there was no irrigation.

Figure 2 shows the volumetric soil water content at different depths (10, 20, 30, 40 and 50 cm), together with the soil water store down to 50 cm, and the irrigations applied during one week in July, in the T-2 treatment. The figure depicts the typical soil water content dynamic. The irrigation events caused a rapid increase in soil water content, which subsequently fell (when the irrigation was turned off) at different velocities until the next irrigation started (Figure 2).

Irrigation caused an increase in the soil water content at all depths, except at the deepest (50 cm), which remained unchanged (mean value of 0.22 m³ m^{— 3}) (Figure 2a). Therefore, it is concluded that under the conditions described, drainage can be considered null, and the decrease in the soil water content, which occurs almost immediately after irrigation is turned off, corresponds mainly to evaporation and root uptake (Green and Clothier, 1999).

The continuous record of the soil water store down to 50 cm during the experimental period (2004) for both irrigation treatments is shown in Figure 3. In the conventionally scheduled irrigation treatment (T-1), the soil water content was low during spring (February-April) and from late summer to the end of the season, indicating an excess of water in June and July, compared with the upper limit of the soil water holding capacity (Figure 3a). Although some drainage below 50 cm might have been expected, this did not occur, because the mean value of the soil water content at this depth remained constant (≈0.20 m³ m^{— 3}, data not shown). Maximum vegetative growth occurred from May to August (Mounzer, 2005).

In treatment T-2 a different pattern of the soil water store was observed (Figure 3b) because the irrigation doses and frequency were scheduled in accordance with the transient soil moisture content and the above mentioned limits (+10% and — 5% field capacity) (Mounzer, 2005). The soil water content remained above the lower limit throughout the experimental period (except during one week of September due to a malfunction in the head unit programmer).

The recorded soil moisture values in T-2 increased beyond the upper limit after irrigation (Figure 3b), although they fell rapidly due to the root water uptake which prevented the water from reaching 50 cm depth (this sensor did not detect significant soil moisture changes at any time during the growing season) (data not shown). During irrigation episodes, confined saturation conditions were promoted near the capacitance sensors, but quickly abated after the end of irrigation as water was infiltrated through the profile (Figure 3b).

The seasonal pattern of stem water potential (Figure 4) was similar in both treatments. The rainfall from January to May (Figure 4) contributed more than 80% of the annual average, and was responsible for the high and almost constant ψ_stem values during this period. The increase in evaporative demand of the atmosphere in summer brought about a decrease in ψ_stem values. These values were lower in T-2 than in T-1 treatment, from June to July, but from August onwards the increase of the frequency of irrigation in the former treatment induced a recovery in ψ_stem values and improved leaf water status compared with T-1 (Figure 4).

Shoot and trunk growth were not affected by the irrigation treatment. Average trunk diameter at the beginning of the experimental period was about 5 cm and grew 2.5 cm during the year, with no significant differences between treatments (Table 2). Annual growth of terminal shoots, with an initial basal diameter of 10 mm and initial length of 30 cm was similar in both treatments (≈552 cm, Table 2). The canopy shaded area was not affected too by the irrigation treatments (Table 2).

No statistically significant differences were found between treatments in the total weight of fruits at harvest or in the number of fruits per tree (Table 3). The yield obtained can be considered adequate for local peach production (Ministerio de Agricultura Pesca y Alimentación, 2003), and slightly higher than that found in early Italian peach trees (Caruso et al., 1997). The analysis of variance also indicated no significant effect of picking date on yield, indicating that harvest was not delayed by the different irrigation treatments (Table 3).

The amount of irrigation water applied between harvesting date of the previous year and the harvest in 2004 amounted to 1405 and 1154.5 m³ ha^-1 in T-1 and T-2, respectively, meaning 17.8% water saving in T-2 compared with the conventionally scheduled irrigation treatment (T-1). Although this saving may not appear very high, it could be added to that obtained using other deficit irrigation strategies, which could be helpful in areas with increasingly scarce water resources (Ruiz-Sánchez and Girona, 1995).

 

CONCLUSIONS

Continuous soil water content measurement by capacitance FDR probe has the advantage over other conventional methods (gravimetric, soil water potential, neutron probe) of allowing access to data in real time, so that variations can be analysed taking into account the soil characteristics, root uptake, climatic conditions and limitations of the capacitance probe itself. The transient soil water content after irrigation was divided into different phases (fitted to polynomial model regressions) according to the effect of different driving forces.

Continuous measurements of soil water content by FDR probe allowed an optimal moisture range to be set for the crop according to the soil properties, from which it was possible to precisely adjust the irrigation dose and frequency.

The scheduling of irrigation in young peach trees to maintain the soil water content in the main root zone (0-50 cm depth) at between +10 % and — 5% of field capacity permitted a water saving of about 18% compared with conventional scheduling based on ETc estimate, with no significant decrease in fruit yield or vegetative growth.

 

ACKNOWLEDGEMENTS

This study was supported by Ministerio de Educación y Ciencia (MEC), (AGL2006-12914-C02-01) and Séneca Foundation (03130/ PI/05) grants to the authors. O. Mounzer and W. Conejero were recipient of research fellowships from MEC, Spain, I. Abrisqueta from I3P-CSIC, Spain and Y. García-Orellana from Fundayacucho (Venezuela).

 

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