Irrigation Water Management and Quality in Two Rocha Pear Orchards ^†

Abstract

Tackling human malnutrition resulting from mineral deficits in foods is currently an agro-industrial problem. To address this problem, an agronomic workflow to enrich Rocha pears with calcium (Ca) was considered in two orchards in Portugal. This study aims to assess quality differences in the irrigation water of two orchards (of Rocha pear) where an agronomic Ca enrichment workflow would be performed and identify possible conditioning to Ca increases in fruits. Thus, electrical conductivity (EC), pH, pHs, cations (Na⁺, K⁺, Ca²⁺, and Mg²⁺), and anions (HCO₃⁻, Cl⁻, and SO₄²⁻) were attained to calculate the Sodium Adsorption Ratio (SAR) index and the Langelier Saturation Index (LSI) and assess the agricultural use. The values of EC, pH, pHs, SAR index and LSI of both orchards varied between 1198 and 1211 µS/cm, 7.4 and 7.5, 7.7 and 8.1, 3.5 and 7.4, and −0.69 and −0.21, respectively. Regarding Piper classification, irrigation waters were classified as sodium bicarbonate (orchard 1) and sodium chloride bicarbonate (orchard 2). Both orchards presented different classifications regarding agricultural use, namely C3S1 (orchard 2) and C3S2 (orchard 1). The water of both orchards presented the same salinity hazard (C3), but the use of these irrigation waters is enabled since these trees can be considered salt-tolerant. However, regarding the alkalinization hazard to soils, the irrigation water from orchard 2 offers less danger (S1) in comparison to orchard 1 (S2). Meanwhile, a slightly inferior LSI (orchard 1) can favor a higher tendency to dissolve calcium carbonate. In conclusion, although slightly different, analysis indicated that the waters of both orchards did not induce toxicity in Rocha pear trees.

1. Introduction

Agroindustries are currently struggling to improve food quality and minimize mineral deficits in human diets, especially with regard to land and hydric shortage [1]. Agronomic biofortification is a practice focusing on the use of soil fertilizers or foliar sprays that increase the levels of mineral elements in the edible parts of plants [2]. In this regard, calcium (Ca) is a mineral element that if in deficit can promote bone deformations or condition mobility [3]. Furthermore, an increase in fruit consumption is expected [1], and Rocha pear (Pyrus communis L.) is a Portuguese variety of importance to the country’s market [4]. However, in comparison to smaller crops where agronomic enrichment trials can first be performed in controlled environment chambers, for tree crops, a field trial is often needed, which emphasizes the importance of monitoring field conditions. Pear trees are not considered a drought-resistant crop, with their production in dry climates being highly dependent on irrigation practices [5]. In Rocha pear orchards, drip irrigation systems can be used and can be one of the most efficient for this crop [6].

Agriculture is the sector that uses the most freshwater globally (about 70%), and with food demand bound to increase, the same is expected to happen to water resources [7]. It is also known that plants’ water supply can affect the soil (moisture and nutrient availability) and ultimately fruit quality, since water is crucial for nutrient absorption as well as the maintenance of evapotranspiration processes, among other factors [8]. Furthermore, the dissolution and precipitation of mineral elements are related to chemical balances, water saturation, and drainage conditions.

The Langelier Saturation Index (LSI) is used to verify the encrusting tendency that water presents in relation to calcium carbonate (positive values indicate encrustation tendencies, while negative values suggest a dissolution tendency of calcium carbonate) [9].

The Wilcox diagram relates salinity (C) and alkalinization (S) hazards, based on conductivity and the Sodium Adsorption Ratio (SAR) index, respectively [10]. The SAR index considers the ratio between the concentration of sodium (Na), Ca, and magnesium (Mg) in the water, and the ionic balance between the soil and water. Thus, waters that are richer in Na than Ca and Mg tend to balance with the soil, dissolving Ca and Mg while precipitating Na. Ultimately, these waters tend to leach Ca and Mg from the soil. In this context, variations in the Ca content in waters (for instance, due to fertirrigation practices) can affect the SAR index, and eventually impact agronomic enrichment workflows, namely those performed with soil fertilizers in comparison to foliar sprays.

In this context, the following study aimed not only to evaluate quality differences between the irrigation water of two different orchards where the Rocha pear variety is produced, but also identify possible conditioning to Ca increases in fruits.

2. Materials and Methods

2.1. Orchards Location

Both orchards are located in the western region of Portugal, in the same district (Leiria). Orchard 1 (GPS coordinates: 39°23′28.997″ N; 9°4′52.48″ O) is located in the Caldas da Rainha region, and further north, orchard 2 is located in the Alcobaça region (GPS coordinates: 39°29′52.641″ N; 9°1′19.604″ O). Both orchards had a drip irrigation system installed.

2.2. Irrigation Water Quality

Water samples were collected from both orchards on 10 September 2019. Electrical conductivity (EC), pH, pHs, cations (Na⁺, K⁺, Ca²⁺, and Mg²⁺), and anions (HCO₃⁻, Cl⁻, and SO₄²⁻) were analyzed as described in Daccak et al. (2023) [11]. The SAR index and LSI were then calculated.

3. Results

In

Table 1. Values of electrical conductivity (EC), pH, Sodium Adsorption Ratio (SAR) index, pHs and Langelier Saturation Index (LSI) of irrigation waters from two Rocha pear orchards.

Parameter	Orchard 1	Orchard 2
EC (at 20 °C)	1211 µS/cm	1198 µS/cm
pH	7.4	7.5
SAR index	7.4	3.5
pHs	8.1	7.7
LSI	−0.69	−0.21

, the EC, pH, SAR index and pHs values of the irrigation water used in both orchards are shown.

The projection of the hydrochemical facies in a Piper’s triangular diagram (Figure 1), reveals that orchard 1 presents higher values of Na⁺, K⁺, SO₄²⁻, and HCO₃⁻ and inferior values of Ca²⁺, Mg²⁺, and Cl⁻ than orchard 2. Water classification for agricultural use (Figure 2) presents similar values of EC, while the SAR index is superior in orchard 1.

The irrigation waters from both orchards differ slightly, with the water from orchard 1 containing sodium bicarbonate according to the Piper classification, while orchard 2 contained sodium chloride bicarbonate (Figure 1). Higher values of the EC, SAR index and pHs were attained for orchard 1 (Table 1), while orchard 2 presented higher values of pH and LSI (Table 1). Based on the EC and SAR index of these waters, both projected in the Wilcox diagram (Table 1, Figure 2), when regarding their agricultural use, orchard 2 was classified as C3S1, while orchard 1 was classified as C3S2.

4. Discussion

Proper irrigation practices play a key role in fruit tree cultivation, and poor management can compromise fruit size and quality [8]. Furthermore, the physical and chemical composition of irrigation waters provide information about its effects on crops (toxicity) and soils (impermeability and/or alkalinization), as well as the maintenance of irrigation equipment (risk of scaling and corrosion).

Orchards for Rocha pear production can be installed near water lines with a pH of 6 to 7.5 [12]. Also, irrigation water used in pome fruit crops should have an EC value lower than 3000 µS cm⁻¹ and a concentration of chloride ions (Cl⁻) inferior to 355 mg·L⁻¹ [6]. The pH, EC, and [Cl⁻] values of both orchards (Table 1) (pH 7.4–7.5; EC–1198–1211 µS cm⁻¹; [Cl⁻]–125 to 143 mg·L⁻¹, for orchard 1 and 2, respectively) are in accordance with these reference values. Additionally, the EC values are related to the total content of dissolved salts in water (salinity hazard), and it is known that the uptake of water in the root zone of crops can be hindered by higher salinity values of the soil solution. Thus, the accumulation of salts in the root zone can be harmful, since the inhibition or reversal of the osmosis process can occur, compromising the acquisition of nutrients by pear trees from the soil [13]. Based on the information in Figure 2, the higher concentration of salts (C3) observed in both orchards indicates that the use of these irrigation waters in soils with poor drainage is unadvisable, and even in soils with adequate drainage, it may be necessary to apply additional measures to control salinization [10]. Thus, regarding salinity hazard, both orchards present very similar values. However, crops such as Rocha pear trees can be considered salt-tolerant (since EC values should be less than 3000 µS cm⁻¹) [6], enabling the use of these irrigation waters for this crop.

Regarding the alkalinization hazard to soils, the irrigation water from orchard 2 offers less danger of alkalinization (S1) in comparison to orchard 1 (S2), because the irrigation water used in orchard 2 has a lower concentration of Na⁺ in comparison to Ca²⁺ and Mg²⁺ ions, which favors its use in almost all soil types [10]. Thus, in orchard 1, an increase in the danger of soil alkalinization is observed, especially if the soils are poorly drained (with a predominance of fine textures such as clay) [10]. The SAR index is calculated based on Ca²⁺, Mg²⁺, and Na⁺ ions. Both Mg and Ca counteract the effects of Na, since waters with high values of SAR can alter soil physical parameters, since the adsorption of Na to soil particles promotes their hardening and compaction [14]. According to the same author [14], soils with finer textures are particularly affected, but the presence of high Ca and Mg contents in agricultural soils can counteract the mentioned effects. Thus, it should be noted that despite the increase in the SAR index, the danger of alkalinization in orchard 1 may have been minimized by higher Ca levels in the soil compared to orchard 2.

The irrigation of both orchards (as previously mentioned) was performed through the localized method, namely drip irrigation, which is an irrigation method authorized for pear crops [6]. Based on the LSI (−0.69 and −0.21), the negative values indicate that these are waters with a non-fouling tendency, with a slight propensity for dissolution because they are subsaturated in calcium carbonate and have a slightly corrosive tendency when circulating in pipes with this composition [9]. Orchard 1 showed a slightly higher tendency to dissolve calcium carbonate, which may favor Ca absorption, through the roots, and accumulation in the plant.

Overall, the values suggest that the water from both orchards cannot induce toxicity to the crop nor damage the soil in which they are located, and thus perhaps not compromise the biofortification route (by not damaging the soil in which the pear trees are located, nor compromising nutrient acquisition).

5. Conclusions

The irrigation waters used in both orchards presented a slight difference, namely when considering the alkalinization hazard to the soils in which the orchards are located.

When considering performing enrichment trials with fertilizer’s applications, fluctuations in the ratio between the concentration of Na, Ca, and Mg in the water, as well as the ionic balance between the soil and the water, should be considered, since they could impact the outcome of these trials.

Since water is crucial for fruit development, this study indicates that the waters of both orchards do not induce toxicity to Rocha pear trees and damage to the soil can be avoided with the monitoring of both the texture and mineral composition of soils. Overall, nutrient absorption appears to not have been conditioned, suggesting that fruit development was not impaired.