Francisco González Valdés
Salt washing as part of the production strategy in Hass avocado
Chili
A key aspect of increasing the efficiency of fertilization programs is the design of irrigation programs under salt-free soil conditions, despite the growing difficulty of applying leaching irrigations during increasingly dry winters.
Scientific models, historical statistical data, glacier retreat, and the progressive decline of the sclerophyllous forest (espino, peumo, quillay, and boldo trees) all demonstrate that rainfall has decreased significantly over the past 15 years. As a reference, in the Maipo River basin, at the elevation of the Olivares River glaciers—one of the sub-basins that contributes most to the Maipo River flow—a glacier mass covering 110 km² existed 150 years ago. Today, these glaciers cover only 54 km² due to rising atmospheric temperatures at higher elevations.
In the future, water resources for agriculture will become increasingly scarce, making irrigation programs ever more efficient. One factor inherently associated with irrigation management is salinity.
Most cultivated plants are sensitive to salinity and may show reductions in productivity even at relatively low salinity levels in irrigation water. In the case of avocado trees, it has been reported that an electrical conductivity (EC) level of 0.8 dS/m in irrigation water can already affect yield potential, while production losses may reach as much as 65% at an EC level of 1.8 dS/m (Figure 2).
An increase in salt concentration in irrigation water, and the subsequent accumulation of salts in the soil solution, inhibits growth and alters water relations within the plant, creating osmotic drought conditions (Figure 1). Stomatal opening, which governs transpiration and gas exchange between the plant and the atmosphere, is dramatically affected. The entire plant experiences reduced transpiration as a consequence of the osmotic effect generated by high salt concentrations in the rhizosphere.
Chloride and sodium ions accumulate in leaves and damage the photosynthetic machinery within chloroplasts. Sodium, in particular, also affects cytoplasmic pH regulation. All these effects are expressed as partial leaf burn and a significant reduction in the photosynthetic capacity of the entire plant.
As a result, leaf area decreases, sudden defoliation may occur during flowering, recently set fruit may drop, fruit drop may intensify during mid-season, and fruit size may be adversely affected. Depending on salinity levels at the beginning of the season and the characteristics of the accumulated ions in the soil, the productive capacity of the plant can be reduced by up to 50%.
Figure 1. Water movement within the plant as a consequence of salt concentration (EC) gradients between the soil solution and plant cell tissues.
Figure 2. Productivity affected by salt levels in irrigation water, assuming that irrigation water EC is similar to soil solution EC (saturated paste). Salt leaching practices are not considered.
Rainfall Events and Salt Leaching
Until a few years ago, rainfall levels considered normal (above 350 mm) were sufficient to provide natural salt leaching during the autumn-winter period, when plants are either dormant (deciduous species) or undergoing fruit maturation and floral induction (evergreen species).
The decline in rainfall across central Chile has resulted in accumulated salts from irrigation water not being adequately leached. Effective salinity management begins with understanding irrigation water quality and how both salt concentration and composition change throughout the year. In rivers of central Chile, increased flows caused by winter rains or summer snowmelt produce a noticeable dilution of salts.
On the other hand, it is not possible to leach salts from the soil when the salt concentration in irrigation water is higher than the salt concentration already present in the soil.
Rainwater, which is virtually salt-free, can partially improve the effectiveness of leaching irrigations, provided that the post-rain wetting front is followed by an early leaching irrigation event.
Conversely, a low-intensity rainfall event may actually intensify the damage caused by accumulated salts if no leaching irrigation is applied afterward.
Salt movement within the soil follows concentration gradients. Therefore, if a rainfall event of approximately 30 mm penetrates about 25 cm into the soil profile, the salt concentration in the upper layer (0–30 cm) would be expected to be lower than in the deeper layer (30–60 cm). Since most fruit trees concentrate their fine absorbing roots in the upper soil layer, the absence of a subsequent leaching irrigation will inevitably cause salts from deeper layers to move upward toward the root zone.
Within three to four weeks, this process may become evident through severe foliage burn caused by salts, characterized by necrotic leaf tips and an overall wilted appearance of the orchard.
Figure 3. Fruit set developing under saline conditions. The apparent good fruit set will be severely affected by the lack of photoassimilates and nutrients resulting from the progressive damage caused by salinity to the foliage.
Figure 4. Severe defoliation that leaves mature fruit susceptible to abortion due to extreme tree weakness. 10/02/2022.
Figure 5. Severe leaf damage in an orchard affected by salinity. Note that although chloride burn affects only about 10% of the leaf surface, toxicity has destroyed chloroplasts throughout most of the leaf, leaving less than 10% of the leaf photosynthetically active. Developing fruit remains exposed and susceptible to abortion. 10/02/2022.
Salt Monitoring and Control Strategies
Currently, moisture and salinity sensors are available that allow growers to monitor electrical conductivity (EC) behavior at different soil depths. These tools also make it possible to verify early on whether a rainfall event has effectively altered EC levels at specific points within the soil profile. Consequently, a leaching irrigation applied within 24 to 72 hours after a rainfall event can effectively regulate electrical conductivity to levels lower than those observed before the rainfall event (Figure 6).
Figure 6. Soil EC measured in April 2022 reached 2.7 dS/m. Following an irrigation event on June 20 combined with 24 mm of rainfall on June 23, EC dropped sharply to 2.0 dS/m by July 1, when the next rainfall event occurred, resulting in a 26% reduction in EC. Each peak represents an irrigation event.
As a complementary practice, taking advantage of the effectiveness of a leaching irrigation combined with a rainfall event, soil amendments can be incorporated to improve some of the salinity components that are most toxic to plants. For example, applying agricultural gypsum during winter months—whose solubility is approximately 2 g per liter of water—can reduce sodium concentrations in the upper soil layers, improving soil permeability for the irrigation season and reducing soil profile electrical conductivity through sodium displacement.
Despite its relatively low solubility compared to most fertilizers, agricultural gypsum can be efficiently incorporated when properly distributed within the irrigation band and followed by rainfall events of approximately 30 mm. The calcium supplied by gypsum remains available for plant nutrition beginning in spring, particularly for newly set fruit. During the early stages of development, fruit absorbs calcium, which becomes part of the cell walls and contributes to improved postharvest quality. In addition, gypsum improves soil particle aggregation, enhances water infiltration, promotes sodium leaching, and reduces competition between sodium and potassium uptake.
The Retrosal Strategy
One formulation that has been tested for salt leaching is Retrosal, particularly considering that large volumes of water are generally unavailable for leaching practices during the production season. Retrosal contains a calcium-based formulation along with other active ingredients that improve plant performance under high-salinity conditions.
The evaluation of Retrosal in an avocado orchard in the Coquimbo Region irrigated with saline water produced the following results:
Table 7. Soil fertility analysis results collected three months after the beginning of treatments. Quilimarí, April 2024. The Retrosal 7.5 treatment was applied at a rate of 7.5 L/ha for four consecutive weeks during January 2024. The Retrosal 5 + Yegún treatment was applied at 5 L/ha for four consecutive weeks and complemented with one application of Yegún Nativa.
The effectiveness of Retrosal at a rate of 7.5 L/ha for four consecutive weeks in January was reflected in a significant reduction in soil EC, whereas lower application rates did not achieve the same result. In addition, a significant reduction in exchangeable sodium was observed. The benefits of mycorrhizal applications as part of a salinity tolerance strategy were reflected in the nutritional status of the plants, although this topic will be discussed in greater detail elsewhere.
Retrosal applications have been recommended for orchards irrigated with water containing high salinity levels (NaCl), particularly at the onset of the fruit’s exponential growth phase. As shown in Table 7, the most effective program consisted of four consecutive applications at 7.5 L/ha, applied at intervals of 7 to 10 days.
Under field conditions, this strategy has been replicated in an avocado orchard planted in 2002, monitoring EC changes throughout the soil profile using FDR probes and a Teros 12 sensor installed at a depth of 30 cm. As illustrated in Figure 7, the reduction in soil EC gradually became established between February and March 2026.
Figure 7. Effect of four consecutive Retrosal applications (7.5 L/ha, blue arrows) between late December 2025 and mid-January 2026 for reducing soil profile salinity. Applications were timed to coincide with the onset of the fruit’s exponential growth phase. The reduction in salinity became fully established by late March.
There are also other groups of biostimulants based on plant growth-promoting rhizobacteria, including formulations containing Bacillus species. These microorganisms have been isolated from avocado orchards and are currently recognized for their ability to promote root growth even under conditions of high salinity (Nadeem et al., 2012).
Several commercial formulations of these microorganisms have already been evaluated by BellotoAgro. Their best performance has been observed when applied during December, and they can be used in combination with Retrosal. In general, they contribute to enhanced root growth and improved plant resilience under saline conditions.
References
Arpaia, Mary Lu. 2008. Salinity Chloride Interactions and Their Influence on Avocado Yields Relative Yield (%) Salinity (EC). California Avocado Society Yearbook 1 (2007).
Crowley, David. 2007. Managing Soils for Avocado Production and Root Health. California Avocado Society Yearbook 90: 107–130.
Crowley, D. 2008. Salinity Management in Avocado Orchards. California Avocado Society Yearbook 91: 83–104.
Nadeem, Sajid M., Baby Shaharoona, Muhammad Arshad, and David E. Crowley. 2012. Population Density and Functional Diversity of Plant Growth Promoting Rhizobacteria Associated with Avocado Trees in Saline Soils. Applied Soil Ecology 62: 147–154.
Francisco González Valdés
Avocado and citrus consultant - Bellotoagro
+56 97478 7535 -
fgonzalez@bellotoagro.cl
Learn more about us at bellotoagro.cl
