Use of RETENTIS® in tomato cultivation

Executive Summary 

In a two-year practical trial in plastic-covered greenhouses in Níjar (Almería, Spain), the biological water-retaining granules RETENTIS® were independently tested in tomato production. The aim was to save water while maintaining or improving yield and plant health.

All percentage and euro figures are based on the yield increases measured in the trial and a model calculation using regional average prices [2].

Introduction

In the southern Spanish region of Almería, vegetables are intensively produced for the European market on around 40,000 ha of plastic-covered greenhouse area—about 60% of the fruit vegetables consumed in the EU come from this region. Water there is an increasingly scarce and expensive resource.

Against this backdrop, a two-year practical trial investigated whether the biological water-retaining granules RETENTIS® can help tomato production use water more efficiently while supplying plants better at the same time. Juvenile development, flowering, vitality (NDVI), stem diameter and yield were assessed under full irrigation (100%) and reduced irrigation (80%).

1. Background

In the greenhouse areas around Almería, tomatoes are typically irrigated with more than 5,600 m³ of water per hectare and production cycle. On many farms, averages are around 10,000 m³/ha. Water is therefore a decisive factor not only ecologically but also economically.

This trial examined whether RETENTIS® (biological water-retaining granules) can store water in the root zone within the sandy top layer of the greenhouse soil profile and gradually release it back to the plants. The aim is to reduce irrigation by 20% while improving tomato yield performance and stress tolerance.

The trial was carried out over two consecutive years (2023 and 2024). In year 1, RETENTIS® was incorporated once; in year 2, no re-application was made in order to assess the product’s multi-year effect.

2. Trial design

All treatments received the same fertilization, crop management and plant-protection measures. Crop management was therefore identical for all treatments, so differences in development and yield can be clearly attributed to irrigation and the use of RETENTIS®.

3. Treatments

In the 2023 trial, six treatments were established. In 2024, the trial was repeated at the same site with a reduced number of treatments, focusing on the practical target rate of 250 g/m² RETENTIS®.

4. Irrigation

Irrigation was carried out via drip irrigation with two emitters per square meter and a flow rate of 3 l/h. Two irrigation levels were used (100% and 80%). The RETENTIS® treatments were run at the reduced irrigation level of 80%. The goal was to demonstrate that, with the help of the water-retaining granules, an equivalent or higher yield is possible despite using less water.

5. Results

5.1 Yield – total yield and individual harvests

Yield is the key parameter across the two trial years. In 2023, the total yield of the untreated control with 100% irrigation was 16.04 t/ha. The RETENTIS® treatments with reduced irrigation showed clear yield increases:

• RETENTIS® 250 g/m² (80% irrigation): 19.09 t/ha (≈ +19% vs. the 100% control).
• RETENTIS® 500 g/m² (80%): 19.34 t/ha (≈ +21%).
• RETENTIS® 1000 g/m² (80%): 20.26 t/ha (≈ +26%).

At the same time, the untreated control with 80% irrigation shows that a pure water reduction without RETENTIS® significantly depresses yield: only 13.72 t/ha were recorded here (≈ −14% vs. the 100% control).

In the second trial year (2024), the picture was as follows: the control with 100% irrigation achieved 25.40 t/ha, while the target rate RETENTIS® 250 g/m² at 80% irrigation reached 26.84 t/ha. This corresponds to a yield increase of about +5.7% vs. the 100% control and about +13.5% vs. the 80% control.

Across both years, 250 g/m² RETENTIS® at 80% irrigation shows a consistent yield increase of around 25% compared with the untreated 80% plots, while using 20% less water than 100% irrigation.

5.2 Stem diameter

Stem diameter was measured on 12 April 2023. The untreated control was 12.47 mm, and the control with 80% irrigation was 11.22 mm. The RETENTIS® treatments were within a narrow corridor around the control. Overall, only minor differences are visible in stem diameter. Importantly, the RETENTIS® treatments at 80% irrigation are at least at the level of the fully irrigated control, indicating that the plants’ mechanical stability does not suffer under reduced irrigation.

5.3 Flowering

Flowering was recorded on several dates in both years; the charts show the mean number of open flowers per plant. In 2023, the fully irrigated control was in the lower range, and the 80% control was consistently below it. Under reduced irrigation, the RETENTIS® treatments sometimes showed higher means and thus a clear tendency toward stronger flowering. In 2024, the RETENTIS® treatments were also above both controls. Overall, the results indicate more stable and more pronounced flowering under RETENTIS® with reduced irrigation.

5.4 NDVI – crop vigor

NDVI (Normalized Difference Vegetation Index) describes crop vigor and active leaf area. In 2023, NDVI values for the 100% control ranged between 0.67 and 0.73 depending on the date. The 80% control was slightly lower. The RETENTIS® treatments reached values up to 0.76:

• RETENTIS® 250–1000 g/m² showed NDVI values of 0.73–0.76 at the last date (27 April), i.e., up to +4% NDVI vs. the 100% control and clearly above the 80% control.
• Across all dates, the RETENTIS® treatments at 80% irrigation show a stable, slightly higher NDVI. The crops remain visibly more vigorous.

This picture is confirmed in 2024: the RETENTIS® treatments again reach NDVI values at least at control level and in some cases slightly above it, even though 80% irrigation is used. RETENTIS® therefore supports a more even water and nutrient supply and visibly mitigates water stress.

6. Conclusion

The two-year greenhouse trial shows clearly: RETENTIS® enables a 20% reduction in water use without harming tomato yield or vigor—on the contrary, both increase.

• Already in the first year (2023), all RETENTIS® treatments at 80% irrigation achieved clear yield increases of around 19–26% compared with the untreated control at 100% irrigation.
• In the second year (2024), without re-application, 250 g/m² RETENTIS® with 80% irrigation still achieved a yield increase of about +5.7% compared with the 100% control.
• RETENTIS® crops appeared more stress-resilient, for example after infestation with Tuta absoluta (tomato leafminer) and subsequent gray mold.

From an agronomic perspective, the treatment with 250 g/m² RETENTIS® and 80% irrigation is particularly interesting: it combines a practical application rate with a clear yield increase, lower disease susceptibility and noticeable water savings.

7. ROI – profitability

In addition to yield and water use, a detailed return-on-investment (ROI) assessment was carried out from the producer’s perspective [2]. The calculation is based on the yield increases measured in the trial (in %) and the water savings with 250 g/m² RETENTIS® at 80% irrigation.

• Yield increase 2023: +20% vs. the control with 100% irrigation, corresponding to an estimated additional revenue of €9,500/ha per year.
• Yield increase 2024 (without re-application): +5.6% vs. the 100% control, with an estimated additional revenue of €4,800/ha per year.
• Water savings: 20% less irrigation, equivalent to ≈ 1.12 million liters/ha or around 7,466 bathtubs.
• A water price of €0.30/m³ was assumed (a moderate assumption—prices for desalinated water are significantly higher), as well as a water use of 5,600 m³/ha.
• Tomato prices: average producer price based on the common tomato types from the Almería region.

8. Appendix – methodology (tomatoes)

The trials were carried out in standard plastic-covered greenhouses in the Níjar region (Almería). The trial plots reflected typical production conditions (soil profile, crop management, fertilization, plant protection) to ensure the results are transferable to practice.

• Yield: Individual harvests were recorded in kg per plot and converted to kg/m² and t/ha. In addition, a total yield per production cycle was calculated.
• Flowering: Number/share of flowers was assessed on representative plants on several dates during the generative phase.
• NDVI: Calculated from reflectance measurements in the near-infrared (NIR) and red spectral ranges using the formula (NIR−R)/(NIR+R). Values typically range between 0.2 (low activity) and 0.9 (high activity). Indicates the share of active leaf area, leaf greenness intensity and photosynthetic performance.
• Stem diameter: Measurement on the main stem at a defined height to reveal differences in vegetative development and stability.
• Diseases/stress: Occurrence of pests (e.g., Tuta absoluta) as well as secondary infections (gray mold) were assessed visually.

The percentage deviations (% vs. 100% or % vs. 80%) shown in the charts refer to the controls displayed in each chart. The differences shown are robust and consistent across both trial years.

9. Sources

1. SICOP Group trial documentation, tomato trial 2023/2024, Níjar (Almería, Spain).
2. Molina-Aiz, F.D., Valera, D.L., López, A., Bouharroud, R. & Fatnassi, H. (2020): Analysis of economic sustainability of tomato greenhouses in Almería (Spain). ISHS Acta Horticulturae 1296 – International Symposium on Advanced Technologies and Management for Innovative Greenhouses: GreenSys2019. DOI: 10.17660/ActaHortic.2020.1296.148 . Keywords: economic viability, production costs, profit, yield, horticulture.