Changyoon Jeong, Carrie Lott, Nolan Glenn and Patrick Colyer

When producers grow tomatoes in greenhouses, they tend to use excess fertilizer and water to maximize yields. This excess leads to the rapid accumulation of nitrate, phosphate and salinity in the growing media. In greenhouse systems, salinity accumulation in the growing media interrupts tomato seed germination, hinders plant growth and decreases yield. The critical mechanism of salinity impact on tomato production is mainly ionic osmotic stress created by the nutrient imbalance in the vascular system in the plant from a high concentration of sodium and chlorine ions in the soil. Accordingly, high salinity causes osmotic unbalance and then decreases plant water uptake leading to transpiration inhibition.

Tomato breeding programs to improve the salinity tolerance have been limited because of the genetic and physiological complexity of the tomato plant. Also, the selection of growing media for tomato production in the greenhouse is an important parameter for supplying water, air and nutrients in the pore spaces in the medium. Several factors affect air and water status in growing media, such as the media components and ratios, the height of the media in the container, and watering practices. The capacity of the media to hold and make available nutrients is affected by the cation exchange capacity (CEC) and the media pH. The CEC is defined as the holding capacity of exchangeable cations, which usually include most nutrients. Since most components of the media are acidic, which means a pH less than 7, the initial pH adjustment is critical for tomato production. The pH range should be between 5.8-6.2.

Traditional growing media for tomato production are peat, perlite, rock wool, shredded pine bark and vermiculite. The preliminary research from the Red River Research Station showed that perlite was easily recyclable as a growing media. Recycling media for tomato production will reduce initial costs for producers. However, tomato growers need more information on accumulation of salts in recycled media and their impact on tomato yield. The objective of the study was to evaluate the impact of salinity on tomato yields comparing recycled and new growing media.

Four varieties – Caiman, Torero, Taymyr and Geronimo – were transplanted to the growing bags with recycled and new perlite in the greenhouse after five weeks of growth. Two transplants were planted at a 3-inch depth and spaced 7 inches apart in each growth bag. Fruit clusters were managed to three or four fruit per cluster unless clipped to remove excess fruit from each pot. Tomato plants were fertilized with irrigation water three times weekly for 10 weeks, from October 5 to December 7, 2015.

The results of potting media analysis after harvest indicate that calcium, magnesium, nitrate and potassium were 1.3, 0.98, 1.6 and 1.4 times higher, respectively, from the recycled potting media than from the new potting media (Figure 1). Additionally, total phosphorus and soluble salt from the recycled potting media were 2.0 and 1.3 times higher, respectively, compared with results from the new potting media (Figure 2). The pH was decreased 0.43 in recycled growing media due to the replacement of salts in binding sites of growing media.

The total yield of tomatoes from a new perlite potting mix ranged between 9.32 and 10.79 pounds per plant, and the yields from an old recycled potting media varied between 9.05 and 10.49 pounds per plant (Table 1). Among the varieties, Caiman was the most sensitive to the salinity stress on total yield. Torero appeared to be the most salt-tolerant. These findings may help producers select the salt-tolerant varieties in tomato production with recycled growing media.

The material cost of tomato production is in Table 2. This study showed some yield reduction when 20-times recycled perlite was used in tomato production. The total savings from using recycled perlite was $13,150 per greenhouse. Further research is required on the threshold for recycling potting media without affecting tomato yield.

Changyoon Jeong, assistant professor; Carrie Lott, research farm supervisor; Nolan Glenn, research associate; and Patrick Colyer, Northwest Region director, are at the Red River Research Station, Bossier City.

Table 1. Influence of salts accumulation in growing media on tomato yields and reduction rate. The growing media was compared between new and recycled perlite for tomato production.

Table 2. Cost of perlite as a growing medium, grow bag and transplant container in 600 tomato plants in a 30-by-96-foot greenhouse at the Red River Research Station. The cost was calculated based on 2016 retail price.

Using recycled perlite saves money in production costs for greenhouse tomatoes, but yields are reduced. Further research is required on the threshold for recycling potting media without impacting tomato yield. These are plants at a greenhouse at the LSU AgCenter Red River Research Station in Bossier City. Photo by Changyoon Jeong

Figure 1. The potting media analysis of calcium, magnesium, nitrate and potassium concentration after tomato harvest.

Figure 2. The potting media analysis of pH, phosphorus and soluble salts concentration after tomato harvest.

Greenhouse at the Red River Research Station in Bossier City, where Changyoon Jeong, in front, conducts research. With him are Carrie Lott, farm supervisor, and Nolan Glenn, research associate.

Greenhouse at the Red River Research Station in Bossier City, where Changyoon Jeong, right, conducts research. Nolan Glenn is a research associate.