In contrast to soil-based cultivation, soilless cultures are prepared using solid substrates as rooting media. Especially the stone wool substrate is widely used in horticultural soilless crop production in controlled environment agriculture (Gruda, 2019). Further, it has confined root zone boundaries and rapidly responds to root zone environment manipulations (Sonneveld, 1991; Saha et al., 2008). Hence, the stone wool culture represents a basic production model, and technical frameworks have long been built on stone wool-based systems.

Typically, the monitored variables in a stone wool-based system include transpiration-dependent irrigation, drainage ratio, root-zone water content, and electrical conductivity (EC) (Shin and Son, 2016). In this system, temporal water control occurs in three major phases (Lee, 2010; Ahn et al., 2020): controlled irrigation commences post sunrise (to compensate for crop water use during the day and the previous night); water is discharged after substrate field capacity is reached; and the controlled irrigation ends around sunset. Furthermore, commercially available sensors such as for measuring radiation, infrared, gutter weight, and substrate moisture, can be utilized for on-farm applications (Shin and Son, 2016; de Koning and Tsafaras, 2017; Montesano et al., 2018). In all, stone wool-based agronomic systems have greatly improved the productivity of soilless crop farming.

Precision control techniques can be applied for the stone wool-based system as it was built on the standardized specifications of the root-zone environment. However, to maximize its performance, it is critical to understand the effect of changing certain parameters of the stone wool substrate. The stone wool substrate usually accommodates three to six plants per slab depending on the cultivation conditions (Jensen, 1997; Shin and Son, 2016), and nutrient and water management processes are carried out in a volume of approximately only 0.01 m³ with a plastic cover wrapping (Sonneveld, 1991; Bussell and Mckennie, 2004). A reduction in the size of the root zone volume lowers the buffering capacity of root environment and requires more stringent management of the system parameters (Van Noordwijk, 1990). Under such conditions, fine-tuning system parameters such as physical shape of the stone wool slab and dripper positions may significantly impact the root-zone environment and yield responses (De Rijck and Schrevens, 1998; Bar-yosef, 2008; Saiful Islam et al., 2008). Indeed, previous studies have analyzed the effects of stone-wool system parameters on the hydraulic properties of the stone wool substrate (Bougoul et al., 2005; Bougoul and Boulard, 2006). Stone wool cover is also of critical importance for drainage and gas exchange performance; however, the cover incision effects on the root-zone environment have rarely been studied as an optimizing factor under stone wool-based water management regime.

In our study, we used the stone wool substrate to test the effect of the plastic cover incision on the root-zone environment and crop yield. Stone wool substrates are formed into slabs and wrapped with a plastic cover. The slabs are used with the cover, and the way the drain hole is cut into the plastic cover can affect the liquid and gas exchange between the root and outside environment. Thus, users generally follow the standard procedures recommended by the manufacturer's guidelines, which primarily focus on favorable drainage performance (Grodan, 2022). Further, root zone temperature is another crucial environmental factor affecting plant growth (Zhao et al., 2021). The stone wool system has lower complexity and fewer variables than the soil system. However, the breathability of the cover, oxygen diffusion in the root zone, drainage rate, thermal condition, and water retention properties of the stone wool system could be sensitively intertwined due to its low buffering capacity, and minor modifications of these components could entail significant environmental variations. The potential cascading effects of modifying these variables can be significant to the root zone environment variability. Thus, in this study, we investigated the series of environmental changes caused by the variations in the slab cover incision levels and their subsequent effect on crop productivity. To our knowledge, this is the first study to investigate the association between slab cover incision and water, thermal, and dissolved oxygen parameters, and the subsequent effect of their variations on crop production. In a greenhouse experiment, four different levels of incisions were made in the stone wool slabs resulting in different levels of cover openness for tomato cultivation. Thereafter, we monitored the root-zone temperature, water content, and dissolved oxygen in the stone wool substrate water, along with the tomato yield.

Stone wool cultivation is amenable to precision control. Various commercially available sensors can be utilized to maximize the efficacy of agronomic technologies (Neupane and Guo, 2019). However, there is a risk of compromising the performance of crop production and water use if the impact of system parameters is neglected. The plastic cover is an important component of the stone wool substrate system, affecting the air and water exchange at the atmosphere and slab interface by modulating its level of openness. However, the systemic effects associated with the plastic cover incision that could potentially interfere with the water use and crop production performance of stone wool-based agronomic techniques have not been studied. Our study is the first to report a series of environmental variations in the slab, aroused by the cover incision modulation and its subsequent impact on crop productivity. The results of this study will improve our understanding of the intertwined relationship between the stone wool cover incision, the environmental root-zone variables, and the associated yield variations.

Previously, the stone wool cover incisions were made based on the grower's expertise or using the methods recommended by the manufacturer, which was sufficient to achieve proper drainage function (Grodan, 2022). To date, the method of making an incision into the stone wool plastic cover for drainage has not been considered as a significant factor, as long as the drainage function is as expected. We found that the stone wool cover incision level modified the stone wool substrate parameters, which substantially affected environmental root-zone variables related to the hydraulic, thermal, and gaseous properties and significantly impacted crop production (Figures 2–4). Our results demonstrate that the stone wool system with high drainage performance could adversely affect its environmental rhizosphere characteristics.

The variations in water content and dissolved oxygen, in our study, demonstrate a general relationship between the liquid and gaseous phases in a porous substrate; increasing the water content in a porous substrate reduces the air content in the medium (Unger et al., 2009). Therefore, high water content can limit oxygen diffusion from the air-filled pores within the substrate (Yanful et al., 2006). Therefore, hours of long-term exposure of the plant roots to flooding conditions can eventually deplete the dissolved oxygen in the liquid phase (Ruperti et al., 2019). The water distribution in Figure 2A shows a clear distinction between the cover incision levels and the maximum achievable water content. Based on the general interactions, the maximum water capacity in this study is related to the field capacity, which is a core parameter of the hydraulic properties of the growth substrate (Assouline and Or, 2014). The plastic stone wool cover could not actually change the physicochemical properties of the slabs; thus, we do not suggest that the cover incision modifies the hydraulic properties of the stone wool slab itself. However, the alterations of the achievable maximum water content triggered by different cover incision levels modified the thermal properties of the stone wool slabs. As shown in Figure 2B, the cover incision treatment affected the outlier temperatures of the root-zone temperature distribution. Although the median and quartile values were similar, the maximum outlier values were 30.2°C in L1 and 36.5°C in L4, exceeding the optimal root zone temperature (<25°C) (Tindall et al., 1990). Typically, the heat inside a greenhouse is induced by solar radiation and subsequently transferred to root-zone (Fitz-Rodríguez et al., 2010). Water, air, and stone wool materials are coexisting components of the root zone that differ in their heat capacity, with water having the highest heat capacity (Rubio et al., 2015). In addition, the root biomass itself is a significant component within the slab, with its own thermal properties. Therefore, the greater water content increased the thermal mass of the slab, conferring a root zone buffer to changes in air temperature.

Specifically, the tendency for increased outlier values could be derived from the elevated susceptibility to the thermal peak during the cultivation period due to a lower heat capacity and water distribution range. This is supported by our data; the correlation test between the greenhouse air temperature and the stone wool temperatures for each treatment identified different sensitivities to the thermal variations of the greenhouse air temperature (Figure 2C). L4, with the favorable drainage performance, had a high positive correlation and slope with air temperature. The slope and coefficient of determination decreased in descending order from L4 to L1. The high correlation associated with L4 suggested rapid responsiveness of the stone wool substrate under this treatment. Consequently, the incision level modifications of the stone wool substrate cover altered the hydraulic, thermal, and gaseous properties of the plastic wrapped stone wool slabs, resulting in significant changes in tomato production (Figure 3). The L1 treatment was applied as an incision level with poor drainage performance. Although water-logged condition could be an unfavorable for normal growth (Striker, 2012), high water conditions, in our study, did not result in hypoxia symptoms such as growth retardation (Fukao and Bailey-Serres, 2004) or a significant yield reduction. On the contrary, the L1 treatment was associated with the highest yield. Although the dissolved oxygen in the water extracted from the stone wool substrate indicated the lowest average value for L1, there were no significant differences between this treatment and the L2 and L3 treatments, which had normal drainage conditions (Grodan, 2022) (Figure 2D). Hypoxia development is a function of exposure time and oxygen conditions (Ruperti et al., 2019). Irrigation is frequently supplied in soilless culture systems (Ta et al., 2012; Shin and Son, 2016). Therefore, oxygen consumption in the root zone might be replenished by a series of irrigation events. Under these experimental conditions, high water content did not act as an inhibitory condition; on the contrary, it significantly enhanced plant growth parameters, such as stem diameter and fruit weight, increasing the tomato production linked to L1 treatment. These results are also supported by previous studies demonstrating the effects of root-zone water content on the plant stem diameter (Nortes et al., 2005; Li et al., 2012).

Root-zone temperature is an important yield-impacting environmental component, other than the water content and dissolved oxygen. In general, the root-zone temperature and yield response have a quadratic curve relation (Díaz-Pérez et al., 2005). Based on this relationship, plant growth can be increased up to the vertex of the curve and decreased thereafter. The normalized yield trends associated with temperature ranges during the experimental period suggested that differences in the heat capacity affected the tomato growth (Figure 4). This finding also suggests a trade-off between drainage performance and heat capacity in stone wool substrate systems during high-temperature weather. However, in this study, we have used a single stone wool product. Therefore, although stone wool products have similar physicochemical characteristics, the variations in the different stone wool products should be additionally considered.

The cover incision method significantly affected the average water content of the stone wool substrate, which is a predictable response typically expected by the modified drainage performance. However, we noted that this process could entail variations in the thermal sensitivity of the stone wool substrate with yield responses to water management. This result reveals another trade-off factor when considering the modulation of drainage and gas exchange performance in a stone wool-based system. Although the difference in average total tomato yield per plant between the highest (L4; 6,160 g/plant) and lowest drainage performance (L1; 6,749 g/plant) treatment was ~589 g/plant, significant differences at the individual plant level can be amplified into very large differences on a commercial scale. Our study reported a series of environmental changes and the resulting crop growth impacts that were not intuitively expected even for a relatively standardized and simplified system such as stone wool slab. In particular, we showed that generally unfavorable drainage conditions can be exploited as a beneficial buffer to changes in air temperature during hot periods. The expanded understanding of the effect associated with the cover incision provides insight into further optimization of water use and crop production in stone wool-based water management systems. For example, monitoring dissolved oxygen concentration, root zone temperature, and air temperature and modeling the relationship between these variables and drainage performance can be helpful for designing a more dynamic stone wool water management program that reduces the cooling energy load during the high-temperature periods. Further, for the technical aspects, developing a drainage performance adjustable gutter might be more effective for implementing this management model.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

TA and JL conceived and supervised the study. J-SY designed and performed the laboratory analysis. Y-HI and YY analyzed the data. TA wrote the manuscript. All authors approved the final version of the manuscript.

This work was supported by National R&D Program through the National Research Foundation funded by Ministry of Science and ICT (2020M3A9I3037807 and Project No. 1711156197), the Korea Institute of Planning and Evaluation for Technology in Food, Agriculture and Forestry (IPET) and the Korea Smart Farm R&D Foundation (KosFarm) through the Smart Farm Innovation Technology Development Program, funded by the Ministry of Agriculture, Food and Rural Affairs (MAFRA) and Ministry of Science and ICT (MSIT), and the Rural Development Administration (RDA) (Nos. 421006-03-2-HD040-KIST and 421039-03-2-HD020-KIST).

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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