Hydroponics involves growing plants solely in a nutrient solution or in soilless substrates (Maher et al., 2007). This method of farming has several benefits, including reduced water use compared to field-based farming (Naresh et al., 2024; Alizaeh et al., 2025), year-round crop production regardless of seasonality (Barbosa et al., 2015; Lee and Lee, 2015; Khatri et al., 2024), and optimized nutrient delivery for rapid growth and shorter crop production cycles (Barbosa et al., 2015; Majid et al., 2021). Moreover, crop productivity (e.g., lettuce) is comparable between hydroponic and field-based farming systems (Lei and Engeseth, 2021). However, economic sustainability is a major challenge in hydroponic farming due to high initial investment and operational costs, leading to low profit margins (Naresh et al., 2024; Souza et al., 2019; Velazquez-Gonzalez et al., 2022).

Customer interest in organic products is rapidly increasing globally due to their benefits to personal wellness and the environment (Michaelidou and Hassan, 2008; Gundala and Singh, 2021; Nafees et al., 2022). Lettuce is one of the major organically grown crops in the US. In 2024, the wholesale value of organic lettuce sold in the US was $382 million (Organic Produce Network, 2024). Generally, organic crops are sold at higher retail prices than conventionally-grown crops (McBride et al., 2015). For example, a meta-analysis of various crops indicated that the price of organically grown produce was 29-32% higher than that of conventional produce in the US (Crowder and Reganold, 2015). Interestingly, hydroponic operations in the US can receive organic certification if they comply with the regulations set by the United States Department of Agriculture (USDA). Accordingly, hydroponic operations must use organic-certified materials, including organic substrates, organic-certified seeds or transplants, and approved organic fertilizers and pesticides (Maher et al., 2007; Park and Williams, 2024), to receive organic certification. Given that organic products are sold at a premium, it may be possible to increase revenue from hydroponic production by obtaining organic certification for hydroponically grown crops. This may potentially increase profits and improve the economic sustainability in hydroponics industry.

Nutrient uptake by plants follows a sigmoidal path, with approximately a third of the nutrients taken during the lag (slow growth) phase and the remaining two-thirds taken during the log (rapid growth) phase (Solis-Toapanta et al., 2020). A key aspect of hydroponic production is the presence of highly soluble nutrients from conventional (synthetic) fertilizers in the root zone to support rapid plant growth. To match the nutrient demand, nutrient release (e.g., controlled-release conventional fertilizers) also follows a similar trend (Chang-wen et al., 2006). Given this, the economic benefit of using organic practices is contingent on matching the nutrient release from organic fertilizers with the nutrient demand from plants. If there is a mismatch between nutrient release from organic fertilizers and nutrient demand from plants, expected high crop yields may not be possible in hydroponic production.

There is limited information related to nutrient release from organic fertilizers in hydroponic systems. Although many studies compared organic and conventional production in field-grown crops, there are limited studies that quantified crop yield responses with organic fertilizers in hydroponic systems. The objectives of our research were to (i) compare differences in nutrient release and their uptake by plants provided with customized organic and conventional fertilizers containing similar amounts of nutrient elements, (ii) quantify yield differences in hydroponically-grown lettuce cultivars supplied with the customized organic and conventional fertilizer mixes, and (iii) develop organic fertilizer management strategies that result in comparable lettuce yield with that of conventional fertilizers in hydroponic production.

Unlike conventional fertilizers, organic fertilizers that can provide both macro and micro nutrients are unavailable. Moreover, available OMRI-certified organic fertilizers do not provide more than three plant nutrients in one compound. Given this, it is difficult to customize organic and conventional mixes with identical amounts of different nutrients. In our study, we prepared a customized organic mix that can supply most of the nutrients required for plant growth. The identified slow-release conventional fertilizer mix (Osmocote 5-9-12) contained most of the elements and was closest in composition to the customized organic mix after addition of gypsum. The quantity of nutrient elements in the conventional and organic mixes in experiment I was almost similar except for lower levels of P₂O₅ and S and higher levels of Fe and B in the organic compared to the conventional fertilizer (Table 3). However, these differences did not result in any observable nutrient deficiency or toxicity symptoms in plants. Elements including Cu and Zn were absent in the organic fertilizer mix, but their levels were within a sufficiency range (Table 4) in the plant tissue. This is likely due to the presence of these elements as impurities in the fertilizer components used to prepare the mix.

Several lettuce cultivars recorded lower FW or economic yield in the organic mix than conventional treatment in our study (Figure 1). These results indicate that the organic fertilizer mix used in our study can reduce lettuce yields in hydroponic production. Further, the average reduction in yield loss can be significant (36%) with our customized organic fertilizer mix in hydroponic crop production. In support, a meta-analysis of field studies indicated that organically grown crop yields were lower by approximately 25% than those of conventionally grown crops (de Ponti et al., 2012; Seufert et al., 2012). Lower yield in organic systems was related to the limited nutrient availability to plants from organic compared to conventional fertilizers (Asadu et al., 2024). Although the organic sources are beneficial for building soil health by adding soil organic matter and improving microbial activity, they are slow at releasing nutrients (Hernández et al., 2016). The slow release of nutrients can potentially cause a mismatch between nutrient demand and supply. This may be the reason for lower yields in organic compared to conventional production systems.

The linear relation between ‘yield gap’ in the organic mix and FW of cultivars in the conventional mix (Figure 2) in our study suggest that production losses can be higher in cultivars with higher growth rate than those with slower growth rate. It is likely that the nutrient demand is more in cultivars with fast than slow growth rate, leading to observed larger yield gap. The large size cultivars likely benefited from the readily available nutrients in the conventional than organic mix. Further, slow nutrient release in the organic mix had a relatively smaller negative effect on growth in red than green cultivars. In general, red leaf cultivars of lettuce have slower growth rate than green leaf cultivars (Miller et al., 2020), which was also observed in our study except for ‘Red sails’ cultivar (Figure 1). The red color of leaves in these cultivars is due to increased biosynthesis of anthocyanin pigment which competes with growth for photosynthates (Son and Oh, 2013; Kong and Nemali, 2023). Due to a relatively slower growth rate, it is likely that the demand for nutrients was relatively lower in red than green cultivars, thereby a smaller negative effect on growth. These results support that organic fertilizer management can be relatively more important when fast growing cultivars are used in crop production.

A higher EC_sub in the conventional than organic mix (Figure 3) indicates an increased level of nutrients available in the root zone of conventionalmix, even when both fertilizer treatments contained a similar quantity of nutrients. This further supports that lower availability of nutrients was the likely reason for lettuce plants exhibiting reduced biomass production in the organic mix used in our study. Organic fertilizers are prepared from plant and animal materials such as manures, blood or bone meal, and fish emulsions (Nygaard Sørensen and Thorup-Kristensen, 2011; Panday et al., 2024; Ullah et al., 2023). These fertilizers are complex forms that require microbes for decomposition into simpler forms. Mineralization converts organic nutrients into inorganic forms for root absorption (Paul, 2015; Harraq et al., 2022). The whole process of nutrient conversion takes time and often fails to keep pace with the plant nutrient demand (Evanylo et al., 2008; Nair et al., 2014). In addition, microbial activities depend on environmental conditions such as moderate soil temperature, adequate moisture, and a neutral to slightly acidic environment (Chapin, 2011; Bian et al., 2022; Wang et al., 2024).

The EC_sub patterns during 1, 10, and 20 DAT (Figure 4) suggest differences in both nutrient release and nutrient uptake by plants in the organic and conventional mixes. The day 10 after transplanting is closer to the end of lag (slow growth) and beginning of log (rapid growth) phase in lettuce (Li et al., 2020). The plant growth and nutrient uptake rates were likely slower during the lag phase (i.e., between 1 and 10 DAT). Therefore, the nutrients released from fertilizers in both treatments likely accumulated in the substrate. This likely led toan increase in EC_sub, albeit at a lower level in the organic mix. The higher EC_sub in conventional than in organic mix during this stage indicates that nutrients were released more rapidly to the substrate in the conventional mix. The higher EC_sub in the conventional than in the organic mix at the start of the log phase likely supported nutrient demand from plants and increased plant growth during this stage. A faster growth rate further likely increased nutrient uptake, which in turn led to an increased depletion of nutrients and EC_sub by 20 DAT. On the other hand, EC_sub in the organic mix showed a gradual decline between 10 and 20 DAT, which is likely due to a relatively slower growth rate from a lower level of EC_sub. This likely reduced nutrient availability and nutrient uptake by plants during the log phase, leading to reduced growth in this treatment. Therefore, the differences in biomass production between conventional and organic treatments in our study are likely due to temporal changes in the availability of nutrients in the substrate, leading to a mismatch between nutrient demand by plants and nutrient supply by the organic compared to conventional mixes used in our study.

Although the levels of several elements (S, Zn, Cu, K, Mn, and B) were significantly different between the two fertilizer treatments, most of the levels were within the sufficiency range described for lettuce (Bryson et al., 2014). The organic mix contained a lower level of S and higher level of B than conventional mix (Table 3), which resulted in a lower S and higher B levels in the plant tissue in this treatment albeit being in the sufficiency range. Moreover, there were no toxicity symptoms from supra-optimal levels of Mn and B observed in plants. Despite Ca being deficient in both treatments, there were no deficiency symptoms (e.g., leaf tip burn) in plants. Although not statistically different between the two fertilizer treatments, Mg was deficient in the organic mix. However, the plants in the organic mix in our study did not exhibit any Mg deficiency symptoms (e.g., interveinal chlorosis). Therefore, it is unlikely these elements caused the observed differences in FW and DW between the fertilizer treatments.

Among different elements, N was the only element that was significantly lower and within deficiency range in plants subjected to the organic mix (Table 4). The plants in the organic mix in our study did exhibit symptoms of N deficiency (e.g., reduced vegetative growth). These results indicate that N deficiency was the major factor responsible for observed biomass differences between the two fertilizer treatments in our study. It is well known that N is a major component of chlorophyll molecule responsible for light absorption (Evans and Terashima, 1987) and constituent of rubisco enzyme involved in carboxylation (Makino, 2003). Plants preferentially lower shoot growth to promote root growth for increased absorption in response to N deficiency (Evans and Poorter, 2001). Another symptom of N deficiency is a decrease in leaf growth (Adhikari and Nemali, 2023), which further lowers light interception by plants. Therefore, we hypothesize that the lower biomass production in the organic compared to conventional mix in our study is due to slow nutrient release from the organic mix, leading to N deficiency and low levels of N_leaf. These differences likely affected photosynthetic performance and leaf growth in plants subjected organic treatment.

Experiment II was designed to test the hypothesis from experiment I and to identify a fertilizer management strategy to minimize the yield gap between conventional and organic mixes. No significant differences in FW and DW between conventional and high-dose organic mixes (Figures 5A, B) suggest that increasing the amount of customized organic mix incorporated into the substrate can increase nutrient availability and biomass production. In support, de Ponti et al. (2012) found that organic productivity relies on organic nutrient management, and abundant application of organic fertilizer increased the yield of field-grown organic crops. Although not statistically different, observed trends in FW and DW were not similar between the two treatments (Figure 5). Specifically, FW was slightly lower while DW was slightly higher in the high-dose organic than conventional mix. Slightly lower FW in the high-dose organic treatment could be due to differences in plant water status (as FW is mostly comprised of water weight) between conventional and high-dose organic mixes. It is possible that a high dose of organic mix in the substrate mildly increased osmotic stress and reduced the overall water uptake by plants. This may have resulted in a numerically lower FW in the high-dose organic than conventional mix. Further, no differences in yield were associated with lack of differences in N_leaf between conventional and high-dose organic mixes (Figure 5C), indicating that a higher dose of organic mix can enhance availability and uptake of N. These results support our hypothesis that low N_leaf was the predominant physiological trait influencing biomass production in organic mix used in our study.

A significantly higher EC_sub in the high-dose organic mix than in the conventional mix with residual fertilizer (Figure 6C) indicates that more nutrients were likely left over in the substrate with high-dose organic mix. Therefore, it is likely that not all of the substrate-incorporated organic fertilizer in the high-dose mix was completely utilized by plants in trial 1. To support this, the average FW of lettuce with residual fertilizer in the high-dose organic mix was higher than that of conventional mix (Figure 6A). Moreover, plants were of good size (approximately 69% of the average FW of the cultivars in trial 1) in the high-dose organic mix. Interestingly, the combined FW of lettuce plants from both trials was higher in the high-dose organic (82.8 g·pot^-1) than in the conventional (64.7 g·pot^-1) mix.

Based on our results, a high-dose of organic mix (similar to the one used in our study) can be used to potentially increase lettuce yield. This is only when the substrate is reused at least in two crop cycles. This approach will likely result in lowering costs and increasing yields and nutrient use efficiency in organic systems. Moreover, reusing the substrate with residual fertilizer can reduce environmental pollution. Precipitation can cause nutrients in the discarded substrate to leach and run off into natural water bodies. However, there are some considerations for reusing the substrate in subsequent production cycles, including: (i) necessity to add fresh substrate material in the second cycle to compensate for loss of substrate from cleaning and removing plant debris/roots, (ii) composition of residual nutrients in the substrate will likely be different from the original mix due to differential nutrient uptake rates by plants (Bugbee, 2004). This may lead to a deficiency of specific nutrients, especially those that are rapidly absorbed by plants, in the second cycle. (iii) reused substrate can harbor pathogens or accumulate phytotoxic substances, which may affect crop growth. However, we did not observe any deficiency or toxicity symptoms using our customized organic mix. Additional research will be needed to thoroughly validate the possibility of reusing residual nutrients and substrate hydroponic production. In addition, certain organic materials (e.g., biochar, Zabaleta Nievas et al., 2023), when incorporated into organic substrates can increase nutrient supply. The potential use of such materials to increase nutrient release from organic mixes should be tested in future studies.

Our research indicates that yield loss in hydroponically-grown lettuce can be significant when using a customized organic fertilizer mix developed in our study. This was due to slower nutrient release in the customized organic than conventional mix. The lower nutrient availability in the substrate resulted in reduced nutrient uptake and N deficiency in plants subjected to organic mix. A lower N_leaf was responsible for reduced biomass production in the organic compared to conventional fertilizer mix. Further, our results suggest that organic fertilizer management can be more important when fast-growing lettuce cultivars are used in hydroponic production due to their higher nutrient demand. Although a high-dose of organic mix improved nutrient availability, uptake, and increased biomass production, the amount of residual nutrients in the substrate with high-dose organic mixes can be significant. Therefore, a high-dose organic fertilization strategy can only be practical when the substrate is reused in subsequent cycles. Future research should focus on strategies that enhance nutrient release from organic fertilizers, such as using organic amendments or microbial supplements.