Biochar is a widely known soil amendment. Here we synthesize the available information on influence of biochar application on different soil properties and crop productivity using meta-analysis. Global data on influence of biochar applications on different soil physical, chemical, microbial properties, and crop productivity were extracted from literature and statistically analyzed. Based on selection criteria, 59 studies from the literature published between 2012 and 2021 were selected for the meta-analysis. Correlations were developed between effect size of biochar application on different soil properties and crop productivity. Application of biochar increased soil pH, cation exchange capacity, and organic carbon by 46%, 20%, and 27%, respectively, with greater effects in coarse and fine-textured soils. Effects on chemical properties were variable among biochar prepared from different feedstocks. Among physical properties, biochar application reduced bulk densities by 29% and increased porosity by 59%. Biochar prepared at higher pyrolytic temperatures (> 500 ℃) improved bulk density and porosity to greater extents (31% and 66%, respectively). Biochar prepared at lower pyrolytic temperatures (< 500 ℃) had a greater effect on microbial diversity (both bacterial and fungal), with more diverse bacterial populations in medium and coarse textured soils, while fungal diversity increased in fine textured soils. Biochar applications increased crop productivity only in fine and coarse textured soil. The effect size of biochar application on crop productivity was correlated with responses to physical properties of soils. The meta-analysis highlighted the need to conduct long-term field experiments to provide better explanations for changes in biochar properties as it undergoes aging, its longer-term effects on soil properties, and timing of re-application of different biochars.

Warm-season legumes have been receiving increased attention as forage resources in the southern United States and other countries. However, the near infrared spectroscopy (NIRS) technique has not been widely explored for predicting the forage quality of many of these legumes. The objective of this research was to assess the performance of NIRS in predicting the forage quality parameters of five warm-season legumes—guar (Cyamopsis tetragonoloba), tepary bean (Phaseolus acutifolius), pigeon pea (Cajanus cajan), soybean (Glycine max), and mothbean (Vigna aconitifolia)—using three machine learning techniques: partial least square (PLS), support vector machine (SVM), and Gaussian processes (GP). Additionally, the efficacy of global models in predicting forage quality was investigated. A set of 70 forage samples was used to develop species-based models for concentrations of crude protein (CP), acid detergent fiber (ADF), neutral detergent fiber (NDF), and in vitro true digestibility (IVTD) of guar and tepary bean forages, and CP and IVTD in pigeon pea and soybean. All species-based models were tested through 10-fold cross-validations, followed by external validations using 20 samples of each species. The global models for CP and IVTD of warm-season legumes were developed using a set of 150 random samples, including 30 samples for each of the five species. The global models were tested through 10-fold cross-validation, and external validation using five individual sets of 20 samples each for different legume species. Among techniques, PLS consistently performed best at calibrating (R²_c = 0.94–0.98) all forage quality parameters in both species-based and global models. The SVM provided the most accurate predictions for guar and soybean crops, and global models, and both SVM and PLS performed better for tepary bean and pigeon pea forages. The global modeling approach that developed a single model for all five crops yielded sufficient accuracy (R²_cv/R²_v = 0.92–0.99) in predicting CP of the different legumes. However, the accuracy of predictions of in vitro true digestibility (IVTD) for the different legumes was variable (R²_cv/R²_v = 0.42–0.98). Machine learning algorithms like SVM could help develop robust NIRS-based models for predicting forage quality with a relatively small number of samples, and thus needs further attention in different NIRS based applications.

Use of hydroponics is increasing because of its ability to be used for year round vegetable production using an environmentally sustainable system. Management of solution pH is an important challenge in hydroponics systems. Our objective was to quantify the effects of various pH modifiers on growth and nutrient uptake of leafy greens and stability of nutrient solution’s pH. Lettuce, basil, and Swiss chard were transplanted into an Ebb and flow system, and nutrient solution pH was maintained using three different pH modifiers (pH Down, lime juice, or vinegar). The nutrient solution’s pH was maintained between 5.5 and 6.5. pH Down resulted in the most stable solution pH and required the least amount of product used when compared to lime juice and vinegar. The cost of using phosphoric acid or lime juice was greater than that of using vinegar. Vinegar reduced the yield of all crops in comparison to pH Down. When compared to pH Down, lime juice reduced the yield of basil and Swiss chard but not that of lettuce. Therefore, growers can use lime juice as an alternative to pH Down in lettuce production but not for basil and Swiss chard, while vinegar would not be recommended for any of the crops studied.

Carbon dioxide (CO₂) concentration is reported to be the most important climate variable in greenhouse production with its effect on plant photosynthetic assimilation. A greenhouse study was conducted using a nutrient film technique (NFT) system to quantify the effect of two different levels of CO₂ (supplemented at an average of 800 ppm and ambient at ~410 ppm) on growth and nutritional quality of basil (Ocimum basilicum L.) ‘Cardinal’, lettuce (Lactuca sativa L.) ‘Auvona’, and Swiss chard (Beta vulgaris L.) ‘Magenta Sunset’ cultivars. Two identical greenhouses were used: one with CO₂ supplementation and the other serving as the control with an ambient CO₂ concentration. The results indicate that supplemented CO₂ could significantly increase the height and width of hydroponically grown leafy greens. Supplemented CO₂ increased the fresh weight of basil ‘Cardinal’, lettuce ‘Auvona’, and Swiss chard ‘Magenta Sunset’ by 29%, 24.7%, and 39.5%, respectively, and dry weight by 34.4%, 21.4%, and 40.1%, respectively. These results correspond to a significant reduction in Soil Plant Analysis Development (SPAD) and atLEAF values, which represent a decrease in leaf chlorophyll content under supplemented CO₂ conditions. Chlorophyll, nitrogen (N), phosphorus (P), and magnesium (Mg) concentrations were generally lower in plants grown in supplemented CO₂ conditions, but the results were not consistent for each species. Supplemented CO₂ reduced tissue N concentration for basil ‘Cardinal’ and lettuce ‘Auvona’ but not Swiss chard, while Mg concentration was reduced in supplemented CO₂ for Swiss chard ‘Magenta Sunset’ only. In contrast, Fe concentration was increased under supplemented CO₂ for basil ‘Cardinal’ only. These findings suggest CO₂ supplementation could increase yield of leafy greens grown with hydroponics and have varying impact on different mineral concentrations among species.

Nutrient-film technique (NFT) trials were conducted to quantify the effect of two different water-soluble hydroponic fertilizers (5N–4.8P–21.6K and 5N–5.2P–21.6K) on different cultivars of lettuce ( Lactuca sativa ), basil ( Ocimum basilicum ), and swiss chard ( Beta vulgaris ). Results indicated swiss chard yield was affected only by cultivars, with Fordhook Giant producing the greatest fresh weight across fertilizer treatments. For lettuce production, interaction between fertilizers and cultivars was significant. ‘Mirlo’ and ‘Rubysky’ had greater growth compared with other cultivars in both fertilizers, whereas Dragoon performed well using 5N–4.8P–21.6K, but not 5N–5.2P–21.6K. For basil, dry weight production showed a significant interaction between fertilizers and cultivars. ‘Largeleaf’ produced greater dry weight with 5N–4.8P–21.6K, whereas ‘Lemon’ produced greater dry weight with 5N–5.2P–21.6K. For nutrient concentration of leaves, the concentrations were within the recommended range for lettuce when fertilized with 5N–5.2P–21.6K. Nutrient concentrations varied by nutrient from the recommended range for basil, but there was no significant difference between fertilizers. For swiss chard, the nutrient concentrations were in the recommended range and there was no difference between fertilizers. Therefore, growers may need to use more than one type of fertilizer for different lettuce and basil cultivars for optimum production, whereas swiss chard cultivars can be selected based on yield regardless of fertilizer.