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Nitrogen (N) is needed by plants in the highest amount among root-zone-derived nutrients. Closed life-support systems for space missions will rely on in-situ food production for long-term sustainability, but high resupply and N fixation costs necessitate N recycling to support human nutrition for extended mission durations. We studied combinations of four N forms [ammonium, nitrate, urea, and N-fixing bacteria (protein)] for their use as an optimal N source for lettuce (Lactuca sativa) grown in peat moss in two container sizes (1.7 L and 8 L). Plants grown with ammonium nitrate consistently had the highest mass. When ammonium, urea, or microbial biomass were added to root-zones, hydrolysis and ammonification likely occurred, which may have led to slower growth. A larger container led to more variable growth among treatments and it was difficult to maintain even moisture throught the root-zone. Our results show that optimization of N source and growth conditions could allow for continuous N recycling in closed systems.

Controlled environment research on tomato (Solanum lycopersicum) is often hindered by large plant size. Desired cultivar traits for research include a compact plant size, fast flowering time, and good fruit set. We grew eight micro-dwarf tomato cultivars to examine their suitability as model plants for research. The cultivar ‘Rejina Red’ had the highest harvest index and most ripe fruits among the red-fruited cultivars; ‘Jochalos’ had the highest harvest index and most ripe fruits among the yellow-fruited cultivars. ‘Microtina’ had a higher harvest index than ‘Rosy Finch’, but fruits were smaller, and growth was less determinate. ‘Monetka’ had a germination rate of only 6% and low fruit set. ‘Rejina Red’ and ‘Jochalos’ had compact growth and excellent fruit set, and we recommend them for use in future tomato-based studies.

pH control is essential to well-managed hydroponics. Here, we describe the selection and interfacing of electrodes, controllers, and loggers.

Steady-state nutrient management is essential in hydroponic culture. Determination of an appropriate refill solution is attained using the principle of mass balance. Optimizing the ratio of nutrient elements and refill solution concentrations requires an understanding of the ratio of nutrients in tissue to water, which is measured as water use efficiency (WUE). This ratio is then multiplied by the desired concentration of nutrients in leaf tissue to determine the refill solution composition. Deep-flow hydroponics enables constant monitoring of solution parameters and root health. We have achieved a steady-state nitrogen concentration in solution by using an automated pH control system that adds a solution of 50 mM nitric acid and 200 mM ammonium sulfate. The pH remains stable throughout the life cycle as roots release a similar ratio of protons and hydroxide ions to balance uptake of either ammonium or nitrate. Daily monitoring of electrical conductivity (EC) guides adjustment of the refill solution concentration. A variable WUE and water to nutrient uptake ratio among species necessitates custom refill solutions. In our preliminary studies, wheat (Triticum aestivum) has a moderate WUE (3 g L-1) but ceases substantial nutrient uptake late in the lifecycle and the EC thus increases over time, causing nutrient accumulation in solution. Lettuce (Lactuca sativa) also has a moderate WUE (3 g L-1), and the EC of the solution remains stable. Tomato (Solanum lycopersicum) has a high WUE (6 g L-1), causing reduced EC and nutrient solution depletion. Monitoring solution compositions has allowed us to optimize the refill solution and acid addition concentration among species and lifecycle stages.

The Utah Hydroponic solutions for monocots and dicots have been developed and refined using mass balance principles coupled with tissue analysis from studies in 25-cm deep, continuously aerated, liquid hydroponics where there is no absorption or desorption with media. These solutions provide guidelines for irrigation of soilless substrates, but adjustments may need to be made to account for the significant cation exchange capacity and thus nutrient absorption and desorption with the substrates. These recipes were developed for greenhouse conditions with ambient CO2 and about 40% humidity. This results in a water use efficiency of about 3 grams per Liter. In high CO2 and higher humidity environments, the water use efficiency can be 6 grams per Liter, so the nutrient concentration must be approximately doubled.

Hydroponically grown corn (Zea mays) is highly susceptible to Fe-chlorosis. Iron (Fe) is essential for chlorophyll synthesis. Metals bound to chelates have reduced bioavailability (activity) compared to free ions.

A critical part of agricultural studies is determining plant stress and growth rate. Modern computer vision provides a series of tools that can be applied to derive this data. In this paper, we will show our findings, analyze their accuracy, and define a system capable of deriving this data with near-human accuracy in a fraction of the time. Denoising techniques applicable to this system will be discussed, as will our discoveries and findings. Finally, suggestions for further research opportunities will be provided.

Consistent delivery of up to 1 mM silicon (Si) to the root-zone is beneficial for plant health, but the maximum solubility of silicon is only 1.7 mM between pH 4 and 8. This makes Si difficult to concentrate and add through a proportioner in liquid fertilizer systems. The solubility of Si begins to increase with increasing pH above 9. We added potassium silicate (K2SiO3) and fumed silica (SiO2) to RO water that was pH adjusted to pH 5, 8, 9, 10, or 11 with either KOH or NaOH. Samples were filtered and dried to obtain the mass of undissolved solids in each treatment. The measured optical turbidity decreased with increasing pH and reached zero at pH 11. We found that this pH is necessary to maintain solubility in a concentrate tank. Below this pH, the concentrated silicon was not completely soluble and the precipitate damaged the injection system of proportioners. Moreover, increasing the pH prior to adding the potassium or fumed silica was necessary to achieve Si solubility. Increasing the pH after adding Si did not result in complete dissolution. The calcium in tap water promoted precipitation at all pH levels and it was necessary to use RO water. KOH and NaOH were equally effective in increasing solubility. By increasing pH of the concentrate tank to 11 prior to adding the Si we maximized solubility and minimized damage to proportioners from precipitate accumulation. The tank concentrate is optically clear with 15 mM Si, or 1700 mg per liter of K2SiO3. With 1 to 50 dilution from proportioners we now add 0.3 mM Si at each watering in our liquid fertilizer system.

Liquid fertilization is often used to supply silicon (Si) (Boldt & Altland, 2021), but Si is minimally soluble. Media components that provide a steady release of Si are valuable. There are many options for Si media amendments, but Si release rate is not well characterized. Our objective was to quantify Si release from media components to achieve a steady-state Si release as mono-silicic acid (H4SiO4) over time.