Updated: Oct 22, 2021
In order to grow food using indoor hydroponics, all you need is rain or tap water (but not untreated ground-water). Plant roots will be happy enough kept at room temperature and refreshed weekly. It's as simple as that. It can also be incredibly complex as described here in a highly complex hydroponic experiment by the National Institute of Health. Plant roots will thrive if we intricately control nutrient solution composition, water supply (rain/ground/tap), nutrient solution temperature, dissolved oxygen concentration, electrical conductivity and pH of the nutrient solution. So while understanding and controlling these variables can be complicated, the average municipal tap water is generally in the range to keep most plants quite happy.
When growing in only water, pH (potential of hydrogen) is key. The target pH for growing plants is different for various plants but in general, a slightly acidic 5.8 pH allows the plants to uptake the maximum amount of nutrients from water. Although 5.8 is the goal, just keeping your plants in the ideal pH range of 5.5 to 6.3 will ensure allow maximum growth. Outside of this range by more than 1 in either direction will start to put the plants under stress and they go into protection mode that will hinder their growth. The reason for this is due to the availability of nutrients that already exist in water. At higher or lower pH levels, the different nutrient molecules form less soluble compounds making it harder for the roots to absorb and use the nutrients available.
Water temperature is also key for maximum root growth and research suggests the maintaining approximately an average of 70F (or 20C) for ideal growth. Water oxygenation can be adequate just by changing it with fresh water weekly. Or just shake a water bottle to introduces oxygen into the water to help the roots health and nutrient uptake. In this short video below and the image above, I use plain tap water which is sourced from Jersey City Municipal Utilities Authority in Lake Hiawathalight and averages a pH of 7. It is not necessary to have the LED light during root regrowth until the plant has regrown some root structure. The water is room temperature and a small pump sprays water onto the celery or lettuce stump to keep it oxygenated. You don't need any fancy equipment to do this. Placing 1 inch of a lettuce or celery stump into a small bowl of water and refresh it every few days will result in root regrowth so you can replant it in the ground or hydroponic system.
Growing food successfully in a home based hobby hydroponic system is as simple as that. Once you have sufficient root growth, you may want to consider other factors like sunlight hours or an artificial light cycle, plus optimized nutrients or fertilizer.
When looking at, and learning from, the history of hydroponics, the start of a sustainable hydroponic food revolution may now be here. In 1627, Francis Bacon wrote Sylva Sylvarum the first documented book discovering what plants needed to survive. There were also records of murals and sightings of water born plants in the gardens of Babylon in 600BC, though not a very scientific discovery per se. It would not be until 1699 when professor John Woodward would scientifically test and prove that hydroponics works. What's true then is still true now. Plants only need water to grow and survive. Further discoveries from subsequent tests show that plants need water + some very specific added mineral nutrients to thrive. Fast forward to 1980s, NASA began researching hydroponic plant growth in more a complete and detailed scientific analysis that sparked today's hydroponic farm revolution. With human missions now planned for the moon and mars within the decade, NASA's ALS (Advanced Life Support) program is now in full swing. Many sustainable hydroponic food supply challenges remain but what is certain is that we can expect rapid advances in our understanding of sustainable food production and technology in the very near future. This new frontier will undoubtedly trigger an exponential and unstoppable advancement of sustainable hydroponically grown food here at home.
Frances Bacon, Sylva Sylvarum: https://www.smile.amazon.com/Sylva-Sylvarum-Natural-History-Centuries/dp/1564596397
John Woodward: https://en.wikipedia.org/wiki/John_Woodward_(naturalist)
NASA hydroponic summary research: https://www.nasa.gov/feature/nasa-plant-researchers-explore-question-of-deep-space-food-crops
NASA (ALS) study: https://www.mdpi.com/2073-4395/10/5/687/pdf