Progress is Sweet (Potatoes)
For years, researchers have searched for a carbohydrate crop that grows well, efficiently, and reliably in simplified hydroponics. It’s possible the search is over.
The Institute of Simplified Hydroponics (ISH) in Fair Play, Missouri, announced a breakthrough in the summer of 2016: researchers managed to produce sweet potatoes with simplified hydroponics.
Simplified hydroponic gardens can be set up without using electricity, fuel-based or mechanical equipment. The use of fertilizers can be limited to those derived from plant waste, composted chicken manure, or bat guano as long as there is no danger of contamination from chemical or non-organic pesticides.
Why Sweet Potato?
The sweet potato is likely a human-engineered adaptation of a plant in the morning glory family. A single one of the plant’s roots can supply three to nine root vegetables, which are usually what humans eat. There are now an estimated 10,000 varieties worldwide, ranging in colors from white to purple.
The orange-fleshed sweet potatoes are one of the best natural sources for vitamin A—half a kilo is likely to have four times the recommended daily requirement. The Okinawa purple variety is higher in protein and loaded with cancer fighting agents.
The green vine tops of the sweet potato are also edible and are used in salads and soups, or as a green vegetable like spinach or turnip greens.
Sweet potatoes are considered one of the world’s super foods. Studies of people living on a diet rich in this vegetable reveal life expectancy is about five years longer than that of a typical person in the US. Also, findings show a huge reduction in the rates of chronic diseases common in the US.
While this is likely not due to sweet potato alone, it does show that recommending this root as a primary staple is a positive step towards human health.
Carbohydrate Progress in Hydroponics
In the 22-year search for a carbohydrate crop that would work in simplified hydroponics, as many as 50 other crops have been tried.
NASA experimented with wheat in the CELSS program for space and found that wheat production for one square meter was just about eight ounces. And this was in high-tech hydroponics with climate control and artificial light.
However, another problem with grains is that only a small part of the plant is edible (15 per cent or less). So, a lot of time, water, and nutrient is used producing part of the product humans do not eat.
Corn is slightly better with 25 per cent of the crop being edible. Hydroponic corn experiments yielded a respectable 1.9 kilogram per square meter (kg/m2) in one season, and it looked like year-round corn could produce 6.8 kg/m2.
Root vegetables, on the other hand, showed more promise in production per space. In Dr. William Gericke’s classic book The Complete Guide to Soilless Gardening, he mentions an expected yield of about 0.25 pounds of Irish potatoes in his hydroponic system. This rate would expect a 13.62 kg/ m2 in 120 days.
Sweet Potato Trials
Helen Keller International (HKI), based in New York City, made a request to ISH to produce sweet potatoes as a substitute for Irish potatoes. This was because the vitamin A in the sweet potato could reduce childhood blindness associated with malnutrition.
The ISH planted two types of sweet potato: an orange-fleshed Beauregard from commercial plant starts and a local heirloom variety provided by a neighbor.
Even though the Beauregard were attacked by insects, likely Japanese beetle, the plants produced about five kg/m2. The local variety suffered no insect attack and produced nine kg/m2. Both varieties were harvested at 120 days growth.
Similar trials in Colombia produced 12 kg/m2 in 95 days. Those conducted in Bangladesh produced four to five kg/m2 in 120 days despite being attacked by insects.
Most of these trials, on average, were higher producing than soil-based sweet potato crops. Globally, the Food and Agriculture Organization of the United Nations found the average production of this tuber in soil to be 14 tons per hectare, or 1.4 kg per m2, although there are reports as high as 45-60 tons per hectare, or 4.5 to 6 kg/m2, in Israel.
Also unlike their dirt counterparts, hydroponic sweet potatoes can be grown in a small space of nine to 20 m2 with small amounts of hydroponic nutrients and water. This and the high yields are encouraging to researchers at ISH as they mean sweet potatoes could provide food security in the form of a dependable source of carbohydrates.
For hydroponic sweet potatoes to become a dependable crop, grow protocols need to be designed.
First, researchers need to discover the ideal depth for growing sweet potatoes in soilless media. Current literature recommends the best soil conditions for sweet potatoes is a softer top soil of about a 9-inch depth, with a deeper depth causing the sweet potatoes to grow long and skinny.
In their 2016 trials, ISH USA and Bangladesh tested varying depths of 12, 18, and 24 inches of substrate. In both trials, the 18- and 24-inch depths showed no increase in yield from the 12-inch. For 2017, however, ISH USA will test the depths of six, nine, 12, and 15 inches.
The depth experiments are crucial as they will help define what type of containers can be used to grow hydroponic sweet potatoes and if the total weight of such a system would rule out its potential as a roof-grown crop.
Next, there’s the question of pest control. In upcoming 2017 trials, ISH will test a garlic spray, a mixed garlic and pepper spray, and an organic commercial insecticide to deter insect pests.
As for nutrients, the experiments and trials so far have only used a grow nutrient. Through literature search and experience, it is likely that a switch to a root nutrient after the vines are established would help increase sweet potato yield.
The experiments in 2017 will test this hypothesis with six root nutrient formulas. Those six will help show how levels of nitrogen and potassium increase root harvest and test how selenium levels are affected. The selenium trials are particularly important because too much selenium can create a toxic plant.
Though there is still work to be done, there is evidence that sweet potatoes could be a reliable source of hydroponically grown carbohydrates. Besides the increasing yields in simplified hydroponics compared to soil, there is an advantage in that soil-borne diseases and bacteria can be avoided in simplified hydroponics. Still, only time will tell if these trials will lead to sweet victory.
Written by Peggy Bradley | Engineer, Founder of Institute of Simplified Hydroponics
Peggy Bradley has a Masters degree in civil engineering and used to own her own hydroponics business until she switched focus and became the founder of the Institute of Simplified Hydroponics, a US non-profit. Since 1995, Peggy has worked on creating and teaching simplified hydroponic systems to help people living in impoverished nations.